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Guidelines for description

Guidelines for soil description

Fourth edition

FOOD AND ORGANIZATION OF THE UNITED NATIONS Rome, 2006 The designations employed and the presentation of material in this information product do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations concerning the legal or development status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.

All rights reserved. Reproduction and dissemination of material in this information product for educational or other non-commercial purposes are authorized without any prior written permission from the copyright holders provided the source is fully acknowledged. Reproduction of material in this information product for resale or other commercial purposes is prohibited without written permission of the copyright holders. Applications for such permission should be addressed to: Chief Publishing Management Service Information Division FAO Viale delle Terme di Caracalla, 00100 Rome, or by e-mail to: [email protected]

© FAO 2006 iii

Contents

Acknowledgements ix List of acronyms x 1. Introduction 1

2. General site information, registration and location 5 Profile number 5 Soil profile description status 5 Date of description 5 Authors 5 Location 6 Elevation 6 Map sheet number and grid reference (coordinates) 7

3. Soil formation factors 9 Atmospheric and weather conditions 9 Soil climate 9 Landform and topography (relief) 10 Major landform 10 Position 10 Slope form 12 Slope gradient and orientation 12 and vegetation 13 Land use 13 Crops 13 Human influence 13 Vegetation 16 16 Age of the land surface 17

4. Soil description 21 Surface characteristics 21 Rock outcrops 21 Coarse surface fragments 21 22 Surface sealing 23 Surface cracks 23 iv

Horizon boundary 24 Depth 24 Distinctness and topography 25 Primary constituents 25 Texture of the fine fraction 25 Rock fragments and artefacts 29 Degree of decomposition and humification of 32 Aeromorphic organic layers on floors 32 Soil colour (matrix) 33 Mottling 35 Colour of mottles 35 Abundance of mottles 35 Size of mottles 35 Contrast of mottles 36 Boundary of mottles 36 Soil redox potential and reducing conditions 36 Determination of redox potential by field method 36 Reducing conditions 37 Carbonates 38 Content 38 Forms 38 39 Content of gypsum 39 Forms of secondary gypsum 39 Readily soluble salts 40 Procedure 40 Field soil pH 41 Soil odour 42 Andic characteristics and volcanic glasses 42 Procedure 42 content 43 Organization of soil constituents 44 44 Consistence 48 Soil-water status 50 Bulk density 50 Voids (porosity) 52 Porosity 52 Type 52 Size 53 Abundance 53 v

Concentrations 53 Coatings 54 and compaction 56 concentrations 58 Biological activity 59 59 Other biological features 60 Human-made materials 60 Artefacts 60 Human-transported material (HTM) 61 and technic hard rock 62 Description of artefacts 63 Description and determination of human-transported material 64 Sampling 64

5. Genetic and systematic interpretation – 67 designation 67 Master horizons and layers 67 Transitional horizons 71 Subordinate characteristics within master horizons and layers 71 Conventions for using letter suffixes 75 Vertical subdivisions 75 Discontinuities 76 Use of the prime 77 Principles of classification according to the WRB 77 Step 1 79 Step 2 79 Step 3 79 Step 4 79 Principles and use of the qualifiers in the WRB 80 Checklist of WRB diagnostic horizons, properties and materials 81 Appending texture and parent material information to the reference soil group 82

References 85

Annexes 1. Explanation of soil temperature regimes 87 2. Explanation of regimes 91 3. Equipment necessary for field work 97 vi

List of tables

1. Soil profile description status 6 2. Codes for weather conditions 9 3. Soil temperature and moisture regime codes 10 4. Hierarchy of major landforms 11 5. Subdivisions for complex landforms 11 6. Classification of slope forms 12 7. Slope gradient classes 12 8. Land-use classification 14 9. Crop codes 15 10. Recommended codes for human influence 15 11. Vegetation classification 16 12. Hierarchy of lithology 18 13. Provisional coding for age of land surface 19 14. Recommended classification of rock outcrops 21 15. Classification of coarse surface fragments 22 16. Classification of erosion, by category 22 17. Classification of total area affected by erosion and deposition 22 18. Classification of erosion, by degree 22 19. Classification of erosion, by activity 23 20. Classification of attributes of surface sealing 23 21. Classification of surface cracks 24 22. Classification of characteristics 24 23. Classification of bleached characteristics 24 24. Classification of horizon boundaries, by distinctness and topography 25 25. Key to the soil textural classes 28 26. Abundance of rock fragments and artefacts, by volume 29 27. Classification of rock fragments and artefacts 30 28. Classification of shape of rock fragments 31 29. Classification of of coarse fragments 31 30. Codes for primary mineral fragments 31 31. Field estimation and coding of the degree of decomposition and humification of peat 32 32. Classification of the abundance of mottles 35 33. Classification of the size of mottles 35 34. Classification of the contrast of mottles 36 35. Classification of boundary between mottle and matrix 36 vii

36. Redoximorphic soil characteristics and their relation to rH values and soil processes 36 37. Reductimorphic colour pattern and occurrence of Fe compounds 37 38. Classification of carbonate reaction in the 38 39. Classification of forms of secondary carbonates 38 40. Classification of gypsum content 39 41. Classification of forms of secondary gypsum 39 42. Classification of salt content of soil 40 43. Dependency of water content of saturation extract on texture and content of for mineral and on decomposition for peat soils 41 44. Classification of pH value 41 45. Classification of soil odour 42 46. Estimation of organic matter content based on Munsell soil colour 43 47. Classification of structure of pedal soil materials 45 48. Classification of types of soil structure 46 49. Codes for types of soil structure 46 50. Size classes for soil structure types 47 51. Combined size classes for soil structure types 47 52. Combinations of soil structures 47 53. Consistence of soil mass when dry 48 54. Consistence of soil mass when moist 49 55. Classification of soil stickiness 49 56. Classification of soil plasticity 49 57. Classification of moisture status of soil 50 58. Field estimation of bulk density for mineral soils 51 59. Field estimation of volume of solids and bulk density of peat soils 52 60. Classification of porosity 52 61. Classification of voids 53 62. Classification of diameter of voids 53 63. Classification of abundance of pores 53 64. Classification of abundance of coatings 55 65. Classification of the contrast of coatings 55 66. Classification of the nature of coatings 55 67. Classification of the form of coatings 56 68. Classification of the location of coatings and accumulation 56 69. Classification of the continuity of cementation/compaction 56 70. Classification of the fabric of the cemented/compacted layer 56 71. Classification of the nature of cementation/compaction 57 72. Classification of the degree of cementation/compaction 57 73. Classification of the abundance of mineral concentrations, by volume 58 viii

74. Classification of the kinds of mineral concentrations 58 75. Classification of the size and shape of mineral concentrations 58 76. Classification of the hardness of mineral concentrations 58 77. Examples of the nature of mineral concentrations 59 78. Colour names of mineral concentrations 59 79. Classification of the diameter of roots 60 80. Classification of the abundance of roots 60 81. Classification of the abundance of biological activity 60 82. Examples of biological features 60 83. Classification of kinds of artefacts 63 84. Determination table and codes for human-made deposits 64 85. Subordinate characteristics within master horizons 72 86. Checklist of WRB diagnostic horizons, properties and materials 81

List of figures

1. The process of soil description, classification, site quality and suitability evaluation 1 2. Slope positions in undulating and mountainous terrain 11 3. Slope forms and surface pathways 12 4. Relation of constituents of fine earth by size, defining textural classes and sand subclasses 27 5. Charts for estimating proportions of coarse fragments and mottles 30 6. Soil structure types and their formation 45 7. Qualification of bulk density 51 8. Charts for estimating size and abundance of pores 54 ix

Acknowledgements

This revision was prepared by R. Jahn (University of Halle-Wittenberg), H.-P. Blume (University of Kiel), V.B. Asio (Leyte State University), O. Spaargaren (ISRIC) and P. Schad (Technische Universität München), with contributions and suggestions from R. Langohr (University Gent), R. Brinkman (FAO), F.O. Nachtergaele (FAO) and R. Pavel Krasilnikov (Universidad Nacional Autónoma de México). x

List of acronyms

EC Electrical conductivity GPS Global Positioning System HDPE High-density polyethylene HTM Human-transported material ISO International Organization for Standardization PVC Polyvinyl chloride RSG Reference Soil Group USDA Department of Agriculture UTM Universal Transverse Mercator WRB World Reference Base for Soil Resources 1

Chapter 1 Introduction

The main objective of research in is the understanding of the nature, properties, dynamics and functions of the soil as part of landscapes and ecosystems. A basic requirement for attaining that objective is the availability of reliable information on and other characteristics obtained through examination and description of the soil in the field. It is important that soil description be done thoroughly; it serves as the basis for soil classification and site evaluation as well as interpretations on the genesis and environmental functions of the soil. A good soil description and the derived knowledge on the genesis of the soil are also powerful tools to guide, help explain and regulate costly laboratory work. It can also prevent errors in soil sampling. Figure 1 shows the role of soil description as an early step to classification, soil and site assessment, and suitability evaluation.

FIGURE 1 The process of soil description, classification, site quality and suitability evaluation

1. Registration, Number, author, date, location description status, locality

2. Soil formation Climate, landform, parent factors material, land use, vegetation, age and history of landscape

3. Horizons and layers Identification of boundries Observations For each horizon/layer: and measurements Characteristics of rock fragments, texture, colour, horizons/layers pH, carbonates, structure, bulk- density, biological activity, ...

4. Interpretation of soil Qualities of 5. formation processes horizons

Genetic and Interpretation Designation systematic Qualities of soil of ecological of horizons interpretation site qualities (classification)

Identification of Site qualities soil unit

Suitability evaluation comparison of land use requirements with site qualities 2 Guidelines for soil description

Soils are affected by human activities, such as industrial, municipal and agriculture, that often result in soil degradation and loss or reduction in . In order to prevent soil degradation and to rehabilitate the potentials of degraded soils, reliable soil data are the most important prerequisite for the design of appropriate land-use systems and practices as well as for a better understanding of the environment. With the present internationalization, the use of a common language is of prime importance, also in soil science. The increasing need for internationally accepted rules and systems of soil description and soil classification led to the development of various soil classification concepts, e.g. the FAO–UNESCO Legend for the of the World (FAO–UNESCO, 1974, 1988) and Soil Taxonomy (USDA Staff 1975, 1999), and soil maps, e.g. the Soil Map of the World (FAO– UNESCO, 1970–1981; FAO, 2002), Soil Map of the European Communities (ECSC–EEC–EAEC, 1985), and Soil Atlas of (EC, 2005). These guidelines are based on the internationally accepted Guidelines for Soil Description (FAO, 1990). Some new international developments in soil information systems and soil classification, such as the Field Book for Describing and Sampling Soils (Schoeneberger et al., 2002) and Keys to Soil Taxonomy (USDA Soil Survey Staff, 2003), Updated Global and National Soils and Terrain Digital Databases (ISRIC, 2005) and the second edition of the World Reference Base for Soil Resources (IUSS Working Group WRB, 2006) are taken into consideration. For practical reasons, the contents of the major sources were modified, shortened and rearranged. Specifically, the various chapters of this field guide were based on the following sources: ¾Chapter 2 on general site description – Guidelines for Soil Description (FAO, 1990). ¾Chapter 3 on the description of soil forming factors – Guidelines for Soil Description (FAO, 1990); updated SOTER (ISRIC, 2005); Field Book for Describing and Sampling Soils (Schoeneberger et al., 2002); and Keys to Soil Taxonomy (USDA Soil Survey Staff, 2003). ¾Chapter 4 on soil description – Guidelines for Soil Description (FAO, 1990) and partly the German Mapping Guide 5 (Kartieranleitung 5; Ad- hoc-AG-Boden, 2005), the material of DVWK (1995), Field Book for Describing and Sampling Soils (Schoeneberger et al., 2002), as well as the personal experiences of the authors. ¾Chapter 5 on horizon designation and soil classification – Guidelines for Soil Description (FAO, 1990), Field Book for Describing and Sampling Soils (Schoeneberger et al., 2002), Keys to Soil Taxonomy (USDA Soil Survey Staff, 2003) and the second edition of the World Reference Base for Soil Resources (IUSS Working Group WRB, 2006). To help beginners, some explanatory notes are included as well as keys based on simple tests and observations for the determination of soil characteristics. The guidelines provide a complete procedure for soil description and for collecting field data necessary for classification according to second edition of the Chapter 1 – Introduction 3

World Reference Base for Soil Resources (WRB) (IUSS Working Group WRB, 2006). Notes for classification purposes are added to each chapter and explain the relevance of the described feature for classification according to the WRB. In order to avoid being excessively lengthy, it is not stated whether the described feature is a required one or is one of two or more options.

5

Chapter 2 General site information, registration and location

Before any actual soil description should be done, it is necessary to take note of some relevant information related to the registration and identification of the soil to be described, such as profile number, description status, date of description, author, location, elevation, map sheet number, and grid reference. This information is necessary for easy referencing and retrieval of the soil description from data storage systems.

PROFILE NUMBER The profile number or profile identification code should be constructed in such a way that it meets local needs and also allows easy and simple retrieval of profile descriptions from computerized data storage systems. The profile identification code should be constructed from a combination of a location letter code and a profile number code. The letter code should consist of a practical selection of codes referring to a country, preferably the internationally accepted International Organization for Standardization (ISO) code, a topographic map reference or any other defined area or town. Example: DE/ST/HAL -0381 = Halle in Saxony- Anhalt in Germany, profile 381.

SOIL PROFILE DESCRIPTION STATUS The status of the soil profile description refers to the quality of the soil description and the analytical data. The status is allocated after completion of the analyses and is indicative of the reliability of soil profile information entered into a database. Table 1 lists the possible descriptions.

DATE OF DESCRIPTION It is important to always indicate the date of description in order to inform future users of the soil data as to how old the data are. The date of description is given as: yymmdd (six digits). For example, 8 January 2006 would be coded 060108.

AUTHORS The persons who perform the description need to be acknowledged properly in future uses of the soil data. In addition, they hold responsibility for the quality of the data. The names or initials of the authors are given. 6 Guidelines for soil description

TABLE 1 Soil profile description status Status 1 Reference profile description No essential elements or details are missing from the description, sampling or analysis. The accuracy and reliability of the description and analytical results permit the full characterization of all soil horizons to a depth of 125 cm, or more if required for classification, or down to a C or R horizon or layer, which may be shallower. 1.1 If soil description is done without sampling. 2 Routine profile description No essential elements are missing from the description, sampling or analysis. The number of samples collected is sufficient to characterize all major soil horizons, but may not allow precise definition of all subhorizons, especially in the deeper soil. The profile depth is 80 cm or more, or down to a C or R horizon or layer, which may be shallower. Additional augering and sampling may be required for lower level classification. 2.1 If soil description is done without sampling. 3 Incomplete description Certain relevant elements are missing from the description, an insufficient number of samples was collected, or the reliability of the analytical data does not permit a complete characterization of the soil. However, the description is useful for specific purposes and provides a satisfactory indication of the nature of the soil at high levels of soil taxonomic classification. 3.1 If soil description is done without sampling. 4 Soil augering description Soil augerings do no permit a comprehensive soil profile description. Augerings are made for routine soil observation and identification in soil mapping, and for that purpose normally provide a satisfactory indication of the soil characteristics. Soil samples may be collected from augerings. 4.1 If soil description is done without sampling. 5 Other descriptions Essential elements are missing from the description, preventing a satisfactory soil characterization and classification. Note: Descriptions from soil augerings or from other observations made for routine soil mapping are either kept on ordinary field data sheets or included in the database, with an appropriate indication of status.

LOCATION A description of the soil location should be given. It should be as precise as possible in terms of the distance (in metres or kilometres) and direction to the site from permanent features that are recognizable in the field and on the topographic map. Distances along or traverses relate to a marked reference point (0.0 km). The description of the location should be such that readers who are unfamiliar with the area are able to locate the approximate position of the site. The administrative units, such as region, province, district, country or locality, are given in the profile number section (above). Example: Agricultural research station Bad Lauchstädt, Sachsen-Anhalt.

ELEVATION The elevation of the site relative to sea level should be obtained as accurately as possible, preferably from detailed contour or topographic maps. Where such information is not available, the best possible estimate is made from general maps or by altimeter readings. At present, determination of elevation by the Global Positioning System (GPS) unit is inaccurate and unacceptable. Elevation is given in metres (1 foot = 0.3048 m). Chapter 2 – General site information, registration and location 7

MAP SHEET NUMBER AND GRID REFERENCE (COORDINATES) The number of the topographic map sheet, preferably at 1:25 000 or 1:50 000 scale, on which the soil observation occurs is given. Example: TK50 L4536 Halle (Saale) = Topographic map 1:50 000 Number L4536 of Halle. The grid reference number, Universal Transverse Mercator (UTM) or the established local system, can be read directly from the topographic map. The latitude and longitude of the site are given as accurately as possible (in degrees, minutes, seconds and decimal seconds); they can be derived directly from topographic maps or a GPS unit. Example: H: 56.95.250 or latitude: 51° 23´ 30.84´´ N; R: 44.91.600 or longitude: 11° 52´ 40.16´´ E. Some countries use their own zero longitude, e.g. Italian topographic maps show the Monte Mario meridian at Rome as zero. For international use, these should be converted to the zero meridian of the Greenwich system.

9

Chapter 3 Soil formation factors

This chapter provides the guidelines for the description of factors that define the kind and intensity of soil formation processes. These factors are also part of the important site qualities. The information may be derived from a combination of field measurements, climate records, field observations and evaluation of climate, topographical, geological and geomorphological maps and documents. For land use and vegetation, the present conditions are reported.

ATMOSPHERIC CLIMATE AND WEATHER CONDITIONS The climate conditions of a site are important site properties that influence plant growth and soil formation. As minimum climate data, the monthly mean temperature (in degrees Celsius) and the monthly mean precipitation (in millimetres) can be taken from the nearest meteorological station. Where available, the length of the growing period (in days) should be specified. The length of the growing period is defined as the period with humid conditions (excess of precipitation over potential evapotranspiration) during the time with temperature ≥ 5 °C (FAO, 1978). The present as well as the weather conditions days or weeks before the description influence soil moisture and structure, hence these should be noted. In addition, the prevailing general weather conditions and the air temperature TABLE 2 at the time of observation as well Codes for weather conditions as that of the near past should be Present weather conditions (Schoeneberger et al., 2002) documented (Table 2). SU sunny/clear PC partly cloudy OV overcast SOIL CLIMATE RA rain Where applicable, the soil climate SL sleet classification should be indicated. SN snow The soil moisture and temperature Former weather conditions (Ad-hoc-AG-Boden, 2005) regimes according to Keys to Soil WC 1 no rain in the last month Taxonomy (USDA Soil Survey WC 2 no rain in the last week Staff, 2003) may be mentioned WC 3 no rain in the last 24 hours (Table 3; explanations in Annexes 1 WC 4 rainy without heavy rain in the last 24 hours WC 5 heavier rain for some days or rainstorm in the and 2). Where such information is last 24 hours not available or cannot be derived WC 6 extremely rainy time or snow melting from representative climate data Note: For example: SU, 25 °C; WC 2 (= sunny, temperature 25 °C, no with confidence, it is preferable to rain in the last week). 10 Guidelines for soil description

TABLE 3 Soil temperature and moisture regime codes Soil temperature regime Soil moisture regime PG = Pergelic AQ = Aquic PQ = Peraquic CR = Cryic DU = Udic PU = Perudic FR = Frigid IF = Isofrigid US = ME = Mesic IM = Isomesic XE = Xeric TH = Thermic IT = Isothermic AR = Aridic and TO = Torric HT = Hyperthermic IH = Isohyperthermic

leave the space blank. Other agroclimate parameters worth mentioning would be a local climate class, the agroclimate zone, length of growing period, etc.

Note for classification purposes 9Soil temperature < 0 °C (pergelic soil temperature regime) A cryic horizon and Gelic qualifier.

LANDFORM AND TOPOGRAPHY (RELIEF) Landform refers to any physical feature on the earth’s surface that has been formed by natural processes and has a distinct shape. Topography refers to the configuration of the land surface described in four categories: ¾ the major landform, which refers to the morphology of the whole landscape; ¾ the position of the site within the landscape; ¾ the slope form; ¾ the slope angle.

Major landform Landforms are described foremost by their morphology and not by their genetic origin or processes responsible for their shape. The dominant slope is the most important differentiating criterion, followed by the relief intensity (Table 4). The relief intensity is the median difference between the highest and lowest point within the terrain per specified distance. The specified distance can be variable. The relief intensity is normally given in metres per kilometre. With complex landforms, the protruding landform should be at least 25 m high (if not it is to be considered mesorelief) except for terraced land, where the main terraces should have elevation differences of at least 10 m. In areas, the major terraces may be very close to each other – particularly towards the lower part of the plain. Finally, the older levels may become buried by down wash. For complex landforms, subdivisions can be used (Table 5). These subdivisions are mainly applicable to level landforms, to some extent to sloping landforms and, in the case of , to intermontane plains.

Position The relative position of the site within the land should be indicated. The position affects the hydrological conditions of the site (external and internal drainage, e.g. Chapter 3 – Soil formation factors 11

TABLE 4 Hierarchy of major landforms 1st level 2nd level Gradient Relief intensity Potential (%) (m km-1) drainage density L level land LP plain < 10 < 50 0–25 LL plateau < 10 < 50 0–25 LD depression < 10 < 50 16–25 LV valley floor < 10 < 50 6–15 S sloping land SE medium-gradient escarpment zone 10–30 50–100 < 6 SH medium-gradient hill 10–30 100–150 0–15 SM medium-gradient 15–30 150–300 0–15 SP dissected plain 10–30 50–100 0–15 SV medium-gradient valley 10–30 100–150 6–15 T steep land TE high-gradient escarpment zone > 30 150–300 < 6 TH high-gradient hill > 30 150–300 0–15 TM high-gradient mountain > 30 > 300 0–15 TV high-gradient valley > 30 > 150 6–15 Notes: Changes proposed at the SOTER meeting at Ispra, October 2004. Potential drainage density is given in number of “receiving” pixels within a 10 × 10 pixels window. Source: Updated SOTER, ISRIC, 2005.

TABLE 5 Subdivisions for complex landforms CU = Cuesta-shaped DO = Dome-shaped RI = Ridged TE = Terraced IN = Inselberg covered (occupying > 1% of level land) DU = Dune-shaped IM = With intermontane plains (occupying > 15%) KA = Strong karst WE = With wetlands (occupying > 15%) Source: Updated SOTER, ISRIC, 2005.

FIGURE 2 Slope positions in undulating and mountainous terrain

CR CR UP UP

MS MS

Channel LS LS TS BO

Note: Position in undulating to mountainous terrain Position in flat or almost flat terrain CR = Crest (summit) HI = Higher part (rise) UP = Upper slope (shoulder) IN = Intermediate part (talf) MS = Middle slope (back slope) LO = Lower part (and dip) LS = Lower slope (foot slope) BO = Bottom (drainage line) TS = Toe slope BO = Bottom (flat) Source: Redrawn from Schoeneberger et al., 2002. 12 Guidelines for soil description

TABLE 6 subsurface runoff), which may be Classification of slope forms interpreted as being predominantly Sstraight water receiving, water shedding or C concave V convex neither of these. Tterraced X complex (irregular) Slope form The slope form refers to the general shape of the slope in both the vertical and horizontal directions FIGURE 3 Slope forms and surface pathways (Figure 3). Table 6 lists the slope form classes.

Slope gradient and orientation The slope gradient refers to the SS SV SC slope of the land immediately surrounding the site. It is measured using a clinometer aimed in the direction of the steepest slope. VS VV VC Where clinometer readings are not possible, field estimates of slope gradient should be matched CS CV CC against calculated gradients from contour maps. Surface flow pathway Slope gradients in almost flat terrain are often overestimated. Source: Redrawn from Schoeneberger et al., 2002. In open plains, slope gradients of 0.2 percent are usually clearly TABLE 7 visible. The proper recording of Slope gradient classes minor slope-gradient variations is Class Description % important, especially for erosion, 01 Flat 0–0.2 02 Level 0.2–0.5 and drainage. 03 Nearly level 0.5–1.0 The slope gradient is recorded 04 Very gently sloping 1.0–2.0 in two ways. The first and most 05 Gently sloping 2–5 important is by means of the 06 Sloping 5–10 actual, measured value, and the 07 Strongly sloping 10–15 second by entering in one of the 08 Moderately steep 15–30 09 Steep 30–60 following classes; they may need 10 Very steep > 60 to be modified to fit the local topography (Table 7). In addition to the attributes of slope in Table 7, both the slope length (particularly above the site) and aspect (orientation) should be recorded. The orientation influences, for example, the precipitation input, the temperature regime, the risk for wind impact and the character of humus formed in higher latitudes. Chapter 3 – Soil formation factors 13

The orientation that a slope is facing is coded N for north, E for east, S for south and W for west; for example, SSW means south-southwest.

LAND USE AND VEGETATION Land use Land use applies to the current use of the land, whether agricultural or non- agricultural, in which the soil is located. Land use has a major influence on the direction and rate of soil formation; its recording enhances the interpretative value of the soil data considerably (Table 8). For arable land use, the dominant crops grown should be mentioned (section on crops [below]), and as much information as possible given on soil management, use of , duration of fallow period, rotation systems and yields.

Crops Crops are plants that are cultivated for their economic value. Information on crops is important because it gives an idea of the nature of soil disturbance as a result of crop management practices as well as the nutrient and soil management requirements of the crop. Information on crops can be given in a general or detailed way as required. Examples for the most common crops with their recommended codes are given in Table 9.

Human influence This item refers to any evidence of human activity that is likely to have affected the landscape or the physical and chemical properties of the soil. Erosion is dealt with separately in Chapter 4. For various environments, it is useful to indicate the degree of disturbance of the natural vegetation. The existing vegetation is described in the section on vegetation (below). Examples of human influences with their recommended codes are given in Table 10.

Note for classification purposes 9Constructed terraces A Escalic qualifier. 9Raised land surfaces A plaggic and terric horizons. 9Ploughing A anthraquic and anthric horizons and Aric qualifier. 9Special depth limits if plough layers are present A , and . 9Special requirements if an eluvial horizon is part of a plough layer A argic and natric horizons. 9Does not form part of a plough layer A cambic horizon. 9Mixing or soil layers or lumps of applied lime A anthric horizon. 9Spade marks A plaggic horizon. 14 Guidelines for soil description

TABLE 8 Land-use classification A = Crop agriculture (cropping) AA = Annual field cropping AA1 = Shifting cultivation AA2 = Fallow system cultivation AA3 = Ley system cultivation AA4 = Rainfed arable cultivation AA5 = Wet cultivation AA6 = Irrigated cultivation AP = Perennial field cropping AP1 = Non-irrigated cultivation AP2 = Irrigated cultivation AT = Tree and shrub cropping AT1 = Non-irrigated tree crop cultivation AT2 = Irrigated tree crop cultivation AT3 = Non-irrigated shrub crop cultivation AT4 = Irrigated shrub crop cultivation Additional codes may be used to further specify the land-use type. For example: AA4 = Rainfed arable cultivation AA4T = Traditional AA4I = Improved traditional AA4M = Mechanized traditional AA4C = Commercial AA4U = Unspecified M = Mixed farming MF = Agroforestry MP = Agropastoralism H = Animal husbandry HE = Extensive HE1 = Nomadism HE2 = Semi-nomadism HE3 = Ranching HI = Intensive grazing HI1 = Animal production HI2 = Dairying F = Forestry FN = Natural forest and woodland FN1 = Selective felling FN2 = Clear felling FP = Plantation forestry P = Nature protection PN = Nature and game preservation PN1 = Reserves PN2 = Parks PN3 = Wildlife management PD = Degradation control PD1 = Without interference PD2 = With interference S = Settlement, industry SR = Residential use SI = Industrial use ST = Transport SC = Recreational use SX = Excavations SD = Disposal sites Y = Military area O = Other land uses U = Not used and not managed Chapter 3 – Soil formation factors 15

TABLE 9 Crop codes Ce = Fo = Fodder plants Fi = Fibre crops CeBa = Barley FoAl = Alfalfa FiCo = Cotton CeMa = FoCl = Clover FiJu = Jute CeMi = Millet FoGr = Grasses Ve = Vegetables CeOa = Oats FoHa = Hay Pu = Pulses PuBe = Beans CePa = Rice, paddy FoLe = Leguminous PuLe = Lentils CeRi = Rice, dry FoMa = Maize PuPe = Peas CeRy = Rye FoPu = Pumpkins Lu = Semi-luxury foods and CeSo = Sorghum Ro = Roots and tubers RoCa = Cassava LuCc = Cocoa CeWh = RoPo = Potatoes LuCo = Oi = Oilcrops OiCc = Coconuts RoSu = Sugar beets LuTe = OiGr = Groundnuts RoYa = Yams LuTo = Tobacco OiLi = Linseed Fr = Fruits and melons Ot = Other crops FrAp = Apples OtSc = Sugar cane OiOl = Olives FrBa = Bananas OtRu = Rubber OiOp = Oil-palm FrCi = Citrus OtPa = Palm (fibres, OiRa = Rape FrGr = Grapes, Wine, kernels) OiSe = Sesame Raisins OiSo = Soybeans FrMa = Mangoes OiSu = Sunflower FrMe = Melons

TABLE 10 Recommended codes for human influence N = No influence BU = Bunding NK = Not known BR = Burning VS = Vegetation slightly disturbed TE = Terracing VM = Vegetation moderately disturbed PL = Ploughing VE = Vegetation strongly disturbed MP = Plaggen VU = Vegetation disturbed (not specified) MR = Raised beds (agricultural purposes) IS = Sprinkler irrigation ME = Raised beds (engineering purposes) IF = Furrow irrigation MS = Sand additions ID = Drip irrigation MU = Mineral additions (not specified) IP = Flood irrigation MO = Organic additions (not specified) IB = Border irrigation PO = Pollution IU = Irrigation (not specified) CL = Clearing AD = Artificial drainage SC = Surface compaction FE = Application of fertilizers SA = Scalped area LF = Landfill (also sanitary) BP = Borrow pit LV = Levelling DU = Dump (not specified) AC = Archaeological (burial mound, midden) MI = Mine (surface, including openpit, and quarries) CR = Impact crater 16 Guidelines for soil description

TABLE 11 Vegetation classification F = Closed forest 1 D = Dwarf shrub FE = Evergreen broad-leaved forest DE = Evergreen dwarf shrub FC = Coniferous forest DS = Semi-deciduous dwarf shrub FS = Semi-deciduous forest DD = Deciduous dwarf shrub FD = Deciduous forest DX = Xermomorphic dwarf shrub FX = Xeromorphic forest DT = Tundra W = Woodland 2 H = Herbaceous WE = Evergreen woodland HT = Tall WS = Semi-deciduous woodland HM = Medium grassland WD = Deciduous woodland HS = Short grassland WX = Xeromorphic woodland HF = Forb S = Shrub M = Rainwater-fed moor peat SE = Evergreen shrub B = -fed bog peat SS = Semi-deciduous shrub SD = Deciduous shrub SX = Xeromorphic shrub 1 Continuous tree layer, crowns overlapping, large number of tree and shrub species in distinct layers. 2 Continuous tree layer, crowns usually not touching, understorey may be present.

Vegetation Vegetation is a dominant factor in soil formation as it is the primary source of organic matter and because of its major role in the nutrient cycling and of a site. There is no uniform acceptance of a system for the description of the natural or semi-natural vegetation. The kind of vegetation can be described using a local, regional or international system. A common example is the vegetation classification according to UNESCO (1973, see updated SOTER; ISRIC, 2005), presented in Table 11 with codes added. In addition, other characteristics of the vegetation, such as height of trees or canopy cover, may be recorded.

PARENT MATERIAL The parent material is the material from which the soil has presumably been derived. The parent material should be described as accurately as possible, indicating its origin and nature. There are basically two groups of parent material on which the soil has formed: unconsolidated materials (mostly ); and weathering materials overlying the hard rock from which they originate. There are transitional cases, such as partly consolidated materials and weathering materials that have been transported, either by water, called alluvium (fluvial if transported by stream), or by gravity, called colluvium. There are also restored natural soil materials or sediments as well as technogenic materials. The reliability of the geological information and the knowledge of the local lithology will determine whether a general or a specific definition of the parent material can be given. Chapter 3 – Soil formation factors 17

For weathered rock, the code WE is first entered, followed by the rock-type code. The code SA for is recommended where the in situ weathered material is thoroughly decomposed, clay-rich but still showing rock structure. Alluvial deposits and colluvium derived from a single rock type may be further specified by that rock type. Where one parent material overlies another, both are indicated. The parent material is coded according to updated SOTER (ISRIC, 2005) at the lowest level of hierarchy as possible. As SOTER was developed to work with maps on a scale of 1:1 000 000, it was a requirement to have not too many rock types. In order to be able to work in smaller scales, some additional natural and anthropogenic parent materials are included in Table 12. For identification in the field, a key to the most important rock types is provided below the extended hierarchical SOTER list.

Note for classification purposes 9Remains intact when a specimen of 25–30 mm is submerged in water for 1 hour; roots cannot penetrate except along vertical cracks that have an average horizontal spacing of ≥ 10 cm and that occupy < 20 percent (by volume); no significant displacement has taken place A continuous rock. 9Differences in lithology A lithological discontinuity. 9Recent sediments above the soil that is classified at the Reference Soil Group (RSG) level A Novic qualifier. 9Sedimentation through human-induced erosion A colluvic material. 9Coprogenous earth or sedimentary peat, diatomaceous earth, marl or gyttja A limnic material. 9Remnants of birds or bird activity A ornithogenic material. 9Organic material consisting of ≥ 75 percent of moss fibres A greater thickness of organic material required for . 9Moor peat saturated predominantly with rainwater A Ombric qualifier. 9Bog peat saturated predominantly with groundwater or flowing surface water A Rheic qualifier.

AGE OF THE LAND SURFACE The age of the landscape is important information from which the possible duration of the occurrence of soil formation processes can be derived. Because many soils are formed from preweathered or moved materials, or may have been derived from an assemblage of autochthonous, fluvial and eolian materials, it is often difficult to obtain precise information. However, an estimate will help to interpret soil data and interaction between different soil forming processes. It may also indicate possible climate changes during soil formation. Table 13 provides a provisional coding. 18 Guidelines for soil description

TABLE 12 Hierarchy of lithology Major class Group Type I IA acid igneous IA1 diorite IA2 grano-diorite IA3 -diorite IA4 rhyolite II intermediate igneous II1 andesite, trachyte, phonolite II2 diorite-syenite IB basic igneous IB1 IB2 IB3 dolerite IU ultrabasic igneous IU1 IU2 pyroxenite IU3 ilmenite, magnetite, , serpentine IP pyroclastic IP1 , tuffite IP2 volcanic scoria/breccia IP3 volcanic ash IP4 ignimbrite M MA acid metamorphic MA1 quartzite MA2 , MA3 slate, phyllite (pelitic rocks) MA4 MB basic metamorphic MB1 slate, phyllite (pelitic rocks) MB2 (green)schist MB3 gneiss rich in Fe–Mg MB4 metamorphic (marble) MB5 amphibolite MB6 eclogite MU ultrabasic metamorphic MU1 , greenstone S SC clastic sediments SC1 conglomerate, breccia (consolidated) SC2 , greywacke, arkose SC3 -, -, claystone SC4 shale SC5 ironstone SO carbonatic, organic SO1 limestone, other carbonate rock SO2 marl and other mixtures SO3 coals, bitumen and related rocks SE evaporites SE1 anhydrite, gypsum SE2 halite U sedimentary rock UR weathered residuum UR1 , (unconsolidated) UF fluvial UF1 sand and gravel UF2 clay, silt and UL lacustrine UL1 sand UL2 silt and clay UM marine, estuarine UM1 sand UM2 clay and silt UC colluvial UC1 slope deposits UC2 lahar UE eolian UE1 UE2 sand UG glacial UG1 moraine UG2 glacio-fluvial sand UG3 glacio-fluvial gravel UK * kryogenic UK1 periglacial rock debris UK2 periglacial layer Chapter 3 – Soil formation factors 19

TABLE 12 Hierarchy of lithology (Continued) Major class Group Type UO organic UO1 rainwater-fed moor peat UO2 groundwater-fed bog peat UA anthropogenic/ UA1 redeposited natural material technogenic UA2 industrial/artisanal deposits UU * unspecified deposits UU1 clay UU2 loam and silt UU3 sand UU4 gravelly sand UU5 gravel, broken rock * Extended. Source: Updated SOTER; ISRIC, 2005.

Materials (natural and anthropogenic/technogenic) deposited by humans are coded: ¾ d... = dumped, ¾ s... = spoiled. Chapter 4 provides more details on human-made materials.

TABLE 13 Provisional coding for age of land surface vYn Very young (1–10 years) natural: with loss by erosion or deposition of materials such as on tidal flats, of coastal dunes, in river valleys, landslides or areas. vYa Very young (1–10 years) anthropogeomorphic: with complete disturbance of natural surfaces (and soils) such as in urban, industrial and mining areas with very early soil development from fresh natural or technogenic or mixed materials. Yn Young (10–100 years) natural: with loss by erosion or deposition of materials such as on tidal flats, of coastal dunes, river valleys, landslides or desert areas. Ya Young (10–100 years) anthropogeomorphic: with complete disturbance of any natural surfaces (and soils) such as in urban, industrial and mining areas with early soil development from fresh natural, technogenic or a mixture of materials, or restriction of flooding by dykes. Hn Holocene (100–10 000 years) natural: with loss by erosion or deposition of materials such as on tidal flats, of coastal dunes, in river valleys, landslides or desert areas. Ha Holocene (100–10 000 years) anthropogeomorphic: human-made relief modifications, such as terracing of forming hills or walls by early civilizations or during the Middle Ages or earlier, restriction of flooding by dykes, or surface raising. lPi Late Pleistocene, ice covered, commonly recent soil formation on fresh materials. lPp Late Pleistocene, periglacial, commonly recent soil formation on preweathered materials. lPf Late Pleistocene, without periglacial influence. oPi Older Pleistocene, ice covered, commonly the recent soil formation on younger over older, preweathered materials. oPp Older Pleistocene, with periglacial influence, commonly the recent soil formation on younger over older, preweathered materials. oPf Older Pleistocene, without periglacial influence. T Tertiary land surfaces, commonly high planes, terraces or peneplains, except incised valleys, frequent occurrence of palaeosoils. O Older, pre-Tertiary land surfaces, commonly high planes, terraces or peneplains, except incised valleys, frequent occurrence of palaeosoils.

21

Chapter 4 Soil description

This chapter presents the procedure to describe the different morphological and other characteristics of the soil. This is best done using a recently dug pit large enough to allow sufficient examination and description of the different horizons. Old exposures such as cuts and ditches may be used, but only after scraping off sufficient material to expose the fresh soil. First, the surface characteristics are recorded. Then, the soil description is done horizon by horizon, starting with the uppermost one. The rules of soil description and the coding of attributes are generally based on the guidelines for soil description according to FAO (1990). Additions have a citation.

SURFACE CHARACTERISTICS Where present, surface characteristics, such as rock outcrops, coarse rock fragments, human-induced erosion, surface sealing and surface cracks, should be recorded. A number of other surface characteristics, such as the occurrence of salts, bleached , litter, worm casts, ant paths, cloddiness, and puddling, may be also be recorded.

Rock outcrops Exposures of may limit the use of modern mechanized agricultural equipment. Rock outcrops should be described in terms of percentage surface cover, together with additional relevant information on the size, spacing and hardness of the individual outcrops. Table 14 lists the recommended classes of percentage of surface cover and of average distance between rock outcrops (single or clusters).

Coarse surface fragments TABLE 14 Coarse surface fragments, Recommended classification of rock outcrops Distance between rock including those partially exposed, Surface cover (%) outcrops should be described in terms of (m) percentage of surface coverage N None 0 and of size of the fragments. V Very few 0–2 1 > 50 F Few 2–5 2 20–50 Classes of occurrence of coarse C Common 5–15 3 5–20 surface fragments are correlated M Many 15–40 4 2–5 with the ones for rock outcrop, as A Abundant 40–80 5 < 2 per Table 15. D Dominant > 80 22 Guidelines for soil description

TABLE 15 Note for classification purposes Classification of coarse surface fragments 9Pavement (consisting of rock Size classes (indicating the Surface cover (%) greatest dimension) outcrops or surface coarse (cm) fragments) that is varnished or N None 0 F Fine gravel 0.2–0.6 includes wind-shaped gravel or V Very few 0–2 M Medium gravel 0.6–2.0 stones or is associated with a F Few 2–5 C Coarse gravel 2–6 vesicular layer A yermic horizon. C Common 5–15 S Stones 6–20 M Many 15–40 B Boulders 20–60 A Abundant 40–80 L Large boulders 60–200 Erosion D Dominant > 80 In describing , emphasis should be given to accelerated or TABLE 16 human-induced erosion. It is not Classification of erosion, by category always easy to distinguish between N No evidence of erosion natural and accelerated erosion W Water erosion or A Wind (aeolian) erosion or deposition deposition as they are often closely related. WS Sheet erosion AD Wind deposition Human-induced erosion is the WR Rill erosion AM Wind erosion and result of irrational use and poor deposition WG Gully erosion AS Shifting sands management, such as inappropriate WT Tunnel erosion AZ Salt deposition agricultural practices, overgrazing WD Deposition by water and removal or overexploitation of WA Water and wind erosion the natural vegetation. M Mass movement (landslides and similar phenomena) NK Not known Main categories Erosion can be classified as water or wind erosion (Table 16), and TABLE 17 Classification of total area affected by erosion and include off-site effects such as deposition deposition; a third major category % is mass movements (landslides and 00related phenomena). 1 0–5 2 5–10 Area affected 3 10–25 4 25–50 The total area affected by erosion 5> 50and deposition is estimated follow- ing the classes defined by SOTER (FAO, 1995) as per Table 17. TABLE 18 Classification of erosion, by degree Degree S Slight Some evidence of damage to surface horizons. Original biotic functions largely intact. It is difficult to define classes of M Moderate Clear evidence of removal of surface horizons. the degree of erosion that would Original biotic functions partly destroyed. be equally appropriate for all soils V Severe Surface horizons completely removed and subsurface horizons exposed. Original biotic and environments and that would functions largely destroyed. also fit the various types of water E Extreme Substantial removal of deeper subsurface horizons (badlands). Original biotic functions and wind erosion. Four classes are fully destroyed. recommended (Table 18), which Chapter 4 – Soil description 23

may have to be further defined TABLE 19 for each type or combination of Classification of erosion, by activity A Active at present erosion and deposition and specific R Active in recent past (previous 50–100 years) environment. For example, in the H Active in historical times case of gully and rill erosion, the N Period of activity not known depth and spacing may need to be X Accelerated and natural erosion not distinguished recorded; for sheet erosion, the loss of ; for dunes, the height; and for deposition, the thickness of the layer.

Activity The period of activity of accelerated erosion or deposition is described using the recommended classes in Table 19.

Note for classification purposes 9Evidence of aeolian activity: rounded or subangular sand particles showing a matt surface; wind-shaped rock fragments; aeroturbation; wind erosion or A aridic properties.

Surface sealing Surface sealing is used to describe crusts that develop at the soil surface after the topsoil dries out. These crusts may inhibit seed germination, reduce water and increase runoff. The attributes of surface sealing are the consistence, when dry, and thickness of the crust as per Table 20.

Note for classification purposes 9Surface crust that does not curl entirely upon drying A takyric horizon. 9Surface crust A Hyperochric qualifier.

Surface cracks Surface cracks develop in shrink–swell clay-rich soils after they dry out. The width (average, or average width and maximum width) of the cracks at the surface is indicated in centimetres. The average distance between cracks may also be indicated in centimetres. Table 21 lists the suggested classes.

Note for classification purposes 9Cracks that open and close TABLE 20 periodically A . Classification of attributes of surface sealing 9Cracks that open and close Thickness Consistence (mm) periodically, ≥ 1 cm wide A N None S Slightly hard vertic properties. F Thin < 2 H Hard 9Polygonal cracks extending ≥ M Medium 2–5 V Very hard 2 cm deep when the soil is dry C Thick 5–20 E Extremely hard A takyric horizon. V Very thick > 20 24 Guidelines for soil description

TABLE 21 Other surface characteristics Classification of surface cracks A number of other surface Width Distance between cracks characteristics, such as the (cm) (m) F Fine < 1 C Very closely spaced < 0.2 occurrence of salts, bleached M Medium 1–2 D Closely spaced 0.2–0.5 sands, litter, worm casts, ant paths, W Wide 2–5 M Moderately widely 0.5–2 cloddiness and puddling, may be spaced V Very wide 5–10 W Widely spaced 2–5 recorded. E Extremely wide > 10 V Very widely spaced > 5 Depth Salt S Surface < 2 The occurrence of salt at the M Medium 2–10 surface may be described in terms D Deep 10–20 of cover, appearance and type of V Very deep > 20 salt. Table 22 lists the classes for

TABLE 22 the percentage of surface cover and Classification of salt characteristics thickness. Cover Thickness (%) (mm) Note for classification purposes 0 None 0–2 N None 9Crust pushed up by salt crystals 1 Low 2–15 F Thin < 2 A Puffic qualifier. 2 Moderate 15–40 M Medium 2–5 3 High 40–80 C Thick 5–20 Bleached sand 4 Dominant > 80 V Very thick > 20 The presence of bleached, loose TABLE 23 sand grains on the surface is typical Classification of bleached sand characteristics for certain soils and influences the % reflection characteristics of the area 0 None 0–2 and, hence, the image obtained 1 Low 2–15 through . Table 23 2 Moderate 15–40 3 High 40–80 lists the classes based on the 4 Dominant > 80 percentage of surface covered.

HORIZON BOUNDARY Horizon boundaries provide information on the dominant soil-forming processes that have formed the soil. In certain cases, they reflect past anthropogenic impacts on the landscape. Horizon boundaries are described in terms of depth, distinctness and topography.

Depth Most soil boundaries are zones of transition rather than sharp lines of division. The depth of the upper and lower boundaries of each horizon is given in centimetres, measured from the surface (including organic and mineral covers) of the soil downwards. Precise notations in centimetres are used where boundaries are abrupt or clear. Rounded-off figures (to the nearest 5 cm) are entered where the boundaries are gradual or diffuse, avoiding the suggestion of spurious levels of accuracy. Chapter 4 – Soil description 25

However, if boundary depths are TABLE 24 near diagnostic limits, rounded- Classification of horizon boundaries, by distinctness and topography off figures should not be used. In Distinctness Topography this case, the depth is indicated as (cm) a medium value for the transitional A Abrupt 0–2 S Smooth Nearly plane surface zone (if it starts at 16 cm and C Clear 2–5 W Wavy Pockets less deep than terminates at 23 cm, the depth wide G Gradual 5–15 I Irregular Pockets more deep than should be 19.5 cm). wide Most horizons do not have a D Diffuse > 15 B Broken Discontinuous constant depth. The variation or irregularity of the surface of the boundary is described by the topography in terms of smooth, wavy, irregular and broken. If required, ranges in depth should be given in addition to the average depth, for example 28 (25–31) cm to 45 (39–51) cm.

Note for classification purposes 9Many diagnostic horizons and properties are found at a certain depth. Important boundary depths are 10, 20, 25, 40, 50, 100 and 120 cm.

Distinctness and topography The distinctness of the boundary refers to the thickness of the zone in which the horizon boundary can be located without being in one of the adjacent horizons (Table 24). The topography of the boundary indicates the smoothness of depth variation of the boundary.

Note for classification purposes 9 A cryic horizon, Cryosols and Turbic qualifier. 9Tonguing of a mollic or umbric horizon into an underlying layer A Glossic qualifier. 9Tonguing of an eluvial albic horizon into an argic horizon A albeluvic tonguing and Glossalbic qualifier. 9Diffuse horizon boundaries A .

PRIMARY CONSTITUENTS This section presents the procedure on the description of and the nature of the primary rock and mineral fragments, which are subdivided into: (i) the fine earth fraction; and (ii) the coarse fragments fraction.

Texture of the fine earth fraction Soil texture refers to the proportion of the various particle-size classes (or soil separates, or fractions) in a given soil volume and is described as soil textural class (Figure 4). The names for the particle-size classes correspond closely with commonly used standard terminology, including that of the system used by the 26 Guidelines for soil description

United States Department of Agriculture (USDA). However, many national systems describing particle-size and textural classes use more or less the same names but different grain fractions of sand, silt and clay, and textural classes. This publication uses the 2000–63–2-μm system for particle-size fractions.

Soil textural classes The names of the textural classes (which describe combined particle-size classes) of the described soil material are coded as in Figure 4. In addition to the textural class, a field estimate of the percentage of clay is given. This estimate is useful for indicating increases or decreases in clay content within textural classes, and for comparing field estimates with analytical results. The relationship between the basic textural classes and the percentages of clay, silt and sand is indicated in a triangular form in Figure 4.

Subdivision of the sand fraction Sands, loamy sands and sandy are subdivided according to the proportions of very coarse to coarse, medium, fine and very fine sands in the sand fraction. The proportions are calculated from the particle-size distribution, taking the total of the sand fraction as being 100 percent (Figure 4).

Field estimation of textural classes The textural class can be estimated in the field by simple field tests and feeling the constituents of the soil (Table 25). For this, the soil sample must be in a moist to weak wet state. Gravel and other constituents > 2 mm must be removed. The constituents have the following feel: ¾Clay: soils fingers, is cohesive (sticky), is formable, has a high plasticity and has a shiny surface after squeezing between fingers. ¾Silt: soils fingers, is non-sticky, only weakly formable, has a rough and ripped surface after squeezing between fingers and feels very floury (like talcum powder). ¾Sand: cannot be formed, does not soil fingers and feels very grainy.

Note for classification purposes Important diagnostic characteristics derived from the textural class are: 9A texture that is loamy sand or coarser to a depth of ≥ 100 cm A Arenosol. 9A texture of loamy fine sand or coarser in a layer ≥ 30 cm thick within 100 cm of the soil surface A Arenic qualifier. 9A texture of silt, silt loam, silty clay loam or silty clay in a layer ≥ 30 cm thick, within 100 cm of the soil surface A Siltic qualifier. 9A texture of clay in a layer ≥ 30 cm thick within 100 cm of the soil surface A Clayic qualifier. 9≥ 30 percent clay throughout a thickness of 25 cm A vertic horizon. 9≥ 30 percent clay throughout a thickness of 15 cm A vertic properties. 9≥ 30 percent clay between the soil surface and a vertic horizon A . Chapter 4 – Soil description 27

FIGURE 4 Relation of constituents of fine earth by size, defining textural classes and sand subclasses Particle-size classes Textural classes 2 000 μm Very coarse sand S Sand (unspecified) 1 250 μm Coarse sand LS Loamy sand 630 μm Medium sand SL Sandy loam 200 μm Fine sand SCL Sandy clay loam 125 μm Very fine sand SiL Silt loam 63 μm Coarse silt SiCL Silty clay loam 20 μm Fine silt CL Clay loam 2 μm 0 Clay 100 L Loam Si Silt SC Sandy clay SiC Silty clay Silt/clay C Clay 0.4 HC Heavy clay (Heavy clay) 0.6

% Clay 50 % Silt <2 μm 50 Clay Silty 2 – 63 μm Sandy clay clay (Vertic horizon) Clay loam Silt clay Sandy clay loam loam Loam (Arenosols) Sandy Silt loam Loamy loam Silt Sand Sand 100 0 100 50 0 % Sand 0.063 – 2 mm 0 100 Subdivisions of sandy textural classes VFS Very fine sand FS Medium sand Fine sand 0.2 – 0.63 mm S MS Medium sand CS Coarse sand US Sand, unsorted Coarse 50 50 sand LVFS Loamy very fine sand LS LFS Loamy fine sand Very coarse + coarse sand 0.63 – 2 mmSand LCS Loamy coarse sand unsorted FSL Fine sandy loam SL Medium CSL Coarse sandy loam Fine sand sand very fine sand 0 100 100 50 0 Very fine + fine sand 0.063 – 0.2 mm

Source: According to FAO (1990) 28 Guidelines for soil description

TABLE 25 Key to the soil textural classes ~% clay 1 Not possible to roll a wire of about 7 mm in diameter (about the diameter of a pencil) 1.1 not dirty, not floury, no fine material in the finger rills: sand S < 5 • if grain sizes are mixed: unsorted sand US < 5 • if most grains are very coarse (> 0.6 mm): very coarse and coarse sand CS < 5 • if most grains are of medium size (0.2–0.6 mm): medium sand MS < 5 • if most grains are of fine size (< 0.2 mm) but still grainy: fine sand FS < 5 • if most grains are of very fine size (< 0.12 mm), tending very fine sand VFS < 5 to be floury: 1.2 not floury, grainy, scarcely fine material in the finger rills, loamy sand LS < 12 weakly shapeable, adheres slightly to the fingers: 1.3 similar to 1.2 but moderately floury: sandy loam SL (clay-poor) < 10 2 Possible to roll a wire of about 3–7 mm in diameter (about half the diameter of a pencil) but breaks when trying to form the wire to a ring of about 2–3 cm in diameter, moderately cohesive, adheres to the fingers 2.1 very floury and not cohesive • some grains to feel: silt loam SiL (clay-poor) < 10 • no grains to feel: silt Si < 12 2.2 moderately cohesive, adheres to the fingers, has a rough and ripped surface after squeezing between fingers and • very grainy and not sticky: sandy loam SL (clay-rich) 10–25 • moderate sand grains: loam L 8–27 • not grainy but distinctly floury and somewhat sticky: silt loam SiL (clay-rich) 10–27 2.3 rough and moderate shiny surface after squeezing sandy clay loam SCL 20–35 between fingers and is sticky and grainy to very grainy: 3 Possible to roll a wire of about 3 mm in diameter (less than half the diameter of a pencil) and to form the wire to a ring of about 2–3 cm in diameter, cohesive, sticky, gnashes between teeth, has a moderately shiny to shiny surface after squeezing between fingers 3.1 very grainy: sandy clay SC 35–55 3.2 some grains to see and to feel, gnashes between teeth • moderate plasticity, moderately shiny surfaces: clay loam CL 25–40 • high plasticity, shiny surfaces: clay C 40–60 3.3 no grains to see and to feel, does not gnash between teeth • low plasticity: silty clay loam SiCL 25–40 • high plasticity, moderately shiny surfaces: silty clay SiC 40–60 • high plasticity, shiny surfaces: heavy clay HC > 60

Note: Field texture determination may depend on clay mineralogical composition. The above key works mainly for soils having illite, chlorite and/or vermiculite composition. Smectite clays are more plastic, and kaolinitic clays are stickier. Thus, clay content can be overestimated for the former, and underestimated for the latter. Source: Adapted from Schlichting, Blume and Stahr, 1995.

9≥ 30 percent clay, < 20 percent change (relative) in clay content over 12 cm to layers immediately above and below, a silt/clay ratio of < 0.4 A nitic horizon. 9Sandy loam or finer particle size A ferralic horizon. 9A texture in the fine earth fraction of very fine sand, loamy very fine sand, or finer A cambic horizon. 9A texture in the fine earth fraction coarser than very fine sand or loamy very fine sand A Brunic qualifier. 9A texture of loamy sand or finer and ≥ 8 percent clay A argic and natric horizons. Chapter 4 – Soil description 29

9A texture of sand, loamy sand, sandy loam or silt loam or a combination of them A plaggic horizon. 9A higher clay content than the underlying soil and relative differences among medium, fine and very fine sand and clay < 20 percent A irragric horizon. 9A texture of sandy clay loam, clay loam, silty clay loam or finer A takyric horizon. 9≥ 8 percent clay in the underlying layer and within 7.5 cm either doubling of the clay content if the overlying layer has less then 20 percent or 20 percent (absolute) more clay A abrupt textural change. 9An abrupt change in particle-size distribution that is not solely associated with a change in clay content resulting from or a relative change of ≥ 20 percent in the ratios between coarse sand, medium sand, and fine sand A lithological discontinuity. 9The required amount of organic carbon depends on the clay content, if the layer is saturated with water for ≥ 30 consecutive days in most years A organic and mineral materials. 9The required amount of organic carbon depends on the texture A aridic properties. 9The depth where an argic horizon starts depends on the texture A , , Luvisols and , and Alic, Acric, Luvic and Lixic qualifiers. 9An argic horizon in which the clay content does not decrease by 20 percent of more (relative) from its maximum within 150 cm A Profondic qualifier. 9An absolute clay increase of ≥ 3 percent A Hypoluvic qualifier. 9A silt/clay ratio < 0.6 A Hyperalic qualifier.

Rock fragments and artefacts The presence of rock fragments influences the nutrient status, water movement, use and management of the soil. It also reflects the origin and stage of development of the soil. Artefacts (sections on artefacts and description of artefacts [below]) are useful for identifying colluviation, human occupation, and industrial processes. Large rock and mineral fragments (> 2 mm) and artefacts are described according to abundance, size, shape, state of weathering, and nature of the fragments. The abundance class TABLE 26 limits correspond with the ones Abundance of rock fragments and artefacts, by volume for surface coarse fragments and % mineral nodules, and the 40- N None 0 percent boundary coincides with V Very few 0–2 the requirement for the skeletic F Few 2–5 C Common 5–15 phase (Table 26 and Figure 5). M Many 15–40 Where rock fragments are not A Abundant 40–80 distributed regularly within a D Dominant > 80 horizon but form a “stone line”, S Stone line any content, but concentrated at a distinct depth of a horizon this should be indicated clearly. 30 Guidelines for soil description

Size of rock fragments and FIGURE 5 Charts for estimating proportions of coarse fragments artefacts and mottles Table 27 indicates the classification for rock fragments and artefacts.

Note for classification purposes Important diagnostic characte- 1 % 3 % 5 % 10 % ristics derived from the amount of rock fragments are: 9< 20 percent (by volume) fine earth averaged over a depth of 15 % 2 % 25 % 30 % 75 cm or to continuous rock A and Hyperskeletic qualifier. 9≥ 40 percent (by volume) gravel or other coarse fragments aver- 40 % 50 % 75 % 90 % aged over: ¾a depth of 100 cm or to continuous rock A Skeletic qualifier; ¾a depth of 50–100 cm A Endoskeletic qualifier; ¾a depth of 20– 50 cm A Coarse Episkeletic qualifier. Medium gravel 9≥ 20 (by volume, by weighted Sand gravel average) artefacts in the upper Fine 10 mm 100 cm A . gravel 9< 40 percent (by volume) of gravels or other coarse fragments in all layers within 100 cm or to a petroplinthic, plinthic or salic horizon A Arenosols. TABLE 27 Classification of rock fragments and artefacts Rock fragments (mm) Artefacts (mm) V Very fine artefacts < 2 F Fine gravel 2–6 F Fine artefacts 2–6 M Medium gravel 6–20 M Medium artefacts 6–20 C Coarse gravel 20–60 S Stones 60–200 C Coarse artefacts > 20 B Boulders 200–600 L Large boulders > 600 Combination of classes FM Fine and medium gravel/artefacts MC Medium and coarse gravel/artefacts CS Coarse gravel and stones SB Stones and boulders BL Boulders and large boulders Chapter 4 – Soil description 31

9Fragmental materials, the TABLE 28 interstices of which are filled Classification of shape of rock fragments with organic material A F Flat A Angular Histosols. S Subrounded R Rounded Shape of rock fragments The general shape or roundness of rock fragments may be described TABLE 29 using the terms in Table 28. Classification of weathering of coarse fragments F Fresh or slightly Fragments show little or no signs of Note for classification purposes weathered weathering. W Weathered Partial weathering is indicated by 9Layers with rock fragments discoloration and loss of crystal form in of angular shape overlying or the outer parts of the fragments while the centres remain relatively fresh and underlying layers with rock the fragments have lost little of their fragments of rounded shape original strength. S Strongly weathered All but the most resistant minerals are or marked differences in size weathered, strongly discoloured and altered throughout the fragments, and shape of resistant minerals which tend to disintegrate under only between superimposed layers A moderate pressure. lithological discontinuity.

State of weathering of rock TABLE 30 fragments and artefacts Codes for primary mineral fragments The state of weathering of the QU Quartz coarse fragments is described as MI Mica per Table 29. FE

Note for classification purposes 9A layer with rock fragments without weathering rinds overlying a layer with rock fragments with weathering rinds A lithological discontinuity.

Nature of rock fragments The nature of rock fragments is described by using the same terminology as for the rock-type description (Table 12). For primary mineral fragments, other codes can be used, e.g. as per Table 30. Fragments of individual weatherable minerals (e.g. and micas) may be smaller than 2 mm in diameter. Nevertheless, where present in appreciable quantities, such fragments should be mentioned separately in the description. For artefacts, see section on artefacts (below).

Note for classification purposes 9Rock fragments that do not have the same lithology as the underlying continuous rock A lithological discontinuity. 32 Guidelines for soil description

TABLE 31 Field estimation and coding of the degree of decomposition and humification of peat Attributes of dry peat Attributes of wet peat Degree of Code decomposition/ Colour Visible plant Goes between the Remnant humification tissues fingers by squeezing in the hand D1 very low white to light only ± clear not muddy brown

D2 low dark brown most brown to muddy water

Fibric D3 moderate dark brown to more than 2/3 muddy muddy D4 strongblack 1/3 to 2/3 1/2 to 2/3 plant structure more visible than before D5.1 moderately 1/6 to 1/3 more or less all only heavy strong decompostable

Hemic remnants D5.2 very strong less than 1/6 no remnant Sapric mud Source: Adapted from Ad-hoc-AG-Boden, 2005

Degree of decomposition and humification of peat In most organic layers, the determination of the texture class is not possible. More valuable is an estimate of the degree of decomposition and humification of the organic material. Colour and percentage of recognizable plant tissue of dry as well as of wet organic material can be used to estimate the degree of decomposition (Table 31).

Note for classification purposes 9Histosols have more than two-thirds (by volume) recognizable plant tissues A Fibric qualifier. 9Histosols have between two-thirds and one-sixth (by volume) recognizable plant tissues A Hemic qualifier. 9Histosols have less than one-sixth (by volume) recognizable plant tissues A Sapric qualifier.

Aeromorphic organic layers on forest floors On forest floors, especially under temperate and cool , organic matter is commonly accumulated in more or less decomposed organic layers under terrestrial conditions. In acidic and nutrient poor mineral soils, the nutrient stock of the organic layers is of vital interest for the vegetation cover. The three major forms, raw humus, moder and mull, are described as follows: ¾ Raw humus (aeromorphic mor): usually thick (5–30 cm) organic matter accumulation that is largely unaltered owing to lack of decomposers. This kind of organic matter layer develops in extremely nutrient-poor and coarse- textured soils under vegetation that produces a litter layer that is difficult to decompose. It is usually a sequence of Oi–Oe–Oa layers over a thin A horizon, easy to separate one layer from another and being very acid with a C/N ratio of > 29. Chapter 4 – Soil description 33

¾ Moder (duff mull): more decomposed than raw humus but characterized by an organic matter layer on top of the mineral soil with a diffuse boundary between the organic matter layer and A horizon. In the sequence of Oi–Oe– Oa layers, it is difficult to separate one layer from another. This develops in moderately nutrient-poor conditions, usually under a cool moist climate. It is usually acidic with a C/N ratio of 18–29. ¾ Mull: characterized by the periodic absence of organic matter accumulation on the surface owing to the rapid decomposition process and mixing of organic matter and the mineral soil material by . It is usually slightly acid to neutral with a C/N ratio of 10–18.

SOIL COLOUR (MATRIX) Soil colour reflects the composition as well as the past and present oxidation- reduction conditions of the soil. It is generally determined by coatings of very fine particles of humified organic matter (dark), (yellow, brown, orange and red), oxides (black) and others, or it may be due to the colour of the . The colour of the soil matrix l of each horizon should be recorded in the moist condition (or both dry and moist conditions where possible) using the notations for hue, value and chroma as given in the Munsell Charts (Munsell, 1975). Hue is the dominant spectral colour (red, yellow, green, blue or violet), value is the lightness or darkness of colour ranging from 1 (dark) to 8 (light), and chroma is the purity or strength of colour ranging from 1 (pale) to 8 (bright). Where there is no dominant soil matrix colour, the horizon is described as mottled and two or more colours are given. In addition to the colour notations, the standard Munsell colour names may be given. For routine descriptions, soil colours should be determined out of direct sunlight and by matching a broken ped with the colour chip of the Munsell Soil Color Charts. For special purposes, such as for soil classification, additional colours from crushed or rubbed material may be required. The occurrence of contrasting colours related to the structural organization of the soil, such as ped surfaces, may be noted. Where possible, soil colour should be determined under uniform conditions. Early morning and late evening readings are not accurate. Moreover, the determination of colour by the same or different individuals has often proved to be inconsistent. Because soil colour is significant with respect to various soil properties, including organic matter contents, coatings and state of oxidation or reduction, and for soil classification, cross-checks are recommended and should be established on a routine basis.

Note for classification purposes Intermediate colours should be recorded where desirable for the distinction between two soil horizons and for purposes of classification and interpretation of the soil profile. Intermediate hues (important for qualifiers, such as Chromic 34 Guidelines for soil description

or Rhodic, and for diagnostic horizons, such as cambic) that may be used are: 3.5, 4, 6, 6.5, 8.5 and 9 YR. For example, when 3.5 YR is noted, it means that the intermediate hue is closer to 2.5 YR than 5 YR; 4 YR means closer to 5 YR, and so on. If values and chromas are near diagnostic limits, rounded-off figures should not be used, but accurate recordings should be made by using intermediate values, or by adding a + or a -. Important diagnostic hues, values and chromas are: 9Abrupt changes in colour not resulting from pedogenesis A lithological discontinuity. 9Redder hue, higher value or higher chroma than the underlying or an overlying layer A cambic horizon. 9Hue redder than 10 YR or chroma ≥ 5 (moist) A ferralic properties, Hypoferralic and Rubic qualifier. 9Hue 7.5 YR or yellower and value ≥ 4 (moist) and chroma ≥ 5 (moist) A Xanthic qualifier. 9Hue redder than 7.5 YR or both hue 7.5 YR and chroma > 4 (moist) A Chromic qualifier. 9Hue redder than 5 YR, value < 3.5 (moist) A Rhodic qualifier. 9Hue 5 YR or redder, or hue 7.5 YR and value ≤ 5 and chroma ≤ 5, or hue 7.5 YR and value ≤ 5 and chroma 5 or 6, or hue 10 YR or neutral and value and chroma ≤ 2, or 10 YR 3/1 (all moist) A spodic horizon. 9Hue 7.5 YR or yellower or GY, B or BG; value ≤ 4 (moist); chroma ≤ 2 (moist) A puddled layer (anthraquic horizon). 9Hue N1 to N8 or 2.5 Y, 5 Y, 5 G or 5 B A reductimorphic colours of the gleyic colour pattern. 9Hue 5 Y, GY or G A gyttja (limnic material). 9Chroma < 2.0 (moist) and value < 2.0 (moist) and < 3.0 (dry) A voronic horizon. 9Chroma ≤ 2 (moist) A . 9Chroma ≤3 (moist) and value ≤ 3 (moist) and ≤ 5 (dry) A mollic and umbric horizon. 9Value and chroma ≤ 3 (moist) A hortic horizon. 9Value ≤ 4 (moist) and ≤ 5 (dry) and chroma ≤ 2 (moist) A plaggic horizon. 9Value > 2 (moist) or chroma > 2 (moist) A fulvic horizon. 9Value ≤ 2 (moist) and chroma ≤ 2 (moist) A melanic horizon. 9Values 4 to 8 and chroma 4 or less (moist) and values 5–8 and chromas 2–3 (dry) A albic horizon. 9Lower value or chroma than the overlying horizon A sombric horizon. 9Value ≥ 3 (moist) and ≥ 4.5 (dry) and chroma ≥ 2 (moist) A aridic properties. 9Value ≤ 4 (moist) A coprogenous earth or sedimentary peat (limnic material). 9Value 3, 4 or 5 (moist) A diatomaceous earth (limnic material). 9Value ≥ 5 (moist) A marl (limnic material). 9Value ≤ 3.5 (moist) and chroma ≤ 1.5 (moist) A Pellic qualifier. 9Value ≥ 5.5 (dry) A Hyperochric qualifier. Chapter 4 – Soil description 35

MOTTLING Mottles are spots or blotches of different colours or shades of colour interspersed with the dominant colour of the soil. They indicate that the soil has been subject to alternate wetting (reducing) and dry (oxidizing) conditions. Mottling of the soil matrix or groundmass is described in terms of abundance, size, contrast, boundary and colour. In addition, the shape, position or any other feature may be recorded.

Note for classification purposes 9Mottles of oxides in the form of coatings or in platy, polygonal or reticulate patterns are diagnostic for the anthraquic (plough pan), hydragric, ferric, plinthic and petroplinthic horizons and for the gleyic colour pattern. 9Mottles of oxides in the form of or nodules are diagnostic for the hydragric, ferric, plinthic, petroplinthic and, pisoplinthic horizons and for the stagnic colour pattern. 9Redox depleted zones in macropores with a value ≥ 4 and a chroma ≤ 2 are diagnostic for the hydragric horizon. 9Mottles or coatings of jarosite or schwertmannite are diagnostic for the thionic horizon and the Aceric qualifier. 9Mottles in the form of yellow concentrations are diagnostic for the thionic horizon.

Colour of mottles It is usually sufficient to describe the colour of the mottles in general terms, corresponding to the Munsell Soil Color Charts.

Abundance of mottles The abundance of mottles is described in terms of classes TABLE 32 indicating the percentage of the Classification of the abundance of mottles exposed surface that the mottles % N None 0 occupy (Table 32). The class limits V Very few 0–2 correspond to those of mineral F Few 2–5 nodules. When the abundance C Common 5–15 of mottles does not allow the M Many 15–40 distinction of a single predominant A Abundant > 40 matrix or groundmass colour, the predominant colours should be determined and entered as soil TABLE 33 Classification of the size of mottles matrix colours. mm V Very fine < 2 Size of mottles F Fine 2–6 Table 33 lists the classes used to M Medium 6–20 indicate the approximate diameters A Coarse > 20 36 Guidelines for soil description

TABLE 34 Classification of the contrast of mottles of individual mottles. They F Faint The mottles are evident only on close examina- correspond to the size classes of tion. Soil colours in both the matrix and mottles mineral nodules. have closely related hues, chromas and values. D Distinct Although not striking, the mottles are readily seen. The hue, chroma and value of the matrix Contrast of mottles are easily distinguished from those of the mottles. They may vary by as much as 2.5 units of The colour contrast between hue or several units in chroma or value. mottles and soil matrix can be P Prominent The mottles are conspicuous and mottling is one described as per Table 34. of the outstanding features of the horizon. Hue, chroma and value alone or in combination are at least several units apart. Boundary of mottles The boundary between mottle and TABLE 35 Classification of boundary between mottle and matrix matrix is described as the thickness mm of the zone within which the S Sharp < 0.5 colour transition can be located C Clear 0.5–2 without being in either the mottle D Diffuse > 2 or matrix (Table 35).

SOIL REDOX POTENTIAL AND REDUCING CONDITIONS Determination of redox potential by field method Soil redox potential is an important physico-chemical parameter used to characterize soil aeration status and availability of some nutrients (Table 36). The redox potential is also used in the WRB classification to classify redoximorphic soils. To measure redox potential (DIN/ISO Draft, DVWK, 1995), drive a hole into the soil using a rigid rod (stainless steel, 20–100 cm long, with a diameter that is 2 mm greater than the redox electrodes) to a depth about 1–2 cm less than the desired depth to be measured. Immediately clean the platinum surface of the redox electrode with sandpaper and insert the electrode about 1 cm deeper than the prepared hole. At least two electrodes should be installed for each depth

TABLE 36 Redoximorphic soil characteristics and their relation to rH values and soil processes

Redoximorphic characteristics rH values and status Processes No redoximorphic characteristics at permanently > 35 strongly aerated permanently high potentials - < 33 NO3 reduction Black Mn concretions temporary < 29 MnII formation Fe mottles and/or brown Fe concretions, in temporary < 20 FeII formation* wet conditions Blue-green to grey colour; Fe2+ always permanently 13–19 formation of FeII/FeIII oxides (green rust)* present Black colour due to metal sulphides, permanently < 13 sulphide formation flammable methane present permanently < 10 methane formation * For field test, see section on reducing conditions (below). Chapter 4 – Soil description 37

being measured. After at least 30 minutes, measure the redox potential with a millivoltmeter against a reference electrode (e.g. Ag/AgCl in KCl of the glass electrode of pH measurements, installed in a small hole on the topsoil that has been filled with 1-M KCl solution). For dry topsoil, a salt bridge (plastic tube 2 cm in diameter and with open ends, filled with 0.5 percent (M/M) agar in KCl solution) should be installed in a hole beside and at the depth of the platinum electrodes. In this tube, the reference electrode should be installed. The measured voltage (Em) is related to the voltage of the standard hydrogen electrode by adding the potential of the reference electrode (e.g. +244 millivolt at 10 °C of Ag/AgCl in 1 M KCl, +287 of Calomel electrode). For interpretation, the results should be transformed to rH values using the formula: rH = 2pH + 2Eh/59 (Eh in mV at 25 °C). Note the rH value on the description sheet.

Reducing conditions Reductimorphic properties of the soil matrix reflect permanently wet or at least reduced conditions (Table 37). They are expressed by neutral (white to black: Munsell N1 to N) or bluish to greenish colours (Munsell 2.5 Y, 5 Y, 5 G, 5 B). The colour pattern will often change by aeration in minutes to days owing to oxidation processes. The presence of FeII ions can be tested by spraying the freshly exposed soil surface with a 0.2-percent (M/V) _,_ dipyridyl solution in 10-percent (V/V) acetic acid solution. The test yields a striking reddish-orange colour in the presence of Fe2+ ions but may not give the strong red colour in soil materials with a neutral or alkaline soil reaction. Care is necessary as the chemical is slightly toxic.

Note for classification purposes 9An rH value of < 20 is diagnostic for reducing conditions in , and , and stagnic and gleyic lower level units of other RSGs. Gaseous emissions (methane, carbon dioxide, etc.) are diagnostic for the Reductic qualifier.

TABLE 37 Reductimorphic colour pattern and occurrence of Fe compounds Colour Munsell colour Formula Mineral Greyish green, light blue 5–GY–5–B2–3/1–3 FeII/FeIII Fe-mix compounds (blue-green rust)

II White, after oxidation brown N7–8 A 10 YR4/5 Fe CO3 siderite II White, after oxidation blue N7–8 A 5–B Fe 3(PO4)2 · 8 H2O vivianite

Bluish black FeS, FeS2 5–10–B1–2/1–3 Fe sulphides (with 10% HCl; H2S- smell) (or Fe3S4) White, after oxidation white N8 A N8 - - Complete loss of Fe compounds Source: Schlichting et al., 1995 38 Guidelines for soil description

TABLE 38 CARBONATES Classification of carbonate reaction in the soil matrix Content % N 0 Non-calcareous No detectable visible or Carbonates in soils are either audible effervescence. residues of the parent material SL 5 0–2 Slightly calcareous Audible effervescence but not visible. or the result of neo-formation MO 5 2–10 Moderately calcareous Visible effervescence. (secondary carbonates). The latter ST 5 10–25 Strongly calcareous Strong visible are concentrated mainly in the form effervescence. Bubbles form a low foam. of soft powdery lime, coatings EX 5 > 25 Extremely calcareous Extremely strong on peds, concretions, surface or reaction. Thick foam forms quickly. crusts, or hard banks. The presence of carbonate

(CaCO3) is established by adding TABLE 39 some drops of 10-percent HCl to Classification of forms of secondary carbonates the soil. The degree of effervescence SC soft concretions of carbon dioxide gas is indicative HC hard concretions HHC hard hollow concretions for the amount of calcium D disperse powdery lime carbonate present. In many soils, it PM pseudomycelia* (carbonate infillings in pores, resembling is difficult to distinguish in the field mycelia) between primary and secondary M marl layer HL hard cemented layer or layers of carbonates (less than carbonates. Classes for the reaction 10 cm thick) of carbonates in the soil matrix are * Pseudomycelia carbonates are not regarded as “secondary defined as per Table 38. carbonates” if they migrate seasonally and have no permanent depth. The reaction to acid depends upon soil texture and is usually more vigorous in sandy material than in fine-textured material with the same carbonate content. Other materials, such as roots, may also give an audible reaction. commonly reacts more slowly and less vigorously than . Secondary carbonates should be tested separately; they normally react much more intensely with HCl.

Forms The forms of secondary carbonates in soils are diverse and are considered to be informative for diagnostics of soil genesis. Soft carbonate concentrations are considered to be illuvial, and hard concretions are generally believed to be of hydrogenic nature. The forms of secondary carbonates should be indicated as per Table 39.

Note for classification purposes Important carbonate contents for classification are: 9≥ 2 percent calcium carbonate equivalent A calcaric material. 9≥ 15 percent calcium carbonate equivalent in the fine earth, at least partly secondary A calcic horizon. 9Indurated layer with calcium carbonate, at least partly secondary A petrocalcic horizon. Chapter 4 – Soil description 39

TABLE 40 Classification of gypsum content %

N 0 Non-gypsiric EC = < 1.8 dS m-1 in 10 g soil/25 ml H2O,

EC = < 0.18 dS m-1 in 10 g soil/250 ml H2O

SL 5 0–5 Slightly gypsiric EC = < 1.8 dS m-1 in 10 g soil/250 ml H2O

MO 5 5–15 Moderately gypsiric EC = > 1.8 dS m-1 in 10 g soil/250 ml H2O

ST 5 15–60 Strongly gypsiric higher amounts may be differentiated by abundance of H2O-soluble pseudomycelia/crystals and soil colour EX 5 > 60 Extremely gypsiric

915–25 percent calcium carbonate equivalent in the fine earth, at least partly secondary A Hypocalcic qualifier. 9≥ 50 percent calcium carbonate equivalent in the fine earth, at least partly secondary A Hypercalcic qualifier. 9Where a soil has a calcic horizon starting 50–10 cm from the soil surface, it is only a if the soil matrix between 50 cm from the soil surface and the calcic horizon is calcareous throughout. 9 and can only have an argic horizon where the argic horizon is permeated with calcium carbonate (Calcisols) or calcium carbonate or gypsum (Gypsisols).

GYPSUM Content of gypsum

Gypsum (CaSO4·2H2O) may be found in the form of residues of gypsiric parent material or new formed features. The latter are pseudomycelia, coarse-sized crystals (individualized, as nests, beards or coatings, or as elongated groupings of fibrous crystals) or loose to compact powdery accumulations. The latter form gives the gypsic horizon a massive structure and a sandy texture. Where more readily soluble salts are absent, gypsum can be estimated in the field by measurements of electrical conductivity (EC in dS m-1) in soil suspensions of different soil–water relations (Table 40) after 30 minutes (in the case of fine- grained gypsum).

Forms of secondary gypsum The forms of secondary gypsum in soils are diverse and are considered to be informative for diagnostics of soil genesis. The forms of secondary carbonates should be indicated as per Table 41. TABLE 41 Classification of forms of secondary gypsum Note for classification purposes SC soft concretions Important contents of gypsum for D disperse powdery gypsum G “gazha” (clayey water-saturated layer with high gypsum classification are: content) 9≥ 5 percent (by volume) gypsum HL hard cemented layer or layers of gypsum (less than 10 cm A gypsiric material. thick) 40 Guidelines for soil description

9≥ 5 percent (by mass) gypsum

TABLE 42 and ≥ 1 percent (by volume) Classification of salt content of soil secondary gypsum A gypsic -1 EC2.5 = dS m (25 ºC) horizon. N (nearly)Not salty < 0.75 9Indurated layer with ≥ SL Slightly salty 0.75–2 5 percent (by mass) gypsum MO Moderately salty 2–4 ST Strongly salty 4–8 and ≥ 1 percent (by volume) VST Very strongly salty 8–15 secondary gypsum A EX Extremely salty > 15 petrogypsic horizon. Source: DVWK, 1995. 915–25 percent (by mass) gypsum and ≥ 1 percent (by volume) secondary gypsum A Hypogypsic qualifier. 9≥ 50 percent (by mass) gypsum and ≥ 1 percent (by volume) secondary gypsum A Hypergypsic qualifier. 9Gypsisols can only have an argic horizon if the argic horizon is permeated with calcium carbonate or gypsum.

READILY SOLUBLE SALTS Coastal or desert soils can be especially enriched with water-soluble salts or salts

more soluble than gypsum (CaSO4·2H2O; log Ks = -4.85 at 25 °C). The salt content of the soil can be estimated roughly from an EC (in dS m-1 = mS cm-1) measured in a saturated soil paste or a more diluted suspension of soil in water (Richards, 1954).

Conventionally, EC is measured in the laboratory in the saturation extract (ECSE). Most classification values and data about salt sensitivity of crops refer to ECSE. An easier and more comfortable method of determining EC in the field is to

use a 20 g soil/50 ml H2O (aqua dest) suspension (EC2.5) and to calculate ECSE depending on the texture and content of organic matter (Table 42).

Procedure Use a transparent plastic cup with marks for 8 cm3 soil (~ 10 g) and 25 ml water and mix carefully with a plastic stick. The EC is measured with a field conductometer after 30 minutes in the clear solution. Use water with an EC < 0.01 dS m-1.

The salt content (NaCl equivalent) can be estimated from EC2.5 by: -1 salt [%] = EC2.5 [mS cm ] · 0.067 · 2.5. The EC2.5 can be converted to ECSE depending on the texture and content of humus according to the formula below and Table 43.

Note for classification purposes -1 9Threshold values of ≥ 8 and ≥ 15 dS m (ECSE, 25 °C) A salic horizon. -1 9≥ 4 dS m (ECSE, 25°C) in at least some layer within 100 cm A Hyposalic qualifier. Chapter 4 – Soil description 41

TABLE 43 Dependency of water content of saturation extract on texture and content of humus for mineral soils and on decomposition for peat soils

Textural class Water content of saturation extract WCSE in g/100 g Mineral soils Content of humus < 0.5% 0.5–1% 1–2% 2–4% 4–8% 8–15% Gravel, CS 5 6 8 13 21 35 MS 8 9 11 16 24 38 FS 10 11 13 18 26 40

LS, SL < 10% clay 14 15 17 22 30 45

SiL < 10% clay 17 18 20 25 34 49 Si 19 20 22 27 36 51

SL 10–20% clay 22 23 26 31 39 55 L 252629344258

SiL 10–27% clay 28 29 32 37 46 62 SCL 32 33 36 41 50 67 CL, SiCL 44 46 48 53 63 80 SC 51 53 55 60 70 88

SiC, C 40–60% clay 63 65 68 73 83 102

HC > 60% clay 105 107 110 116 126 147 Peat soils Decomposition stage (see section 3.3.3) D1 fibric D2 low D3 moderate D4 strong D5 sapric 80 120 170 240 300 Source: Adapted from DVWK (1995), recalculated to FAO textural classes.

-1 9≥ 30 dS m (ECSE, 25°C) in at least some layer within 100 cm A Hypersalic qualifier.

FIELD SOIL PH Soil pH expresses the activity of the hydrogen ions in the soil solution. It affects the availability of mineral nutrients to plants as well as many soil processes. When the pH is measured in the field, the method used should be indicated on the field data sheet. The field soil pH should not be a substitute for a laboratory determination. Field soil pH measurements should be correlated with laboratory determinations where possible. In the field, pH is either estimated using indicator papers, indicator liquids (e.g. Hellige), or measured with a portable pH meter in a soil suspension (1 part soil and

2.5 parts 1 M KCl or 0.1 M CaCl2 solution). After shaking the solution and waiting for 15 minutes, the pH value can be read. For the measurement, use a transparent 50- ml plastic cup with marks for 8 cm3 soil (~ 10 g) and 25 ml solution. TABLE 44 Classification of pH value

Note for classification purposes pHCaCl of < 5.1, if > 15% OM is an indication for a Dystric 2 qualifier (= base saturation As the pH value in many soils < 4.6, if 4–15% OM <50%), otherwise Eutric < 4.2, if < 4% OM A correlates with the base saturation, qualifier it may be used in the field for < 3.6, if > 15% OM is an indication for a base saturation of less than 10% preliminary classification purposes < 3.4, if 4–15% OM and for a high Al saturation < 3.2, if < 4% OM A (Table 44). However, proof in the Hyperalic qualifier laboratory is necessary. Source: Adapted from Schlichting, Blume and Stahr, 1995. 42 Guidelines for soil description

SOIL ODOUR TABLE 45 Classification of soil odour Record the presence of any strong Odour – kind Criteria smell (Table 45), by horizon. No N None No odour detected entry implies no odour. P Petrochemical Presence of gaseous or liquid gasoline, oil, creosote, etc. ANDIC CHARACTERISTICS AND S Sulphurous Presence of H2S (hydrogen sulphide; “rotten eggs”); commonly associated with strongly VOLCANIC GLASSES reduced soil containing sulphur compounds. Soils formed from young volcanic materials often have andic properties: a bulk density of 0.9 kg dm-3 or less, and a smeary consistence (owing to higher contents of allophane and/or ferrihydrite). Surface horizons with andic characteristics are normally black because of high humus contents. Andic characteristics may be

identified in the field using the pHNaF field test developed by Fieldes and Perrott (1966). A pHNaF of more than 9.5 indicates the presence of abundant allophanic products and/or organo- complexes. The method depends on active aluminium sorbing fluoride ions with subsequent release of OH+ ions. The test is indicative for most layers with andic properties, except for those very rich in organic matter. However, the same reaction occurs in spodic horizons and in certain acid clayey soils that are rich in aluminium-interlayered clay minerals; soils with free carbonates also react. Before applying a field NaF test, it is important to check soil pH (the test is not suitable for alkaline soils) and the presence of free carbonates (using HCl field test).

Procedure Place a small amount of soil material on a filter paper previously soaked in phenolphthalein and add some drops of 1 M NaF (adjusted to pH 7.5). A positive reaction is indicated by a fast change to an intense red colour. Alternatively, measure the pH of a suspension of 1 g soil in 50 ml 1 M NaF (adjusted to pH 7.5) after 2 minutes. If the pH is more than 9.5, it is a positive indication. Note in the description sheet the sign of a + or a -. In addition, soil material with andic characteristics may exhibit thixotropy; the soil material changes under pressure or by rubbing from a plastic solid into a liquefied stage and back into the solid condition.

Note for classification purposes 9Positive field test for allophanic products and/or organo-aluminium complexes A andic properties. 9Thixotropy A Thixotropic qualifier. In many young volcanic materials, volcanic glasses, glassy aggregates and other glass-coated primary minerals occur. Coarser fractions may be checked by a ×10 hand-lens; finer fractions may be checked by microscope. Chapter 4 – Soil description 43

Note for classification purposes 9≥ 5 percent (by grain count) volcanic glass, glassy aggregates and other glass- coated primary minerals, in the fraction 0.05–2 mm, or in the fraction 0.02– 0.25 mm A vitric properties. 9≥ 30 percent (by grain count) volcanic glass, glass-coated primary minerals, glassy materials, and glassy aggregates in the 0.02–2 mm particle-size fraction A tephric material.

ORGANIC MATTER CONTENT Organic matter refers to all decomposed, partly decomposed and undecomposed organic materials of plant and animal origin. It is generally synonymous with humus although the latter is more commonly used when referring to the well- decomposed organic matter called humic substances. The content of organic matter of mineral horizons can be estimated from the Munsell colour of a dry and/or moist soil, taking the textural class into account (Table 46). This estimation is based on the assumption that the soil colour (value) is due to a mixture of dark coloured organic substances and light coloured minerals. This estimate does not work very well in strongly coloured . It tends to overestimate organic matter content in soils of dry regions, and to underestimate the organic matter content in some tropical soils. Therefore, the organic matter values should always be locally checked as they only provide a rough estimate.

TABLE 46 Estimation of organic matter content based on Munsell soil colour

Colour Munsell Moist soil Dry S LS, SL, L SiL, Si, SiCL, CL, S LS, SL, L SiL, Si, SiCL, CL, SCL, SC, SiC, C SCL, SC, SiC, C (%) Light grey 7 < 0.3 < 0.5 < 0.6 Light grey 6.5 0.3–0.6 0.5–0.8 0.6–1.2 Grey 6 0.6–1 0.8–1.2 1.2–2 Grey 5.5 < 0.3 1–1.5 1.2–2 2–3 Grey 5 < 0.3 < 0.4 0.3–0.6 1.5–2 2–4 3–4 Dark grey 4.5 0.3–0.6 0.4–0.6 0.6–0.9 2–3 4–6 4–6 Dark grey 4 0.6–0.9 0.6–1 0.9–1.5 3–5 6–9 6–9 Black grey 3.5 0.9–1.5 1–2 1.5–3 5–8 9–15 9–15 Black grey 3 1.5–3 2–4 3–5 8–12 > 15 > 15 Black 2.5 3–6 > 4 > 5 > 12 Black 2 > 6 Note: If chroma is 3.5–6, add 0.5 to value; if chroma is > 6, add 1.0 to value. Source: Adapted from Schlichting, Blume and Stahr, 1995. 44 Guidelines for soil description

Note for classification purposes 9If saturated with water for ≥ 30 consecutive days in most years (unless drained): ≥ [12 + (clay percentage of the mineral fraction × 0.1)]% organic carbon or ≥ 18 percent organic carbon, else ≥ 20 percent organic carbon A organic material. 9Organic material saturated with water for ≥ 30 consecutive days in most years (unless drained) A histic horizon. 9Organic material saturated with water for < 30 consecutive days in most years A folic horizon. 9Weighted average of ≥ 6 percent organic carbon, and ≥ 4 percent organic carbon in all parts A fulvic and melanic horizon. 9Organic carbon content of ≥ 0.6 percent A mollic and umbric horizon. 9Organic carbon content of ≥ 1.5 percent A voronic horizon. (Note: the ratio of organic carbon to organic matter is about 1:1.7–2.) Write the range or average value in the description sheet.

ORGANIZATION OF SOIL CONSTITUENTS This section describes the primary physical organization of arrangement of the soil constituents, together with the consistence of the constituents. Primary organization is considered as being the overall arrangement of the soil mass without concentrations, reorientations and biological additions. It will not always be possible to make clear distinctions between primary and secondary elements of the organization. Voids (pores), which relate to the structural organization of soil, are described in a later section.

Soil structure Soil structure refers to the natural organization of soil particles into discrete soil units (aggregates or peds) that result from pedogenic processes. The aggregates are separated from each other by pores or voids. It is preferred to describe the structure when the soil is dry or slightly moist. In moist or wet conditions, it is advisable to leave the description of structure to a later time when the soil has dried out. For the description of soil structure, a large lump of the soil should be taken from the profile, from various parts of the horizon if necessary, rather than observing the soil structure in situ. Soil structure is described in terms of grade, size and type of aggregates. Where a soil horizon contains aggregates of more than one grade, size or type, the different kinds of aggregates should be described separately and their relationship indicated.

Grade In describing the grade or development of the structure, the first division is into apedal soils (lacking soil structure) and pedal soils (showing soil structure). In apedal or structureless soil, no aggregates are observable in place and there is no definite arrangement of natural surfaces of weakness. Structureless soils are subdivided into single grain and massive (see below). Single-grain soil material Chapter 4 – Soil description 45

has a loose, soft or very friable consistence and consists on rupture of more than 50 percent discrete mineral particles. Massive soil material normally has a stronger consistence and is more coherent on rupture. Massive soil material may be further defined by consistence (below) and porosity (below). Grades of structure of pedal soil materials are defined as per Table 47.

TABLE 47 Classification of structure of pedal soil materials WE Weak Aggregates are barely observable in place and there is only a weak arrangement of natural surfaces of weakness. When gently disturbed, the soil material breaks into a mixture of few entire aggregates, many broken aggregates, and much material without aggregate faces. Aggregate surfaces differ in some way from the aggregate interior. MO Moderate Aggregates are observable in place and there is a distinct arrangement of natural surfaces of weakness. When disturbed, the soil material breaks into a mixture of many entire aggregates, some broken aggregates, and little material without aggregates faces. Aggregates surfaces generally show distinct differences with the aggregates interiors. ST Strong Aggregates are clearly observable in place and there is a prominent arrangement of natural surfaces of weakness. When disturbed, the soil material separates mainly into entire aggregates. Aggregates surfaces generally differ markedly from aggregate interiors. Combined classes may be constructed as follows: WM Weak to moderate MS Moderate to strong

FIGURE 6 Soil structure types and their formation

Pedogenic ped formation? No Caronates Yes Single Massive Layered Gypsum grain (coherent) (coherent) Formed by cementation from precipitates of Humus Iron Silica Formed by assemble Formed by separation Formed by fragmentation (biotic) (abiotic) or compaction

Granular Worm- Blocky Blocky Prismatic Columnar Platy Crumbly Lumpy Clody casts subangular angular 46 Guidelines for soil description

TABLE 48 Classification of types of soil structure Blocky Blocks or polyhedrons, nearly equidimensional, having flat or slightly rounded surfaces that are casts of the faces of the surrounding aggregates. Subdivision is recommended into angular, with faces intersecting at relatively sharp angles, and subangular blocky faces intersecting at rounded angles. Granular Spheroids or polyhedrons, having curved or irregular surfaces that are not casts of the faces of surrounding aggregates. Platy Flat with vertical dimensions limited; generally oriented on a horizontal plane and usually overlapping. Prismatic the dimensions are limited in the horizontal and extended along the vertical plane; vertical faces well defined; having flat or slightly rounded surfaces that are casts of the faces of the surrounding aggregates. Faces normally intersect at relatively sharp angles. Prismatic structures with rounded caps are distinguished as Columnar. Rock structure Rock structure includes fine stratification in unconsolidated , and pseudomorphs of weathered minerals retaining their positions relative to each other and to unweathered minerals in saprolite from consolidated rocks. Wedge-shaped Elliptical, interlocking lenses that terminate in sharp angles, bounded by slickensides; not limited to vertic materials. Crumbs, lumps and clods Mainly created by artificial disturbance, e.g. tillage.

TABLE 49 Type Codes for types of soil structure The basic natural types of structure RS Rock structure (Figure 6) are defined as per SS Stratified structure SG Single grain Table 48. MA Massive Where required, special cases PM Porous massive or combinations of structures BL Blocky AB Angular blocky may be distinguished, which are AP Angular blocky (parallelepiped) subdivisions of the basic structures. AS Angular and subangular blocky The recommended codes are given AW Angular blocky (wedge-shaped) SA Subangular and angular blocky in Table 49. SB Subangular blocky SN Nutty subangular blocky Size PR Prismatic PS Subangular prismatic Size classes vary with the structure WE Wedge-shaped type. For prismatic, columnar and CO Columnar platy structures, the size classes GR Granular WC Worm casts refer to the measurements of the PL Platy smallest dimension of the aggregate CL Cloddy (Table 50). CR Crumbly LU Lumpy Combined classes may be constructed as per Table 51. Where a second structure is present, its relation to the first structure is described. The first and second structures may both be present (e.g. columnar and prismatic structures). The primary structure may break down into a secondary structure (e.g. prismatic breaking into angular blocky). The first structure may merge into the second structure (e.g. platy merging into prismatic). These can be indicated as per Table 52. Chapter 4 – Soil description 47

TABLE 50 Size classes for soil structure types Granular/platy Prismatic/columnar/wedge- Blocky/crumbly/lumpy/ shaped cloddy (mm) (mm) (mm) VF Very fine/thin < 1 < 10 < 5 FI Fine/thin 1–2 10–20 5–10 ME Medium 2–5 20–50 10–20 CO Coarse/thick 5–10 50–100 20–50 VC Very coarse/thick > 10 100–500 > 50 EC Extremely coarse – > 500 –

Note for classification purposes TABLE 51 9Soil structure, or absence Combined size classes for soil structure types of rock structure (the term FF Very fine and fine VM Very fine to medium “rock structure” also applies FM Fine and medium to unconsolidated sediments FC Fine to coarse in which stratification is still MC Medium and coarse visible) in half of the volume or MV Medium to very coarse more of the fine earth A cambic CV Coarse and very coarse horizon. 9Soil structure sufficiently strong TABLE 52 that the horizon is not both Combinations of soil structures massive and hard or very hard CO + PR Both structures present when dry (prisms larger than PR A AB Primary breaking to secondary structure 30 cm in diameter are included PL / PR One structure merging into the other in the meaning of massive if there is no secondary structure within the prisms) A mollic, umbric and anthric horizons. 9Granular or fine subangular blocky soil structure (and worm casts) A voronic horizon. 9Columnar or prismatic structure in some part of the horizon or a blocky structure with tongues of an eluvial horizon A natric horizon. 9Moderate to strong, angular blocky structure breaking to flat-edged or nut- shaped elements with shiny ped faces A nitic horizon. 9Wedge-shaped structural aggregates with a longitudinal axis tilted 10–60 ° from the horizontal A vertic horizon. 9Wedge-shaped aggregates A vertic properties. 9Platy structure A puddled layer (anthraquic horizon). 9Uniformly structured A irragric horizon. 9Separations between structural soil units that allow roots to enter have an average horizontal spacing of ≥ 10 cm A fragic horizon. 9Platy or massive structure A takyric horizon. 9Platy layer A yermic horizon. 48 Guidelines for soil description

9Strong structure finer than very coarse granular A Grumic qualifier. 9Massive and hard to very hard in the upper 20 cm of the soil A Mazic qualifier. 9A platy structure and a surface crust A Hyperochric qualifier. 9Stratification in ≥ 25 percent of the soil volume A fluvic material.

Consistence Consistence refers to the degree of cohesion or adhesion of the soil mass. It includes soil properties such as friability, plasticity, stickiness and resistance to compression. It depends greatly on the amount and type of clay, organic matter and moisture content of the soil. For reference descriptions (Status 1, Chapter 2), a recording of consistence is required for the dry, moist and wet (stickiness and plasticity) states. Where applicable, the smeariness (thixotropy) and fluidity may also be recorded. For routine descriptions, the soil consistence in the natural moisture condition of the profile may be described. Wet consistence can always be described, and moist conditions where the soil is dry, by adding water to the soil sample.

Consistence when dry The consistence when dry (Table 53) is determined by breaking an air-dried mass of soil between thumb and forefinger or in the hand.

Consistence when moist Consistence when moist (Table 54) is determined by attempting to crush a mass of moist or slightly moist soil material.

Consistence when wet: maximum stickiness and maximum plasticity Soil stickiness depends on the extent to which soil structure is destroyed and on the amount of water present. The determination of stickiness should be performed under standard conditions on a soil sample in which structure is completely destroyed and which contains enough water to express its maximum stickiness. In this way, the maximum stickiness will be determined and comparison between degrees of stickiness of various soils will be feasible. The same principle applies to soil plasticity.

TABLE 53 Consistence of soil mass when dry LO Loose Non-coherent. SO Soft Soil mass is very weakly coherent and fragile; breaks to powder or individual grains under very slight pressure. SHA Slightly hard Weakly resistant to pressure; easily broken between thumb and forefinger. HA Hard Moderately resistant to pressure; can be broken in the hands; not breakable between thumb and forefinger. VHA Very hard Very resistant to pressure; can be broken in the hands only with difficulty. EHA Extremely hard Extremely resistant to pressure; cannot be broken in the hands. Note: Additional codes, needed occasionally to distinguish between two horizons or layers, are: SSH, soft to slightly hard; SHH, slightly hard to hard; and HVH, hard to very hard. Chapter 4 – Soil description 49

TABLE 54 Consistence of soil mass when moist LO Loose Non-coherent. VFR Very friable Soil material crushes under very gentle pressure, but coheres when pressed together. FR Friable Soil material crushes easily under gentle to moderate pressure between thumb and forefinger, and coheres when pressed together. FI Firm Soil material crushes under moderate pressure between thumb and forefinger, but resistance is distinctly noticeable. VFI Very firm Soil material crushes under strong pressures; barely crushable between thumb and forefinger. EFI Extremely firm Soil material crushes only under very strong pressure; cannot be crushed between thumb and forefinger.

Note: Additional codes are: VFF, very friable to friable; FRF, friable to firm; and FVF, firm to very firm.

Stickiness is the quality of TABLE 55 adhesion of the soil material to Classification of soil stickiness NST Non-sticky After release of pressure, practically no soil other objects determined by noting material adheres to thumb and finger. the adherence of soil material when SST Slightly sticky After pressure, soil material adheres to both thumb and finger but comes off it is pressed between thumb and one or the other rather cleanly. It is not finger (Table 55). appreciably stretched when the digits are separated. Plasticity is the ability of ST Sticky After pressure, soil material adheres to soil material to change shape both thumb and finger and tends to stretch somewhat and pull apart rather continuously under the influence than pulling free from either digit. of an applied stress and to retain VST Very sticky After pressure, soil material adheres strongly to both thumb and finger and the compressed shape on removal is decidedly stretched when they are of stress. Determined by rolling separated. Note: Additional codes are: SSS, slightly sticky to sticky; and SVS, sticky the soil in the hands until a wire to very sticky. about 3 mm in diameter has been formed (Table 56).

TABLE 56 Note for classification purposes Classification of soil plasticity 9Extremely hard consistence when NPL Non-plastic No wire is formable. dry A petrocalcic horizon. SPL Slightly plastic Wire formable but breaks immediately if bent into a ring; soil mass deformed by 9Surface crust with very hard very slight force. consistence when dry, and very PL Plastic Wire formable but breaks if bent into a ring; slight to moderate force required plastic and sticky consistence for deformation of the soil mass. when wet A takyric horizon. VPL Very plastic Wire formable and can be bent into a ring; moderately strong to very strong 9Air-dry clods, 5–10 cm in force required for deformation of the diameter, slake or fracture in soil mass. water within 10 minutes A Note: Additional codes are: SPP, slightly plastic to plastic; and PVP, plastic to very plastic. fragic horizon. 9Penetration resistance at field capacity of ≥ 50 kN m-1 A fragic horizon. 9Penetration resistance of ≥ 450 N cm-2 A petroplinthic horizon. 50 Guidelines for soil description

TABLE 57 Classification of moisture status of soil Rubbing Crushing Forming (to a ball) Moistening Moisture pF* (in the hand) dusty or hard not possible, seems to be warm going very dark not lighter very dry 5 makes no dust not possible, seems to be warm going dark hardly lighter dry 4 makes no dust possible (not sand) going slightly dark obviously lighter slightly moist 3 finger moist and cool, weakly is sticky no change of colour obviously lighter moist 2 shiny free water drops of water no change of colour wet 1 free water drops of water without crushing no change of colour very wet 0 * pF (p = potential, F = free energy of water) is log hPa.

Soil-water status Soil-water status is the term used for the moisture condition of a horizon at the time the profile is described. The moisture status can be estimated in the field as per Table 57.

Note for classification purposes 9The definitions of mineral and organic materials and of the histic, folic and cryic horizons depend on the soil-water status. 9Temporarily water-saturated A Gelistagnic, Oxyaquic and Reductaquic qualifiers. 9Organic material floating on water A Floatic qualifier. 9Permanently submerged under water < 2 m A Subaquatic qualifier. 9Flooded by tidewater, but not covered at mean low tide A Tidalic qualifier. 9Artificially drained histic horizon A Drainic qualifier.

BULK DENSITY Bulk density is defined as the mass of a unit volume of dry soil (105 °C). This volume includes both solids and pores and, thus, bulk density reflects the total soil porosity. Low bulk density values (generally below 1.3 kg dm-3) generally indicate a porous soil condition. Bulk density is an important parameter for the description of and ecosystem function. High bulk density values indicate a poorer environment for growth, reduced aeration, and undesirable changes in hydrologic function, such as reduced water infiltration. There are several methods of determining soil bulk density. One method is to obtain a known volume of soil, dry it to remove the water, and weigh the dry mass. Another uses a special coring instrument (cylindrical metal device) to obtain a sample of known volume without disturbing the natural soil structure, and then to determine the dry mass. For surface horizons, a simple method is to dig a small hole and fill it completely with a measured volume of sand. Field determinations of bulk density may be obtained by estimating the force required to push a knife into a soil horizon exposed at a field moist pit wall (Table 58). Chapter 4 – Soil description 51

TABLE 58 Field estimation of bulk density for mineral soils Observation Frequent ped shape Bulk density (kg dm-3) Code Sandy, silty and loamy soils with low clay content Many pores, moist materials drop easily out of the auger; materials with granular < 0.9 vesicular pores, mineral soils with andic properties. BD1 Sample disintegrates at the instant of sampling, many pores visible on single grain, granular 0.9–1.2 the pit wall. BD1 Sample disintegrates into numerous fragments after application of weak single grain, subangular, 1.2–1.4 pressure. angular blocky BD2 Knife can be pushed into the moist soil with weak pressure, sample subangular and angular 1.4–1.6 disintegrates into few fragments, which may be further divided. blocky, prismatic, platy BD3 Knife penetrates only 1–2 cm into the moist soil, some effort required, prismatic, platy, (angular 1.6–1.8 sample disintegrates into few fragments, which cannot be subdivided blocky) BD4 further. Very large pressure necessary to force knife into the soil, no further prismatic > 1.8 disintegration of sample. BD5 Loamy soils with high clay content, clayey soils When dropped, sample disintegrates into numerous fragments, further angular blocky 1.0–1.2 disintegration of subfragments after application of weak pressure. BD1 When dropped, sample disintegrates into few fragments, further angular blocky, prismatic, 1.2–1.4 disintegration of subfragments after application of mild pressure. platy, columnar BD2 Sample remains mostly intact when dropped, further disintegration coherent, prismatic, 1.4–1.6 possible after application of large pressure. platy, (columnar, angular BD3 blocky, platy, wedge– shaped) Sample remains intact when dropped, no further disintegration after coherent (prismatic, >1.6 application of very large pressure. columnar, wedge– BD4, 5 shaped) Note: If organic matter content is > 2%, bulk density has to be reduced by 0.03 kg dm-3 for each 1% increment in organic matter content.

Note for classification purposes 9Bulk density of 0.90 kg dm-3 or less A andic properties. 9In the plough pan, a bulk FIGURE 7 density ≥20 percent (relative) Qualification of bulk density higher than that of the puddled Si SiL SiCL SiC layer A anthraquic horizon. L CL C HC SL SCL SC Root penetration is not only LS Texture classes S limited by bulk density, but also by 2.0 BD5 g cm-3 Very firm (PD5) BD4 texture. Fine-textured soils contain Firm (PD4) BD3 1.5 Intermediate (PD3) fewer pores in size and abundance BD2 Loose (PD2) Bulk densty BD1 than needed for unrestricted 1.0 Very loose (PD1) root growth. Therefore, the evaluation of bulk density has to 0 50 100 % Clay content take soil texture into account. For evaluation purposes the “packing Source: according to Ad-hoc-AG-Boden, 2005. density” (PD = BD + 0.009 ·% clay) can also be used (Figure 7). 52 Guidelines for soil description

TABLE 59 Field estimation of volume of solids and bulk density of peat soils Drainage conditions Solid volume Bulk density Bog FenPeat characteristics Classes of decomposition Vol. (%) g cm-3 Code Undrained Undrained Almost swimming D1 Very low (fibric) < 3 < 0.04 SV1 Weakly drained Weakly Loose D2 Low (fibric) 3– < 5 0.04–0.07 drained SV2 Moderately drained Weakly Rather loose D3 Moderate (fibric) 5– < 8 0.07–0.11 drained SV3 Well drained Moderately Rather dense D4 Strong (hemic) 8– < 12 0.11–0.17 drained SV4 Well drained Well drained Dense D5 Very strong (sapric) ≥ 12 > 0.17 SV5 Source: Adapted from Ad-hoc-AG-Boden, 2005.

Bulk density and volume of solids of organic soils may be estimated after the decomposition stage or the extent of peat drainage. Weakly drained and weakly decomposed peat materials are characterized by a lower bulk density and a lower solid volume than well-drained and strongly decomposed peat materials (Table 59). Organic surface horizons of mineral soils may be treated like strongly decomposed peat layers.

VOIDS (POROSITY) Voids include all empty spaces in the soil. They are related to the arrangement of the primary soil constituents, rooting patterns, burrowing of animals or any other soil-forming processes, such as cracking, translocation and . The term void is almost equivalent to the term pore, but the latter is often used in a more restrictive way and does not, for example, include fissures or planes. Voids are described in terms of type, size and abundance. In addition, continuity, orientation or any other feature may also be recorded.

Porosity The porosity is an indication of the total volume of voids discernible with a ×10 hand-lens measured by area and recorded as the percentage of the surface occupied by pores (Table 60).

Type TABLE 60 Classification of porosity There is a large variety in the shape % and origin of voids. It is impractical 1 Very low < 2 and usually not necessary to 2 Low 2–5 describe all different kinds of 3 Medium 5–15 voids comprehensively. Emphasis 4 High 15–40 should be given to estimating the 5 Very high > 40 continuous and elongated voids. Chapter 4 – Soil description 53

TABLE 61 Classification of voids I Interstitial Controlled by the fabric, or arrangement, of the soil particles, also known as textural voids. Subdivision possible into simple packing voids, which relate to the packing of sand particles, and compound packing voids, which result from the packing of non-accommodating peds. Predominantly irregular in shape and interconnected, and hard to quantify in the field. B Vesicular Discontinuous spherical or elliptical voids (chambers) of sedimentary origin or formed by compressed air, e.g. gas bubbles in slaking crusts after heavy rainfall. Relatively unimportant in connection with plant growth. V Vughs Mostly irregular, equidimensional voids of faunal origin or resulting from tillage or disturbance of other voids. Discontinuous or interconnected. May be quantified in specific cases. C Channels Elongated voids of faunal or floral origin, mostly tubular in shape and continuous, varying strongly in diameter. When wider than a few centimetres (burrow holes), they are more adequately described under biological activity. P Planes Most planes are extra-pedal voids, related to accommodating ped surfaces or cracking patterns. They are often not persistent and vary in size, shape and quantity depending on the moisture condition of the soil. Planar voids may be recorded, describing width and frequency.

The major types of voids may be classified in a simplified way as per Table 61. In most cases, it is recommended that only the size and abundance of the channels, which are mostly continuous tubular pores, be described (Figure 8). For the other types of voids, the following size and abundance classes should serve as a guide for the construction of suitable classes for each category.

Size The diameter of the elongated or tubular voids is described as per Table 62.

Abundance The abundance of very fine and fine elongated pores as one group, TABLE 62 and of medium and coarse pores as Classification of diameter of voids another group is recorded as the mm number per unit area in a square V Very fine < 0.5 decimetre (Table 63). F Fine 0.5–2 M Medium 2–5 C Coarse 5–20 Note for classification purposes VC Very coarse 20–50 9Vesicular layer below a platy Note: Additional codes are: FM, fine and medium; FF fine and very layer or pavement with a vesicular fine; and MC, medium and coarse. layer A yermic horizon. 9Sorted soil aggregates and vesicular pores A anthraquic TABLE 63 horizon. Classification of abundance of pores < 2 mm (number) > 2 mm N None 0 0 CONCENTRATIONS V Very few 1–20 1–2 This section deals with the most F Few 20–50 2–5 common concentrations of soil C Common 50–200 5–20 materials, including secondary M Many > 200 > 20 enrichments, cementations and reorientations. 54 Guidelines for soil description

FIGURE 8 Charts for estimating size and abundance of pores

Abundance Very few Few Common Size

Very fine (<0.5 mm)

Fine (0.5–2 mm)

Medium (2–5 mm)

1 cm

Coatings This section describes clay or mixed-clay illuviation features, coatings of other composition (such as calcium carbonate, manganese, organic or silt), reorientations (such as slickensides and pressure faces), and concentrations associated with surfaces but occurring as stains in the matrix (“hypodermic coatings”). All these features are described according to their abundance, contrast, nature, form and location.

Abundance For coatings, an estimate is made of how much of the ped or aggregate faces is covered (Table 64). Corresponding criteria should be applied when the cutanic feature is related to other surfaces (voids, and coarse fragments) or occurs as lamellae.

Contrast Table 65 shows the classification of the contrast of coatings. Chapter 4 – Soil description 55

Nature TABLE 64 The nature of coatings may be Classification of abundance of coatings described as per Table 66. % N None 0 V Very few 0–2 Form F Few 2–5 For some coatings, the form C Common 5–15 may be informative for their M Many 15–40 genesis (Table 67). For example, A Abundant 40–80 manganese and iron–manganese D Dominant > 80 coatings of dendroidal form indicate their formation owing to TABLE 65 poor infiltration and periodically Classification of the contrast of coatings reductive conditions because of F Faint Surface of coating shows only little contrast in percolating water. colour, smoothness or any other property to the adjacent surface. Fine sand grains are readily apparent in the cutan. Lamellae are less than Location 2 mm thick. D Distinct Surface of coating is distinctly smoother or The location of the coatings or different in colour from the adjacent surface. Fine sand grains are enveloped in the coating clay accumulation is indicated but their outlines are still visible. Lamellae are (Table 68). For pressure faces and 2–5 mm thick. slickensides, no location is given P Prominent Surface of coatings contrasts strongly in smoothness or colour with the adjacent surfaces. because they are by definition Outlines of fine sand grains are not visible. located on pedfaces. Lamellae are more than 5 mm thick.

TABLE 66 Classification of the nature of coatings C Clay S Sesquioxides H Humus CS Clay and sesquioxides CH Clay and humus (organic matter) CC Calcium carbonate GB HC Hypodermic coatings (Hypodermic coatings, as used here, are field-scale features, commonly only expressed as hydromorphic features. Micromorphological hypodermic coatings include non-redox features [Bullock et al., 1985].) JA Jarosite MN Manganese SL Silica (opal) SA Sand coatings ST Silt coatings SF Shiny faces (as in nitic horizon) PF Pressure faces SI Slickensides, predominantly intersecting (Slickensides are polished and grooved ped surfaces that are produced by aggregates sliding one past another.) SP Slickensides, partly intersecting SN Slickensides, non intersecting

Source: Adapted from Schoeneberger et al, 2002. 56 Guidelines for soil description

Note for classification purposes TABLE 67 Classification of the form of coatings 9Evidence of silica accumulation, C Continuous e.g. as coatings A petroduric CI Continuous irregular (non-uniform, heterogeneous) horizon. DI Discontinuous irregular 9Slickensides A vertic horizon DE Dendroidal and vertic properties. DC Discontinuous circular O Other 9Evidence of clay illuviation A argic and natric horizons. 9Cracked coatings on sand grains

TABLE 68 A spodic horizon. Classification of the location of coatings and clay 9Uncoated sand and silt grains A accumulation Greyic qualifier. P Pedfaces 9Clay coatings in the argic PV Vertical pedfaces horizon A Cutanic qualifier. PH Horizontal pedfaces CF Coarse fragments 9Illuviation in the form of LA Lamellae (clay bands) lamellae in the argic, natric and VO Voids spodic horizon A Lamellic BR Bridges between sand grains qualifier. NS No specific location 9Coatings that have a different colour from the matrix (section on mottling [above]). TABLE 69 Classification of the continuity of cementation/compaction Cementation and compaction B Broken The layer is less than 50 percent cemented The occurrence of cementation or or compacted, and shows a rather irregular appearance. compaction in pans or otherwise is D Discontinuous The layer is 50–90 percent cemented or described according to its nature, compacted, and in general shows a regular appearance. continuity, structure, agent and C Continuous The layer is more than 90 percent degree. cemented or compacted, and is only interrupted in places by cracks or fissures. Compacted material has a firm or stronger consistence when moist and a close packing of particles. Cemented material does TABLE 70 not slake after 1 hour of immersion Classification of the fabric of the cemented/compacted in water. layer P Platy The compacted or cemented parts are plate- like and have a horizontal or subhorizontal Continuity orientation. Table 69 indicates the classification V Vesicular The layer has large, equidimensional voids that may be filled with uncemented material. of the continuity of cementation/ P Pisolithic The layer is largely constructed from compaction. cemented spherical nodules. D Nodular The layer is largely constructed from cemented nodules or concretions of irregular Structure shape. The fabric or structure of the cemented or compacted layer may be described as per Table 70. Chapter 4 – Soil description 57

Nature TABLE 71 The nature of cementation or Classification of the nature of cementation/compaction compaction is described according K Carbonates to the cementing agent or Q Silica compacting activity, as indicated in KQ Carbonates–silica Table 71. F Iron FM Iron–manganese (sesquioxides) FO Iron–organic matter Degree I Ice Table 72 indicates the classification GY Gypsum of the degree of cementation/ C Clay compaction. CS Clay–sesquioxides M Mechanical Note for classification purposes P Ploughing 9Ice overlain by organic material NK Not known A Histosols. 9Cementation by ice or readily visible ice crystals A cryic horizon. 9≥ 75 percent ice (by volume) A Glacic qualifier. 9Cementation by organic matter and aluminium A spodic horizon. 9Cemented spodic horizon A Ortsteinic qualifier. 9Iron pan that is 1–25 mm thick and is continuously cemented by a combination of organic matter, iron and/or aluminium A Placic qualifier. 9Strongly cemented or indurated A petrocalcic, duric, gypsic and plinthic horizons, Petric, Petrogleyic and Petrosalic qualifiers. 9Cementation on repeated wetting and drying A plinthic horizon. 9Roots cannot penetrate except along vertical fractures that have an average horizontal spacing of ≥ 10 cm and occupy < 20 percent (by volume) of the layer A petrocalcic, petroduric and petrogypsic horizons. 9Strongly cemented or indurated horizon consisting of clods with an average horizontal length of < 10 cm A Fractipetric and Fractiplinthic qualifiers.  Natural or artificial compaction A Densic qualifier.

TABLE 72 Classification of the degree of cementation/compaction

N Non-cemented and non-compacted Neither cementation nor compaction observed (slakes in water). Y Compacted but non-cemented Compacted mass is appreciably harder or more brittle than other comparable soil mass (slakes in water). W Weakly cemented Cemented mass is brittle and hard, but can be broken in the hands. M Moderately cemented Cemented mass cannot be broken in the hands but is discontinuous (less than 90 percent of soil mass). C Cemented Cemented mass cannot be broken in the hands and is continuous (more than 90 percent of soil mass). I Indurated Cemented mass cannot be broken by body weight (75-kg standard soil scientist) (more than 90 percent of soil mass). 58 Guidelines for soil description

TABLE 73 Mineral concentrations Classification of the abundance of mineral concentrations, Mineral concentrations cover by volume a large variety of secondary % N None 0 crystalline, microcrystalline and V Very few 0–2 amorphous concentrations of non- F Few 2–5 organic substances as infillings, C Common 2–15 soft concretions, irregular M Many 15–40 concentrations (mottles), nodules A Abundant 40–80 D Dominant > 80 of mainly pedogenetically formed materials. Gradual transitions exist with mottles (above), some

TABLE 74 of which may be considered as Classification of the kinds of mineral concentrations weak expressions of nodules. T Crystal The mineral concentrations are C A discrete body with a concentric described according to their internal structure, generally cemented. abundance, kind, size, shape, SC Soft concretion S Soft segregation Differs from the surrounding soil mass hardness, nature and colour. (or soft in colour and composition but is not accumulation) easily separated as a discrete body. Abundance (by volume) N Nodule Discrete body without an internal organization. Table 73 describes the classification IP Pore infillings Including pseudomycelium of of the abundance of mineral carbonates or opal. concentrations. IC Crack infillings R Residual rock Discrete impregnated body still fragment showing rock structure. Kind O Other Table 74 describes the classification of the kinds of mineral concen- trations. TABLE 75 Classification of the size and shape of mineral concentrations Size and shape Size (mm) Shape Table 75 describes the classification V Very fine < 2 R Rounded (spherical) of the size and shape of mineral F Fine 2–6 E Elongated concentrations. M Medium 6–20 F Flat C Coarse > 20 I Irregular A Angular Hardness Table 76 describes the classification of the hardness of mineral concen- TABLE 76 trations. Classification of the hardness of mineral concentrations H Hard Cannot be broken in the fingers. Nature S Soft Can be broken between forefinger and thumb nail Mineral concentrations are described B Both hard and soft. according to the composition or impregnating substance. Table 77 provides some examples. Chapter 4 – Soil description 59

Colour TABLE 77 The general colour names given in Examples of the nature of mineral concentrations Table 78 are usually sufficient to K Carbonates (calcareous) KQ Carbonates–silica describe the colour of the nodules C Clay (argillaceous) (similar to mottles) or of artefacts. CS Clay–sesquioxides GY Gypsum (gypsiferous) SA Salt (saline) Note for classification purposes GB Gibbsite 9≥ 10 percent (by volume) of JA Jarosite weakly cemented to indurated, S Sulphur (sulphurous) Q Silica (siliceous) silica-enriched nodules F Iron (ferruginous) (durinodes) A duric horizon. FM Iron–manganese (sesquioxides) 9Reddish to blackish nodules of M Manganese (manganiferous) NK Not known which at least the exteriors are at least weakly cemented or indurated A ferric horizon. TABLE 78 9Firm to weakly cemented Colour names of mineral concentrations nodules or mottles with a WH White stronger chroma or redder hue RE Red RS Reddish than the surrounding material YR Yellowish red A plinthic horizon. BR Brown 9Strongly cemented or indurated BS Brownish RB Reddish brown reddish to blackish nodules A YB Yellowish brown pisoplinthic horizon. YE Yellow RY Reddish yellow BIOLOGICAL ACTIVITY GE Greenish GR Grey In this section, evidence of past or GS Greyish present biological activity, including BU Blue human activity, is recorded. BB Bluish-black BL Black MC Multicoloured Roots The recording of both the size and the abundance of the roots is in general sufficient to characterize the distribution of roots in the profile. In specific cases, additional information can be noted, such as a sudden change in root orientation. The abundance of roots can only be compared within the same size class. The abundance of fine and very fine roots may be recorded similarly as for voids (Figure 8), expressed in the number of roots per decimetre square.

Size (diameter) Table 79 indicates the classification of the size of roots.

Abundance Table 80 indicates the classification of the abundance of roots. 60 Guidelines for soil description

TABLE 79 Other biological features Classification of the diameter of roots Biological features, such as mm krotovinas, termite burrows, insect VF Very fine < 0.5 nests, worm casts and burrows of F Fine 0.5–2 M Medium 2–5 larger animals, are described in C Coarse > 5 terms of abundance and kind. Note: Additional codes are: FF, very fine and fine; FM, fine and In addition, specific locations, medium; and MC, medium and coarse. patterns, size, composition or any other characteristic may be TABLE 80 recorded. Classification of the abundance of roots < 2 mm > 2 mm N None 0 0 Abundance V Very few 1–20 1–2 Abundance of biological activity is F Few 20–50 2–5 recorded in the general descriptive C Common 50–200 5–20 terms indicated in Table 81. M Many > 200 > 20 Kind

TABLE 81 Examples of biological features are Classification of the abundance of biological activity given in Table 82. N None F Few Note for classification purposes C Common 9≥ 50 percent (by volume) of M Many wormholes, casts or filled animal burrows A voronic horizon and Vermic qualifier. TABLE 82 Examples of biological features 9≥ 25 percent (by volume) of A Artefacts animal pores, coprolites or B Burrows (unspecified) other traces of animal activity BO Open large burrows A hortic and irragric horizons. BI Infilled large burrows C Charcoal E channels HUMAN-MADE MATERIALS P Pedotubules With the growing human influence T Termite or ant channels and nests in the world, especially in urban I Other insect activity and mining areas, it becomes increasingly important to document the type and degree of influence. Of particular importance are the human-made materials found in soils. Their age, amount, state and composition determine to a large extent the duration of human influence and the environmental impact.

Artefacts Artefacts (IUSS Working Group WRB, 2006) are solid or liquid substances that are: (i) created or modified substantially by humans as part of an industrial or artisanal manufacturing process; or (ii) brought to the surface by human activity Chapter 4 – Soil description 61

from a depth where they were not influenced by surface processes. They have properties substantially different from the environment where they are placed, and they have substantially the same properties as when first manufactured, modified or excavated. This definition has several implications: ¾ “Liquid” includes chemicals of industrial origin. ¾ It does not include mining overburden that has been influenced by surface processes or transported soil. ¾ It includes excavated natural solids and liquids, such as coal, spilled crude oil and bitumen. ¾ The human origin must be evident in the material itself, not from written records or inference. ¾ If it has been transformed so that its origin is no longer identifiable, it is no longer an artefact. Some examples of artefacts are: ¾ synthetic solids (compounds not found in nature): slag and plastic; ¾ synthetic liquids: creosote and refined hydrocarbons; ¾ waste liquids: sludges (e.g. brewery and municipal); ¾ natural materials recognizably reworked by humans: flint knives and arrowheads; ¾ natural materials processed by humans into a form or composition not found in nature: pottery, bricks, concrete, and shot; ¾ mixed materials: building rubble; ¾ industrial dusts (both natural and synthetic); ¾ pavements and paving stones; ¾ natural materials minimally processed but mixed in a way not found in nature: organic garbage. ¾ mine spoil or crude oil

Note for classification purposes 9≥ 20 percent (by volume, by weighted average) artefacts A Technosols.

Human-transported material (HTM) Human-transported material (HTM) is any material in the soil to be classified brought from “outside”, often by machinery. This can be for agricultural purposes (e.g. large-scale terracing, mine spoil re-vegetation), for human settlement, or simply to dispose of material that is unwanted in its original location (e.g. dredgings). It is a parent material for pedogenesis, by analogy to fluvial sediments and colluvium. It has been defined as: “Human-transported material (abbreviation ‘HTM’): Any solid or liquid material moved into the soil from a source area outside of its immediate vicinity by intentional human activity, usually with the aid of machinery, without substantial reworking or displacement by natural forces” (Rossiter, 2004). 62 Guidelines for soil description

The definition has several implications: ¾ The restriction to “intentional” excludes dusts from wind erosion or mass movement (e.g. slumps) caused by human activity. The intention must be inferred from the type of material and manner of deposition, not from historical records. ¾ “Liquids” can be of any viscosity and include slurries, liquid manures, hydrocarbons and other industrial chemicals transported by humans. ¾ If material originally transported by humans has been further moved by natural forces, such as erosion (water or wind) or flooding, the human influence is reduced, and it is no longer HTM. It is a different substrate and could be referred to as e.g. “colluvium from HTM”. ¾ Similarly, if the material is substantially reworked in situ (e.g. by frost), the human influence is reduced, and so it is no longer HTM. It could be referred to as “cyroturbated soil material originally human-transported”. ¾ The requirement that materials be moved farther than from the “immediate vicinity” excludes materials from ditching, terracing, etc. where the transported material is placed as close as possible to the source; the “transportation” is too local. HTM may be mixed with non-transported material, e.g. spoil that is partially ploughed into underlying natural soil. Thus, a soil layer may consist of part HTM and part non-transported (but reworked in situ) material. HTM may have substantial pedogenesis and still be identified as such. HTMs may be identified in several ways: ¾ by evidence of deposition processes after transportation (e.g. voids, compaction, and disorganized fragments of diagnostic horizons); ¾ by artefacts (not always present), although isolated artefacts may be mixed into non-transported soil by ploughing or bioturbation; ¾ by absence of evidence of transportation by natural forces (e.g. layering from flooding) or reworking in situ (e.g. cryoturbation); ¾ by absence of pedogenesis that masks evidence of deposition. In each case, the classifier must state the specific evidence for HTM. Historical evidence, e.g. site plans, may be used as an indication of where to find HTM but it is not diagnostic; this is the same as for fluvic sediments, which must be identified only from morphology, not from records of flooding.

Note for classification purposes 9HTM A Transportic qualifier.

Geomembranes and technic hard rock A (IUSS Working Group WRB, 2006) is a synthetic membrane laid on the surface or into the soil or any other substrate. Many geomembranes are made of polyvinyl chloride (PVC) or high-density polyethylene (HDPE). Technic hard rock (IUSS Working Group WRB, 2006) is consolidated material resulting Chapter 4 – Soil description 63

from an industrial process, with properties substantially different from those of natural materials.

Note for classification purposes 9A continuous, very slowly permeable to impermeable, constructed geomembrane starting within 100 cm of the soil surface A Technosols with the Linic qualifier. 9Technic hard rock starting within 5 cm of the soil surface and covering ≥ 95 percent of the horizontal extent of the soil A Technosols with the Ekranic qualifier.

Description of artefacts Artefacts are described according to their abundance, kind, size, hardness, weathering stage, and colour, if applicable.

Abundance Abundance is described with the same rules as for rock fragments (above).

Kind Table 83 lists the kinds of artefacts classified.

Size Size is described with the same rules as for rock fragments (above) or mineral nodules (above).

Hardness Hardness is described with the same rules as for mineral nodules (above).

Weathering State of weathering of the material is described with the same rules as for rock fragments (above).

Colour Colour is described with the same rules as for mineral nodules (above).

Note for classification purposes TABLE 83 9≥ 35 percent of the artefacts Classification of kinds of artefacts consisting of organic waste AN Artesanal natural material ID Industrial dust materials A Garbic qualifier. MM Mixed material 9≥ 35 percent of the artefacts OG Organic garbage consisting of industrial PS Pavements and paving stones waste materials (mine spoil, SL Synthetic liquid dredgings, rubble, etc.) A SS Synthetic solid WL Waste liquid Spolic qualifier. 64 Guidelines for soil description

TABLE 84 Determination table and codes for human-made deposits 1 Observation at the profile a) stratified (spoiled materials) go to step 2 s... b) not stratified but clods of different colour, texture and/or artefacts go to step 3 d... (dumped substrate) 2 Test for colour and texture a) light to dark grey, fine sand to silt, coarser grains have vesicular pores fly and bottom ash ...UA2 b) dark grey to black, visible particles of coal coke mud ...UA2 c) light to dark brown, fine sand to silt, small Fe/Mn concretions dredge mud of rivers ...UA1

d) dark grey to black, H2S smell dredge mud of ...UA1

e) dark grey to black, NH3 smell, artefacts sewage sludge ...UA2 f) dark grey to black, faecal smell, artefacts faecal sludge ...UA2 3 Test for texture, consistence and colour a) earthy, humic (grey to blackish grey) topsoil material ...UA1 b) loamy, with carbonates calcareous loam ...UU3 c) mainly sandy sand ...UU3 d) clayey clay ...UU1 e) mixture of sand, silt and clay loam ...UU2 f) mainly gravel gravel ...UU5 g) mainly broken rock broken rock ...UU5 h) > 30 percent pieces of grey to reddish-brown slag slag ...UA2 i) > 30 percent pieces of bricks and mortar and concrete construction rubble ...UA2

j) grey to black, H2S smell, > 30 percent artefacts (glass, ceramic, leather, waste ...UA2 wood, plastic, metals) Source: According to Meuser (1996), shortened.

9≥ 35 percent of the artefacts consisting of rubble and garbage of human settlements A Urbic qualifier.

Description and determination of human-transported material Where HTM is dominant, i.e. it occupies more than 50 percent (by volume) of the soil, it is sufficient to identify the type of HTM. Use the determination table (Table 84) and record the code.

SAMPLING The sample code and sampling depth are given. It is recommended that the number given to the sample be the profile number followed by an additional capital letter (A, B, C, D, etc.) and depth range at which each sample has been collected from top to bottom, regardless of the horizon they are taken from (some may not be sampled while others may be sampled twice). Samples are never taken across horizon boundaries. The weight of material taken for each sample is usually 1 kg. Horizon symbols should not be used as sample codes because the horizon classifications may be changed later. Chapter 4 – Soil description 65

There are basically two methods of collecting samples: ¾ To collect the sample in equal proportions over the whole horizon. This is the recommended method and should be used for reference (Status 1) descriptions where a dense sampling is required. ¾ To take the sample in equal proportions within a depth of 20 cm, either from the centre (area of maximum expression) of the horizon, or, if more than one sample is to be taken from the same horizon, at balanced intervals. In both methods, the boundary area itself should not be sampled. In detailed descriptions of soils with horizons no more than 30–40 cm thick, there will be little difference between the two methods in practice. It is recommended that the topsoil be sampled within the first 20 cm of the surface, or shallower where the horizon depth is less. This will facilitate comparison of topsoil characteristics in soil inventories and land evaluation. If the presence of a mollic horizon is assumed, the sampling depth for a soil with a more than 60 cm thick may be more than 20 cm but not exceeding 30 cm. Depth criteria of diagnostic horizons and properties should be taken into account in determining the depth of sampling. To indicate the occurrence of an argic horizon that is defined as having a specified clay increase over a vertical distance of 15 or 30 cm, the samples are preferably taken at that depth interval (e.g. A 0–20 cm, B 20–30 cm or 30–50 cm). Another example is for the classification of Nitisols: a sample should be taken at a depth of 140–160 cm, in addition to the one taken from that part of the B horizon where the clay content is assumed to be highest.

67

Chapter 5 Genetic and systematic interpretation – soil classification

SOIL HORIZON DESIGNATION The soil horizon designation summarizes many observations of the soil description and gives an impression about the genetic processes that have formed the soil under observation. In this chapter, the soil morphological and other characteristics are presented as they are described by horizon. Horizon symbols consist of one or two capital letters for the master horizon and lower case letter suffixes for subordinate distinctions, with or without a figure suffix. For the presentation and understanding of the soil profile description, it is essential that correct horizon symbols be given.

Master horizons and layers The capital letters H, O, A, E, B, C, R, I, L and W represent the master horizons or layers in soils or associated with soils. The capital letters are the base symbols to which other characters are added in order to complete the designation. Most horizons and layers are given a single capital letter symbol, but some require two. Currently, ten master horizons and layers and seven transitional horizons are recognized. The master horizons and their subdivisions represent layers that show evidence of change and some layers that have not been changed. Most are genetic soil horizons, reflecting a qualitative judgement about the kind of changes that have taken place. Genetic horizons are not equivalent to diagnostic horizons, although they may be identical in soil profiles. Diagnostic horizons are quantitatively defined features used in classification. Three additional layers associated with some soils are identified, viz. I for ice, L for limnic materials, and W for water layers.

H horizons or layers These are layers dominated by organic material formed from accumulations of undecomposed or partially decomposed organic material at the soil surface, which may be underwater. All H horizons are saturated with water for prolonged periods, or were once saturated but are now drained artificially. An H horizon may be on top of mineral soils or at any depth beneath the surface if it is buried. 68 Guidelines for soil description

O horizons or layers These are layers dominated by organic material consisting of undecomposed or partially decomposed litter, such as leaves, needles, twigs, moss and lichens, that has accumulated on the surface; they may be on top of either mineral or organic soils. O horizons are not saturated with water for prolonged periods. The mineral fraction of such material is only a small percentage of the volume of the material and is generally much less than half of the weight. An O layer may be at the surface of a mineral soil or at any depth beneath the surface where it is buried. A horizon formed by illuviation of organic material into mineral subsoil is not an O horizon, although some horizons formed in this manner contain much organic matter.

A horizons These are mineral horizons that formed at the surface or below an O horizon, in which all or much of the original rock structure has been obliterated and which are characterized by one or more of the following: ¾ an accumulation of humified organic matter intimately mixed with the mineral fraction and not displaying properties characteristic of E or B horizons (see below); ¾ properties resulting from cultivation, pasturing, or similar kinds of disturbance; ¾ a morphology that is different from the underlying B or C horizon, resulting from processes related to the surface. If a surface horizon (or epipedon) has properties of both A and E horizons but the dominant feature is an accumulation of humified organic matter, it is designated an A horizon. In some places, where warm and arid climates prevail, the undisturbed surface horizon is less dark than the underlying horizon and contains only small amounts of organic matter. It has a morphology distinct from the C layer, although the mineral fraction may be unaltered or only slightly altered by weathering. Such a horizon is designated A because it is at the surface. Examples of epipedons that may have a different structure or morphology owing to surface processes are Vertisols, soils in pans or playas with little vegetation, and soils in . However, recent alluvial or aeolian deposits that retain fine stratification are not considered to be an A horizon unless cultivated.

E horizons These are mineral horizons in which the main feature is loss of silicate clay, iron, aluminium, or some combination of these, leaving a concentration of sand and silt particles, and in which all or much of the original rock structure has been obliterated. An E horizon is usually, but not necessarily, lighter in colour than an underlying B horizon. In some soils, the colour is that of the sand and silt particles, but in many soils coatings of iron oxides or other compounds mask the colour of Chapter 5 – Genetic and systematic interpretation – soil classification 69

the primary particles. An E horizon is most commonly differentiated from an underlying B horizon in the same soil profile: by colour of higher value or lower chroma, or both; by coarser texture; or by a combination of these properties. An E horizon is commonly near the surface, below an O or A horizon and above a B horizon. However, the symbol E may be used without regard to position in the profile for any horizon that meets the requirements and that has resulted from soil genesis.

B horizons These are horizons that formed below an A, E, H or O horizon, and in which the dominant features are the obliteration of all or much of the original rock structure, together with one or a combination of the following: ¾ illuvial concentration, alone or in combination, of silicate clay, iron, aluminium, humus, carbonates, gypsum or silica; ¾ evidence of removal of carbonates; ¾ residual concentration of sesquioxides; ¾ coatings of sesquioxides that make the horizon conspicuously lower in value, higher in chroma, or redder in hue than overlying and underlying horizons without apparent illuviation of iron; ¾ alteration that forms silicate clay or liberates oxides or both and that forms a granular, blocky or prismatic structure if volume changes accompany changes in moisture content; ¾ brittleness. All kinds of B horizons are, or were originally, subsurface horizons. Included as B horizons are layers of illuvial concentration of carbonates, gypsum or silica that are the result of pedogenetic processes (these layers may or may not be cemented) and brittle layers that have other evidence of alteration, such as prismatic structure or illuvial accumulation of clay. Examples of layers that are not B horizons are: layers in which clay films either coat rock fragments or are on finely stratified unconsolidated sediments, whether the films were formed in place or by illuviation; layers into which carbonates have been illuviated but that are not contiguous to an overlying genetic horizon; and layers with gleying but no other pedogenetic changes.

C horizons or layers These are horizons or layers, excluding hard bedrock, that are little affected by pedogenetic processes and lack properties of H, O, A, E or B horizons. Most are mineral layers, but some siliceous and calcareous layers, such as shells, coral and diatomaceous earth, are included. The material of C layers may be either like or unlike that from which the solum presumably formed. A C horizon may have been modified even where there is no evidence of pedogenesis. Plant roots can penetrate C horizons, which provide an important growing medium. Included as C layers are sediments, saprolite, and unconsolidated bedrock and other geological materials that commonly slake within 24 hours when air dry or 70 Guidelines for soil description

drier chunks are placed in water and when moist can be dug with a spade. Some soils form in material that is already highly weathered, and such material that does not meet the requirements of A, E or B horizons is designated C. Changes not considered pedogenetic are those not related to overlying horizons. Layers having accumulations of silica, carbonates or gypsum, even if indurated, may be included in C horizons unless the layer is obviously affected by pedogenetic processes; then it is a B horizon.

R layers These consist of hard bedrock underlying the soil. , basalt, quartzite and indurated limestone or sandstone are examples of bedrock that are designated R. Air-dry or drier chunks of an R layer when placed in water will not slake within 24 hours. The R layer is sufficiently coherent when moist to make hand digging with a spade impractical, although it may be chipped or scraped. Some R layers can be ripped with heavy power equipment. The bedrock may contain cracks, but these are so few and so small that few roots can penetrate. The cracks may be coated or filled with clay or other material.

I layers These are ice lenses and wedges that contain at least 75 percent ice (by volume) and that distinctly separate organic or mineral layers in the soil. Ice comes and goes in soils in areas affected by . Ice bodies in soils can grow to such an extent that they form lenses of wedges that separate entire soil layers. In case such, where ice concentrations occur within the depth of soil description, they can be designated as an I layer. The I symbol is not used in transitional horizon designations.

L layers These are sediments deposited in a body of water (subaqueous) composed of both organic and inorganic materials, also known as limnic material. Limnic material is either: (i) deposited by precipitation or through action of aquatic organisms, such as algae or ; or (ii) derived from underwater and floating aquatic plants and subsequently modified by aquatic animals (USDA Soil Survey Staff, 2003). L layers include coprogenous earth or sedimentary peat (mostly organic), diatomaceous earth (mostly siliceous), and marl (mostly calcareous). The L symbol is not used in transitional horizon designations.

W layers These are water layers in soils or water submerging soils, either permanently or cyclic within the time frame of 24 hours. Some organic soils float on water. In such cases, the W symbol may be used at the end of the soil description to indicate the floating character. In other cases, shallow water (i.e. water not deeper than 1 m) may cover the soil permanently, as in the case of shallow lakes, or cyclic, as in tidal flats. The symbol W is then used Chapter 5 – Genetic and systematic interpretation – soil classification 71

to indicate the depth of submergence at the start of the horizon or layer sequence. The occurrence of tidal water can be indicated by (W).

Transitional horizons There are two kinds of transitional horizons: those with properties of two horizons superimposed; and those with the two properties separate. For horizons dominated by properties of one master horizon but having subordinate properties of another, two capital letter symbols are used, such as AB, EB, BE and BC. The master horizon symbol that is given first designates the kind of horizon whose properties dominate the transitional horizon. For example, an AB horizon has characteristics of both an overlying A horizon and an underlying B horizon, but it is more like the A than like the B. In some cases, a horizon can be designated as transitional even if one of the master horizons to which it is apparently transitional is not present. A BE horizon may be recognized in a truncated soil if its properties are similar to those of a BE horizon in a soil in which the overlying E horizon has not been removed by erosion. An AB or a BA horizon may be recognized where bedrock underlies the transitional horizon. A BC horizon may be recognized even if no underlying C horizon is present; it is transitional to assumed parent material. A CR horizon can be used for weathered bedrock that can be dug with a spade although roots cannot penetrate except along fracture planes. Horizons in which distinct parts have recognizable properties of two kinds of master horizons are indicated as above, but the two capital letters are separated by a virgule (/), such as E/B, B/E, B/C and C/R. Commonly, most of the individual parts of one of the components are surrounded by the other. The I, L and W symbols are not used in transitional horizon designations.

Subordinate characteristics within master horizons and layers Designations of subordinate distinctions and features within the master horizons and layers are based on profile characteristics observable in the field and are applied during the description of the soil at the site. Lower case letters are used as suffixes to designate specific kinds of master horizons and layers, and other features. The list of symbols and terms is shown in Table 85 and explanations of them are given below: a. Highly decomposed organic material: Used with H and O horizons only, to indicate the state of decomposition of the organic material. Highly decomposed organic material has less than one-sixth (by volume) visible plant remains. b. Buried genetic horizon: Used in mineral soils to indicate identifiable buried horizons with major genetic features that were formed before burial. Genetic horizons may or may not have formed in the overlying materials, which may be either like or unlike the assumed parent materials of the buried soil. The symbol is not used in organic soils or to separate an organic layer from a mineral layer, in cryoturbated soils, or with C horizons. 72 Guidelines for soil description

TABLE 85 Subordinate characteristics within master horizons Suffix Short description Used for a Highly decomposed organic material H and O horizons b Buried genetic horizon mineral horizons, not cryoturbated c Concretions or nodules mineral horizons c Coprogenous earth L horizon d Dense layer (physically root restrictive) mineral horizons, not with m d Diatomaceous earth L horizon e Moderately decomposed organic material H and O horizons f Frozen soil not in I and R horizons g Stagnic conditions no restriction h Accumulation of organic matter mineral horizons i Slickensides mineral horizons i Slightly decomposed organic material H and O horizons j Jarosite accumulation no restriction k Accumulation of pedogenetic carbonates no restriction l Capillary fringe mottling (gleying) no restriction m Strong cementation or induration (pedogenetic, massive) mineral horizons m Marl L horizon n Pedogenetic accumulation of exchangeable no restriction o Residual accumulation of sesquioxides (pedogenetic) no restriction p Ploughing or other human disturbance no restriction, E, B or C as Ap q Accumulation of pedogenetic silica no restriction r Strong reduction no restriction s Illuvial accumulation of sesquioxides B horizons t Illuvial accumulation of silicate clay B and C horizons u Urban and other human-made materials H, O, A, E, B and C horizons v Occurrence of no restriction w Development of colour or structure B horizons x characteristics no restriction y Pedogenetic accumulation of gypsum no restriction z Pedogenetic accumulation of salts more soluble than gypsum no restriction @ Evidence of cryoturbation no restriction

c. Concretions or nodules: In mineral soil, it indicates a significant accumulation of concretions or nodules. The nature and consistence of the nodules is specified by other suffixes and in the horizon description. Coprogenous earth: With limnic material L it denotes coprogenous earth, i.e. organic materials deposited under water and dominated by faecal material from aquatic animals. d. Dense layer: Used in mineral soils to indicate a layer of relatively unaltered, mostly earthy material that is non-cemented, but that has such bulk density or internal organization that roots cannot enter except in cracks; the symbol is not used in combination with the symbols m (cementation) and x (fragipan). Diatomaceous earth: In combination with limnic material L, it is used Chapter 5 – Genetic and systematic interpretation – soil classification 73

to indicate diatomaceous earth, i.e. materials deposited under water and dominated by the siliceous remains of diatoms. e. Moderately decomposed organic materials: Used with H and O horizons only, to indicate the state of decomposition of the organic material. Moderately decomposed organic material has between one-sixth and two- thirds (by volume) visible plant remains. f. Frozen soil: Designates horizons or layers that contain permanent ice or are perennially colder than 0 °C. It is not used for seasonally frozen layers or for bedrock layers (R). If needed, “dry frozen soil” layers may be labelled (f). g. Stagnic conditions: Designates horizons in which a distinct pattern of mottling occurs that reflects alternating conditions of oxidation and reduction of sesquioxides, caused by seasonal surface waterlogging. If aggregates are present, the interiors of the aggregates show oxidizing colours and the surface parts reducing colours. h. Accumulation of organic matter: Designates the accumulation of organic matter in mineral horizons. The accumulation may occur in surface horizons, or in subsurface horizons through illuviation. i. Slickensides: Denotes in mineral soils the occurrence of slickensides, i.e. oblique shear faces 20–60 º of horizontal owing to the shrink–swell action of clay; wedge-shaped peds and seasonal surface cracks are commonly present. Slightly decomposed organic material: In organic soils and used in combination with H or O horizons, it indicates the state of decomposition of the organic material; slightly decomposed organic material has in more than two-thirds (by volume) visible plant remains. j. Jarosite: Indicates the presence of jarosite mottles, coatings or hypodermic coatings. k. Accumulation of pedogenetic carbonates: Indicates an accumulation of alkaline earth carbonates, commonly calcium carbonate. l. Capillary fringe mottling: Indicates mottling caused by ascending groundwater. If aggregates are present, the interiors of the aggregates show reducing colours and the surface parts oxidizing colours. Strong cementation or induration: Indicates in mineral soils continuous or nearly continuous cementation, and is used only for horizons that are more than 90 percent cemented, although they may be fractured. The layer is root restrictive and roots do not enter except along fracture planes. The single predominant or codominant cementing agent may be indicated using defined letter suffices single or in pairs. If the horizon is cemented by carbonates km is used; by silica, qm; by iron, sm; by gypsum, ym; by both lime and silica, kqm; and by salts more soluble than gypsum, zm. Marl: In combination with limnic material it is used to indicate marl, i.e. materials deposited under water and dominated by a mixture of clay and calcium carbonate; typically grey in colour. n. Pedogenetic accumulation of exchangeable sodium: Indicates an accumulation of exchangeable sodium. 74 Guidelines for soil description

o. Residual accumulation of sesquioxides: Indicates residual accumulation of sesquioxides. It differs from the use of symbol s, which indicates illuvial accumulation of organic matter and sesquioxide complexes. p. Ploughing or other human disturbance: Indicates disturbance of the surface layer by ploughing or other tillage practices. A disturbed organic horizon is designated Op or Hp. A disturbed mineral horizon, even though clearly once an E, B or C, is designated Ap. q. Accumulation of pedogenetic silica: Indicates an accumulation of secondary silica. If silica cements the layer and cementation is continuous or nearly continuous, qm is used. r. Strong reduction: Indicates presence of iron in reduced state. If r is used with B, pedogenetic change in addition to reduction is implied; if no other change has taken place, the horizon is designated Cr. s. Illuvial accumulation of sesquioxides: Used with B to indicate the accumulation of illuvial, amorphous, dispersible organic matter–sesquioxide complexes if the value and chroma of the horizon are more than 3. The symbol is also used in combination with h as Bhs if both the organic matter and sesquioxides components are significant and both value and chroma are about 3 or less. t. Accumulation of silicate clay: Used with B or C to indicate an accumulation of silicate clay that either has formed in the horizon or has been moved into it by illuviation, or both. At least some part should show evidence of clay accumulation in the form of coatings on ped surfaces or in pores, as lamellae, or as bridges between mineral grains. u. Urban and other human-made materials: Used to indicate the dominant presence of human-made materials, including technogenic ones. The symbol can be used in combination with H, O, A, E, B and C. v. Occurrence of plinthite: Indicates the presence of iron-rich, humus-poor material that is firm or very firm when moist and that hardens irreversibly when exposed to the atmosphere. When hardened, it is no longer called plinthite but a , ironstone, a petroferric or a skeletic phase. In that case, v is used in combination with m. w.Development of colour or structure in B: Used with B only to indicate development of colour or structure, or both. It is not used to indicate a transitional horizon. x. Fragipan characteristics: Used to indicate genetically developed firmness, brittleness or high bulk density. These features are characteristic of , but some horizons designated x do not have all the properties of a fragipan. y. Pedogenetic accumulation of gypsum: Indicates an accumulation of gypsum. z. Pedogenetic accumulation of salts more soluble than gypsum: Indicates an accumulation of salts more soluble than gypsum. @ Evidence of cryoturbation: irregular or broken boundaries, sorted rock fragments (patterned ground), or organic matter in the lower boundary Chapter 5 – Genetic and systematic interpretation – soil classification 75

between the active layer and permafrost layer. The suffix is always used last, e.g. Hi@.

Conventions for using letter suffixes Many master horizons and layers that are symbolized by a single capital letter will have one or more lowercase letter suffixes. More than three suffixes are rarely used. The following rules apply: ¾ Letter suffixes should follow the capital letter immediately. ¾ When more than one suffix is needed, the following letters, if used, are written first: r, s, t, u and w. The symbol t has precedence over all other symbols, e.g. Btr, Btu. In all other combinations, the symbols are listed alphabetically, e.g. Cru. ¾ If more than one suffix is needed and the horizon is not buried, these symbols, if used, are written last: c, f, g, m, v and x. Some examples: Btc, Bkm, and Bsv. ¾ If a horizon is buried, the suffix b is written last. ¾ A B horizon that has significant accumulation of clay and also shows evidence of development of colour or structure, or both, is designated Bt (t has precedence over w, s and h). A B horizon that is gleyed or that has accumulations of carbonates, sodium, silica, gypsum, salts more soluble than gypsum, or residual accumulation or sesquioxides carries the appropriate symbol g, k, n, q, y, z or o. If illuvial clay is also present, t precedes the other symbols, e.g. Bto. ¾ Suffixes h, s and w are normally not used with g, k, n, q, y, z or o unless needed for explanatory purposes. ¾ Suffixes a and e are used only in combination with H or O. ¾ Suffixes c, d, i and m have two different meanings, depending on the master horizon designation they are coupled to. The different combinations are mutually exclusive, e.g. Bi indicates presence of slickensides in the B horizon, whereas Hi indicates a slightly decomposed H horizon. Similarly, Bd indicates a dense B horizon, and Ld diatomaceous earth in a limnic layer. ¾ Suffix @ is always used last, and cannot be combined with b. ¾ Unless otherwise indicated, suffixes are listed alphabetically.

Vertical subdivisions Horizons or layer designated by a single combination of letter symbols can be subdivided using Arabic numerals, which follow all the letters. For example, within a C, successive layers could be C1, C2, C3, etc.; or if the lower part is gleyed and the upper part is not, the designations could be C1-C2-Cg1-Cg2 or C-Cg1-Cg2-R. These conventions apply whatever the purpose of subdivision. A horizon identified by a single set of letter symbol may be subdivided on the basis of evident morphological features, such as structure, colour or texture. These subdivisions are numbered consecutively. The numbering starts with 1 at whatever level in the 76 Guidelines for soil description

profile. Thus, Bt1-Bt2-Btk1-Btk2 is used, not Bt1-Bt2-Btk3-Btk4. The numbering of vertical subdivisions within a horizon is not interrupted at a discontinuity (indicated by a numerical prefix) if the same letter combination is used in both materials: Bs1-Bs2-2Bs3-2Bs4 is used, not Bs1-Bs2-2Bs1-2Bs2. A and E horizons can be subdivided similarly, e.g. Ap, A1, A2; Ap1, Ap2; A1, A2, A3; and E1, E2, Eg1, Eg2.

Discontinuities In mineral soils, Arabic numerals are used as prefixes to indicate discontinuities. Wherever needed, they are used preceding A, E, B, C and R. They are not used with I and W, although these symbols clearly indicate a discontinuity. These prefixes are distinct from Arabic numerals used as suffixes to denote vertical subdivisions. A discontinuity is a significant change in particle-size distribution or mineralogy that indicates a difference in the material from which the horizons formed or a significant difference in age or both, unless that difference in age is indicated by the suffix b. Symbols to identify discontinuities are used only when they will contribute substantially to the reader’s understanding of relationships among horizons. The stratification common in soils formed in alluvium is not designated as discontinuities unless particle-size distribution differs markedly from layer to layer even though genetic horizons have formed in the contrasting layers. Where a soil has formed entirely in one kind of material, a prefix is omitted from the symbol; the whole profile is material 1. Similarly, the uppermost material in a profile having two or more contrasting materials is understood to be material 1, but the number is omitted. Numbering starts with the second layer of contrasting material, which is designated 2. Underlying contrasting layers are numbered consecutively. Even where a layer below material 2 is similar to material 1, it is designated 3 in the sequence. The numbers indicate a change in the material, not the type of material. Where two or more consecutive horizons formed in one kind of material, the same prefix number applies to all of the horizon designations in that material: Ap-E-Bt1-2Bt2-2Bt3-2BC. The number suffixes designating subdivisions of the Bt horizon continue in consecutive order across the discontinuity. If an R layer is below a soil that formed in residuum and the material of the R layer is judged to be like that from which the material of the soil weathered, the Arabic number prefix is not used. If the R layer would not produce material like that in the solum, the number prefix is used, as in A-Bt-C-2R or A-Bt-2R. If part of the solum formed in residuum, R is given the appropriate prefix: Ap-Bt1-2Bt2- 2Bt3-2C1-2C2-2R. Buried horizons (designated b) are special problems. A buried horizon is not the same deposit as horizons in the overlying deposit. However, some buried horizons formed in material lithological like that of the overlying deposit. A prefix is not used to distinguish material of such buried horizons. If the material in which a horizon of a buried soil formed is lithological unlike that of the Chapter 5 – Genetic and systematic interpretation – soil classification 77

overlying material, the discontinuity is designated by number prefixes and the symbol for a buried horizon is used as well: Ap-Bt1-Bt2-BC-C-2ABb-2Btb1- 2Btb2-2C. In organic soils, discontinuities between different kinds of layers are not identified. In most cases, the differences are shown by the letter suffix designations, if the different layers are organic, or by the master symbol if the different layers are mineral.

Use of the prime Identical designations may be appropriate for two or more horizons or layers separated by at least one horizon or layer of a more different kind in the same pedon. The sequence A-E-Bt-E-Btx-C is an example – the soil has two E horizons. To make communication easier, a prime is used with the master horizon symbol of the lower of two horizons having identical letter designations: A-E-Bt-E’-Btx-C. The prime is applied to the capital letter designation, and any lower case symbol follows it: B’t. The prime is not used unless all letters of the designations of two different layers are identical. Rarely, three layers have identical letter symbols; a double prime can be used: E’’. The same principle applies in designating layers of organic soils. The prime is used only to distinguish two or more horizons that have identical symbols: O-C-C’-C’’. The prime is added to the lower C layer to differentiate it from the upper.

PRINCIPLES OF CLASSIFICATION ACCORDING TO THE WRB The surveyor should attempt to classify the soil in the field as precisely as possible on the basis of the soil morphological features that have been observed and described. The final classification is made after the analytical data have become available. It is recommended that the occurrence and depth of diagnostic horizons, properties and materials identified be listed (below). The general principles on which the classification according to the WRB is based (IUSS Working Group WRB 2006)can be summarized as follows: ¾ The classification of soils is based on soil properties defined in terms of diagnostic horizons, properties and materials, which to the greatest extent possible should be measurable and observable in the field. ¾ The selection of diagnostic characteristics takes into account their relationship with soil forming processes. It is recognized that an understanding of soil- forming processes contributes to a better characterization of soils but that they should not, as such, be used as differentiating criteria. ¾ To the extent possible at a high level of generalization, diagnostic features are selected that are of significance for soil management. ¾ Climate parameters are not applied in the classification of soils. It is fully realized that they should be used for interpretation purposes, in dynamic combination with soil properties, but they should not form part of soil definitions. 78 Guidelines for soil description

¾ The WRB is a comprehensive classification system that enables people to accommodate their national classification system. It comprises two tiers of categorical detail: • the Reference Base, limited to the first level only and having 32 RSGs; • the WRB Classification System, consisting of combinations of a set of prefix and suffix qualifiers that are uniquely defined and added to the name of the RSG, allowing very precise characterization and classification of individual soil profiles. ¾ Many RSGs in the WRB are representative of major soil regions so as to provide a comprehensive overview of the world’s soil cover. ¾ The Reference Base is not meant to substitute for national soil classification systems but rather to serve as a common denominator for communication at an international level. This implies that lower-level categories, possibly a third category of the WRB, could accommodate local diversity at country level. Concurrently, the lower levels emphasize soil features that are important for land use and management. ¾ The Revised Legend of the FAO/UNESCO Soil Map of the World (FAO, 1988) has been used as a basis for the development of the WRB in order to take advantage of international soil correlation that has already been conducted through this project and elsewhere. ¾ The first edition of the WRB, published in 1998, comprised 30 RSGs; the second edition, published in 2006, has 32 RSGs. ¾ Definitions and descriptions of soil units reflect variations in soil characteristics both vertically and laterally in order to account for spatial linkages within the landscape. ¾ The term Reference Base is connotative of the common denominator function that the WRB assumes. Its units have sufficient width to stimulate harmonization and correlation of existing national systems. ¾ In addition to serving as a link between existing classification systems, the WRB also serves as a consistent communication tool for compiling global soil databases and for the inventory and monitoring of the world’s soil resources. ¾ The nomenclature used to distinguish soil groups retains terms that have been used traditionally or that can be introduced easily in current language. They are defined precisely in order to avoid the confusion that occurs where names are used with different connotations. Although the basic framework of the FAO Legend (with its two categorical levels and guidelines for developing classes at a third level) was adopted, it has been decided to merge the lower levels. Each RSG of the WRB is provided with a listing of possible prefix and suffix qualifiers in a priority sequence, from which the user can construct the second-level units. The broad principles that govern the WRB class differentiation are: ¾ At the higher categorical level, classes are differentiated mainly according to the primary pedogenetic process that has produced the characteristic soil features, except where special soil parent materials are of overriding importance. Chapter 5 – Genetic and systematic interpretation – soil classification 79

¾ At the second level, soil units are differentiated according to any secondary soil-forming process that has affected the primary soil features significantly. In certain cases, soil characteristics that have a significant effect on use may be taken into account. It is recognized that a number of RSGs may occur under different climate conditions. However, it was decided not to introduce separations on account of climate characteristics so that the classification of soils is not subordinated to the availability of climate data. Including the soil description, classification is done in four steps.

Step 1 The profile description is checked to find references to soil-forming processes (qualitatively) and express them in the horizon designation. Examples may be: ¾ Darkening of topsoil in comparison to subsoil A enrichment with organic material A Ah-horizon. ¾ Browning and finer texture in the middle part of a soil profile in comparison to the parent material A enrichment of Fe-oxides and clay A weathering A Bw-horizon.

Step 2 The profile description and the horizon designation are to be checked whether the expression, thickness and depth of certain soil characteristics correspond with the requirements of WRB diagnostic horizons, properties and materials. These are defined in terms of morphological characteristics and/or analytical criteria (IUSS Working Group WRB, 2006). In line with the WRB objectives, attributes are described as much as possible to support field identification.

Step 3 The described combination of diagnostic horizons, properties and materials is compared with the WRB Key (IUSS Working Group WRB, 2006) in order to find the RSG, which is the first level of WRB classification. The user should go through the Key systematically, starting at the beginning and excluding one by one all RSGs for which the specified requirements are not met. The soil belongs to the first RSG for which it meets all specified requirements.

Step 4 For the second level of WRB classification, qualifiers are used. The qualifiers are listed in the Key with each RSG as prefix and suffix qualifiers. Prefix qualifiers comprise those that are typically associated to the RSG and the intergrades to other RSGs. All other qualifiers are listed as suffix qualifiers. For classification at the second level, all applying qualifiers have to be added to the name of the RSG. Redundant qualifiers (the characteristics of which are included in a previously set qualifier) are not added. 80 Guidelines for soil description

Specifiers can be used to indicate the degree of expression of qualifiers. Buried layers can be indicated by the Thapto- specifier, which can be used with any qualifier, listed in IUSS Working Group WRB (2006). Where a soil is buried under new material, the following rules apply: 1. The overlying new material and the buried soil are classified as one soil if both together qualify as , , Cryosol, , Vertisol, , , , , or Arenosol. 2. Otherwise, the new material is classified at the first level if the new material is 50 cm or more thick or if the new material, if it stood alone, fits the requirements of a RSG other than a . 3. In all other cases, the buried soil is classified at the first level. 4. If the overlying soil is classified at the first level, the buried soil is recognized with the Thapto- specifier and -ic added to the RSG name of the buried soil. The whole is placed in brackets after the name of the overlying soil, e.g. Technic (Greyic) (Thapto-Podzolic). If the buried soil is classified at the first level, the overlying material is indicated with the Novic qualifier.

Principles and use of the qualifiers in the WRB A two-tier system is used for the qualifier level, comprising: ¾ Prefix qualifiers: typically associated qualifiers and intergrade qualifiers; the sequence of the intergrade qualifiers follows that of the RSGs in the WRB Key, with the exception of Arenosols; this intergrade is ranked with the textural suffix qualifiers (see below). Haplic closes the prefix qualifier list indicating that neither typically associated nor intergrade qualifiers apply. ¾ Suffix qualifiers: other qualifiers, sequenced as follows: (1) qualifiers related to diagnostic horizons, properties or materials; (2) qualifiers related to chemical characteristics; (3) qualifiers related to physical characteristics; (4) qualifiers related to mineralogical characteristics; (5) qualifiers related to surface characteristics; (6) qualifiers related to textural characteristics, including coarse fragments; (7) qualifiers related to colour; and (8) remaining qualifiers. Prefix qualifier names are always put before the RSG; suffix qualifier names are always placed between brackets following the RSG name. Combinations of qualifiers that indicate a similar status or duplicate each other are not permitted, such as combinations of Thionic and Dystric, Calcaric and Eutric, or Rhodic and Chromic. Specifiers such as Epi-, Endo-, Hyper-, Hypo-, Thapto-, Bathy-, Para-, Proto-, Cumuli- and Ortho- are used to indicate a certain expression of the qualifier. When classifying a soil profile, all applying qualifiers of the listing must be recorded. For mapping purposes, the scale will determine the number of qualifiers used. In that case, prefix qualifiers have priority over the suffix qualifiers. The qualifier listing for each RSG accommodates most cases. Where not listed qualifiers are needed, the cases should be documented and reported to the WRB Working Group. Chapter 5 – Genetic and systematic interpretation – soil classification 81

The field classification provides a preliminary assessment using all observable or easily measurable properties and features of the soil and associated terrain. The final classification is made when analytical data are available. It is recommended that Procedures for Soil Analysis (Van Reeuwijk, 2006) be followed in determining chemical and physical characteristics.

Example of WRB soil classification A soil has a ferralic horizon; texture in the upper part of the ferralic horizon changes from sandy loam to sandy clay within 15 cm. The pH is between 5.5 and 6, indicating moderate to high base saturation. The B horizon is dark red; below 50 cm, mottling occurs. The field classification of this soil is: Lixic Ferralsol (Ferric, Rhodic). If subsequent laboratory analysis reveals that the cation exchange -1 capacity of the ferralic horizon is less than 4 cmolc kg clay, the soil finally classifies as: Lixic Vetic Ferralsol (Ferric, Rhodic).

CHECKLIST OF WRB DIAGNOSTIC HORIZONS, PROPERTIES AND MATERIALS While still in the field, it is advisable to determine or estimate, for each horizon, the diagnostic characteristics that apply to the classification system used. Table 86 provides a checklist of diagnostic horizons, properties and materials in the order as they appear in the WRB (IUSS Working Group WRB, 2006).

TABLE 86 Checklist of WRB diagnostic horizons, properties and materials Diagnostic horizons Diagnostic properties Diagnostic materials Albic horizon Natric horizon Abrupt textural change Artefacts Anthraquic horizon Nitic horizon Albeluvic tonguing Calcaric material Anthric horizon Petrocalcic horizon Andic properties Colluvic material Argic horizon Petroduric horizon Aridic properties Fluvic material Calcic horizon Petrogypsic horizon Continuous rock Gypsiric material Cambic horizon Petroplinthic horizon Ferralic properties Limnic material Cryic horizon Pisoplinthic horizon Geric properties Mineral material Duric horizon Plaggic horizon Gleyic colour pattern Organic material Ferralic horizon Plinthic horizon Lithological discontinuity Ornithogenic material Ferric horizon Salic horizon. Reducing conditions Sulphidic material Folic horizon Sombric horizon Secondary carbonates Technic hard rock Fragic horizon Spodic horizon Stagnic colour pattern Tephric material Fulvic horizon Takyric horizon Vertic properties Gypsic horizon Terric horizon Vitric properties Histic horizon Thionic horizon Hortic horizon Umbric horizon Hydragric horizon Vertic horizon Irragric horizon Voronic horizon Melanic horizon Yermic horizon Mollic horizon 82 Guidelines for soil description

APPENDING TEXTURE AND PARENT MATERIAL INFORMATION TO THE REFERENCE SOIL GROUP In its present state, the WRB (IUSS Working Group WRB, 2006) is, by history and practical purposes, mixing information about soil genesis (e.g. podzolization – , gleysation – Gleysol), texture (e.g. Arenosol, skeletic, arenic, siltic, and clayic subunits), parent materials (e.g. , Fluvisols, calcaric, and gypsiric subunits) and others. The system distinguishes second-level units only in a generalized manner by texture and only in the parent material of some RSGs. In order to overcome this problem and to provide users with more systematic and precise information about texture, parent material and layering, the following framework for a reference is recommended (Jahn, 2004).

Example A to which only the Dystric qualifier applies and which has variations in texture and in which the upper part has developed from loess with some fluvial gravelly sand and the lower part has developed from glacio-fluvial gravelly sand. The full description is: Haplic Cambisol (Dystric); silt loam from loess with glacio-fluvial gravelly sand over sandy skeleton from glacio-fluvial gravel It is coded: CMdy; SiL(UE2, UG2)/SSK(UG3) 1 2 3 4 5 1 = Coding of soil unit according to the WRB (IUSS Working Group WRB, 2006). 2 = Coding of texture class for the upper part of the soil body. The texture class for the fine earth is used according to Chapter 4 and combined with four classes of coarse fragments, e.g.: SiL = silt loam with coarse fragments < 10 percent (by volume); skSiL = skeletal silt loam with coarse fragments of from 10 to < 40 percent (by volume); silSK = silt loamy skeleton with coarse fragments of from 40 to <80 percent (by volume); SK = skeleton with coarse fragments of 80 percent or more (by volume). 3 = The parent material is given in descending order of importance from left to right within brackets. For coding, an extended hierarchical lithology list, based on updated SOTER (ISRIC, 2005) is used. 4 = A change of material with depth (either by texture or by parent material or by both is coded with: ...\... as “shallow … over …” where occurring at a depth of 0–3 dm; .../... as “… over …” where occurring at a depth of 3–7 dm; (the intermediate of 5 dm is corresponding with the WRB-epi and -endo); ...//... as “… over deep …” where occurring at a depth of 7–12 dm. 5 = The lower part of the soil body is described according to 2 and 3. Chapter 5 – Genetic and systematic interpretation – soil classification 83

Further rules in describing texture and parent material are: ¾ e.g.: skSiL(UE2, UG2/UG3) where no change in texture but in parent material; ¾ e.g.: SiL/skSiL(UE2, UG3) where no change in parent material but in texture; ¾ e.g.: .../R(and lithology) means: over massive rock; ¾ Horizons are combined to one complex and described with the average where not more than one of the three parameters: (1) texture (fine earth); (2) coarse fragments; and (3) lithology differs for one class. Thin (extension < 2 cm ) horizons are neglected.

85

References

Ad-hoc-AG-Boden. 2005. Bodenkundliche Kartieranleitung – 5. Auflage. Hannover, Germany. 438 pp. Bullock, P., Federoff, N., Jongerius, A., Stoops, G., Tursina, T. & Babel, U. 1985. Handbook for soil thin section description. Waine publications. 152 pp. DVWK. 1995. Bodenkundliche Untersuchungen im Felde zur Ermittlung von Kennwerten zur Standortscharakterisierung. Teil I: Ansprache von Böden. DVWK Regeln 129. Bonn, Germany, Wirtschafts- und Verlagsges. Gas und Wasser. ECSC–EEC–EAEC. 1985. Soil map of the European Communities 1:1 000 000. Luxembourg. 124 pp. and paper maps. European Commission Joint Research Centre. 2005. Soil atlas of Europe. FAO. 1978. Report on the agro-ecological zones project. Vol. 1. Methodology and results for . World Soil Resources Report No. 48/1. Rome. FAO. 2002. FAO/UNESCO Digital Soil Map of the World and derived soil properties. Land and Water Digital Media Series #1 rev 1. FAO, Rome. FAO–ISRIC. 1990. Guidelines for profile description. 3rd Edition. Rome. FAO–UNESCO. 1970–1981. Soil map of the world 1:5 000 000. Vol. 2–9. Paris. FAO–UNESCO. 1974. Soil map of the world. Vol. I – legend. Paris. 59 pp. FAO–UNESCO. 1988. Soil map of the world. Revised legend. World Soil Resources Report No. 60. Rome. Fieldes, M. & Perrott, K.W. 1966. The nature of allophane soils: 3. Rapid field and laboratory test for allophane. N. Z. J. Sci., 9: 623–629. International Soil Reference and Information Centre (ISRIC). 2005. Updated global and national soils and terrain digital databases (SOTER). IUSS Working Group WRB. 2006. World reference base for soil resources 2006 – A framework for international classification, correlation and communication. World Soil Resources Reports No. 103. Rome, FAO. Jahn, R. 2004. Research needs and new developments in soil classification and mapping: meeting the changing demands for soil information. Proceedings of International Conference on Innovative Techniques in Soil Survey, pp. 207–222. Cha-Am, . Meuser, H. 1996. Ein Bestimmungsschlüssel für natürliche und technogene Substrate in Böden städtisch-industrieller Verdichtungsräume. Z. Pflanzenernähr. Bodenk., 159: 305–312. Munsell. 1975. Standard soil color charts. Richards, L.A. 1954. Diagnosis and improvement of saline and alkali soils. Agriculture Handbook No. 60. USDA Rossiter, D.G. 2004. Proposal: classification of urban and industrial soils in the World Reference Base for Soil Resources (WRB). In: Abstracts Eurosoil 2004. Freiburg im Breisgau, Germany. 86 Guidelines for soil description

Schlichting, E., Blume, H.-P. & Stahr, K. 1995. Bodenkundliches Praktikum. 2nd edition. Berlin, Vienna, Blackwell. 295 pp. Schoeneberger, P.J. Wysocki, D.A, Benham, E.C. & Broderson, W.D. 2002. Field book for describing and sampling soils. Version 2.0. Lincoln, USA, National Soil Survey Center, Natural Resources Conservation Service, USDA. UNEP–ISSS–ISRIC–FAO. 1995. Global and national soils and terrain digital database (SOTER). World Soil Resources Report No. 74 Rev. 1. Rome. United States Department of Agriculture (USDA) Soil Survey Staff. 1975. Soil taxonomy. Agricultural Handbook No. 436. Washington, DC. 754 pp. United States Department of Agriculture (USDA) Soil Survey Staff. 1999. Soil taxonomy, a basic system of soil classification for making and interpreting soil surveys. 2nd edition. Agricultural Handbook No. 436. Washington, DC. 869 pp. United States Department of Agriculture (USDA) Soil Survey Staff. 2003. Keys to soil taxonomy. 9th edition. Washington, DC, Natural Resources Conservation Service, USDA. 332 pp. Van Reeuwijk, L.P. 2006. Procedures for soil analysis. 7th edition. Technical Report 9. Wageningen, Netherlands, ISRIC – World Soil Information. 87

Annex 1 Explanation of soil temperature regimes1

The temperature of a soil is one of its important properties. Within limits, temperature controls the possibilities for plant growth and for soil formation. Below freezing point, there is no biotic activity, water no longer moves as a liquid, and, unless there is frost heaving, time stands still for the soil. Between 0 and 5 °C, root growth of most plant species and germination of most seeds are impossible. A horizon as cold as 5 °C is a thermal pan to the roots of most plants. Each pedon has a characteristic temperature regime that can be measured and described. For most practical purposes, the temperature regime can be described by: the mean annual soil temperature; the average seasonal fluctuations from that mean; and the mean warm or cold seasonal temperature gradient within the main rootzone, which is the zone from a depth of 5 to 100 cm.

MEAN ANNUAL SOIL TEMPERATURE Each pedon has a mean annual temperature that is essentially the same in all horizons at all depths in the soil and at depths considerably below the soil. The measured mean annual soil temperature is seldom the same in successive depths at a given location. However, the differences are so small that it seems valid and useful to take a single value as the mean annual temperature of a soil. The mean annual soil temperature is related most closely to the mean annual air temperature, but this relationship is affected to some extent by: the amount and distribution of rain; the amount of snow; the protection provided by shade and by O horizons in ; the slope aspect and gradient; and irrigation. Other factors, such as soil colour, texture, and content of organic matter, have negligible effects.

FLUCTUATIONS IN SOIL TEMPERATURE The mean annual temperature of a soil is not a single reading but the average of a series of readings. Near the surface, the readings may fluctuate from the mean fully as much as those of the air temperature, especially where there is no insulating cover. The fluctuations occur as daily and annual cycles, which weather events make somewhat irregular in most places. The fluctuations decrease with increasing depth and are ultimately damped out in the substrata

1 Adapted from USDA, 1999. 88 Guidelines for soil description

in a zone where the temperature is constant and is the same as the mean annual soil temperature.

ESTIMATION OF SOIL TEMPERATURE Soil temperature can often be estimated from climatological data with a precision that is adequate for the present needs of soil surveys. Where it is not possible to make reasonably precise estimates, the measurement of soil temperature need not be a difficult or a time-consuming task. Frequently, the mean annual soil temperature for much of the United States of America is estimated by adding 1 °C to the mean annual air temperature. The mean summer soil temperature at a specific depth can also be estimated. To make this estimate, it is possible to take the average summer temperatures of the upper 100 cm and correct for the temperature-depth gradient by adding or subtracting 0.6 °C for each 10 cm above or below a depth of 50 cm. The mean winter temperature of many mid-latitude soils can be estimated from the difference between the mean annual temperatures and the mean summer temperatures because the differences are of the same magnitude but have opposite signs.

CLASSES OF SOIL TEMPERATURE REGIMES The following is a description of the soil temperature regimes used in defining classes at various categoric levels in the soil taxonomy of the United States of America. PG – Pergelic (Latin per, throughout in time and space, and gelare, to freeze; meaning permanent frost). Soils with a pergelic temperature regime have a mean annual temperature lower than 0 °C. These are soils that have permafrost if they are moist, or dry frost if there is no excess water. CR – Cryic (Greek kryos, coldness; meaning very cold soils). Soils in this temperature regime have a mean annual temperature lower than 8 °C but do not have permafrost. 1. In mineral soils, the mean summer soil temperature (June, July and August in the Northern Hemisphere, and December, January, and February in the Southern Hemisphere) either at a depth of 50 cm from the soil surface or at a densic, lithic or paralithic contact, whichever is shallower, is as follows: a. if the soil is not saturated with water during some part of the summer and: (1) If there is no O horizon: lower than 15 °C; or (2) If there is an O horizon: lower than 8 °C; or b. if the soil is saturated with water during some part of the summer and: (1) If there is no O horizon: lower than 13 °C; or (2) If there is an O horizon or a histic epipedon: lower than 6 °C. 2. In organic soils, the mean annual soil temperature is lower than 6 °C. Cryic soils that have an aquic moisture regime are commonly churned by frost. Annex 1 – Explanation of soil temperature regimes 89

Isofrigid soils could also have a cryic temperature regime. A few with organic materials in the upper part are exceptions. The concepts of the soil temperature regimes described below are used in defining classes of soils in the low categories.

FR – Frigid A soil with a frigid temperature regime is warmer in summer than a soil with a cryic regime. However, its mean annual temperature is lower than 8 °C and the difference between mean summer (June, July and August) and mean winter (December, January and February) soil temperatures is more than 6 °C either at a depth of 50 cm from the soil surface or at a densic, lithic or paralithic contact, whichever is shallower.

IF – Isofrigid The mean annual soil temperature is lower than 8 °C and the mean summer and mean winter soil temperatures differ by less than 6 °C at a depth of 50 cm or at a densic, lithic or paralithic contact, whichever is shallower.

ME – Mesic The mean annual soil temperature is 8 °C or higher but lower than 15 °C, and the difference between mean summer and mean winter soil temperatures is more than 6 °C either at a depth of 50 cm from the soil surface or at a densic, lithic or paralithic contact, whichever is shallower.

IM – Isomesic The mean annual soil temperature is 8 °C or higher but lower than 15 °C and the mean summer and mean winter soil temperatures differ by less than 6 °C at a depth of 50 cm or at a densic, lithic or paralithic contact, whichever is shallower.

TH – Thermic The mean annual soil temperature is 15 °C or higher but lower than 22 °C, and the difference between mean summer and mean winter soil temperatures is more than 6 °C either at a depth of 50 cm from the soil surface or at a densic, lithic or paralithic contact, whichever is shallower.

IT – Isothermic The mean annual soil temperature is 15 °C or higher but lower than 22 °C and the mean summer and mean winter soil temperatures differ by less than 6 °C at a depth of 50 cm or at a densic, lithic or paralithic contact, whichever is shallower.

HT – Hyperthermic The mean annual soil temperature is 22 °C or higher, and the difference between mean summer and mean winter soil temperatures is more than 6 °C at a depth of 90 Guidelines for soil description

50 cm from the soil surface or at a densic, lithic or paralithic contact, whichever is shallower.

IH – Isohyperthermic The mean annual soil temperature is 22 °C or higher and the mean summer and mean winter soil temperatures differ by less than 6 °C at a depth of 50 cm or at a densic, lithic or paralithic contact, whichever is shallower. 91

Annex 2 Explanation of soil moisture regimes1

The term “soil moisture regime” refers to the presence or absence either of groundwater or of water held at a tension of less than 1 500 kPa (pF 4.2) in the soil or in specific horizons during periods of the year. Water held at a tension of 1 500 kPa or more is not available to keep most mesophytic plants alive. The availability of water is also affected by dissolved salts. Where a soil is saturated with water that is too salty to be available to most plants, it is considered salty rather than dry. Consequently, a horizon is considered dry when the moisture tension is 1 500 kPa or more, and it is considered moist when water is held at a tension of less than 1 500 kPa but more than zero. A soil may be continuously moist in some or all horizons either throughout the year or for some part of the year. It may be either moist in winter and dry in summer or the reverse. In the Northern Hemisphere, summer refers to June, July and August, and winter refers to December, January and February.

SIGNIFICANCE TO SOIL CLASSIFICATION The moisture regime of a soil is an important property of the soil as well as a determinant of processes that can occur in the soil. During geological time, there have been significant changes in climate. Soils that could have formed only in a humid climate are now preserved in an arid climate in some areas. Such soils have relict features that reflect the former moisture regime and other features that reflect the present moisture regime. Each of the moisture regimes in the history of a soil is a factor in the genesis of that soil and is the cause of many accessory characteristics. However, most of the accessory characteristics and those most important for interpretations are associated with the present moisture regime, even if the present regime differs widely from some of the earlier. More importantly, the present climate determines use and management of the soil. It is a property of the soil. Furthermore, the moisture regimes of most soils are inferred from the present climate, and small-scale maps can be interpreted in terms of the many accessory characteristics that are common to most of the soils that have a common climate. These characteristics include: the amount, nature and distribution of organic matter; the base status of the soil; and the presence or absence of salts.

1 Adapted from USDA, 1999. 92 Guidelines for soil description

NORMAL YEARS In the discussions that follow and throughout the keys, the term “normal years” is used. A normal year is defined as a year that has plus or minus one standard deviation of the long-term mean annual precipitation (long term refers to 30 years or more). In addition, the mean monthly precipitation in a normal year must be plus or minus one standard deviation of the long-term monthly precipitation for 8 of the 12 months. Normal years can usually be calculated from the mean annual precipitation. However, when catastrophic events occur during a year, the standard deviations of the monthly means should also be calculated.

ESTIMATION The landscape position of every soil is subject to extremes in climate. While no two years have exactly the same weather conditions, the moisture status of the soil must be characterized by probability. Weather probabilities can be determined from long-term weather records and observations of how each soil responds to weather conditions as modified by its landscape position. A number of methods have been devised to relate soil moisture to meteorological records. To date, all these methods have some shortcomings, even for gently sloping soils that depend primarily on precipitation for their moisture. Dew and fog can add appreciable amounts of moisture to some soils, but quantitative data are rare.

SOIL MOISTURE CONTROL SECTION The intention in defining the soil moisture control section is to facilitate estimation of soil moisture regimes from climate data. The upper boundary of this control section is the depth to which a dry (tension of more than 1 500 kPa, but not air-dry) soil will be moistened by 2.5 cm of water within 24 hours. The lower boundary is the depth to which a dry soil will be moistened by 7.5 cm of water within 48 hours. These depths do not include the depth of moistening along any cracks or animal burrows that are open to the surface. The boundaries for the soil moisture control section correspond to the rooting depths for many crops. However, there are natural plant communities that have their roots either above or below the control section. Attempts are currently being made to improve the parameters of the soil moisture control section. If 7.5 cm of water moistens the soil to a densic, lithic, paralithic or petroferric contact or to a petrocalcic or petrogypsic horizon or a duripan, the contact or the upper boundary of the cemented horizon constitutes the lower boundary of the soil moisture control section. The concept of the soil moisture control section does not apply well to the cracking clays, because these clays remoisten from both the surface and the bases of the cracks. The soil moisture patterns of these soils are defined in terms of the pattern of cracking over time. If moistening occurs unevenly, the weighted average depth of moistening in a pedon is used for the limits of the moisture control section. Annex 2 – Explanation of soil moisture regimes 93

The moisture control section of a soil extends approximately: ¾from 10 to 30 cm below the soil surface if the particle-size class of the soil is fine-loamy (> 15 percent particles 0.1–75 mm and 18–35 percent clay), coarse-silty (< 15 percent particles 0.1–75 mm and < 18 percent clay in fine earth), fine-silty (< 15 percent particles 0.1–75 mm and 18–35 percent clay in fine earth), or clayey (> 35 percent clay); ¾from 20 to 60 cm if the particle-size class is coarse-loamy silty (>15 percent particles 0.1–75 mm and < 18 percent clay in fine earth); ¾from 30 to 90 cm if the particle-size class is sandy (texture of sand or loamy sand). If the soil contains rock and pararock fragments that do not absorb and release water, the limits of the moisture control section are deeper. The limits of the soil moisture control section are affected not only by the particle-size class but also by differences in soil structure or pore-size distribution or by other factors that influence the movement and retention of water in the soil.

CLASSES OF SOIL MOISTURE REGIMES The soil moisture regimes are defined in terms of the level of groundwater and in terms of the seasonal presence or absence of water held at a tension of less than 1 500 kPa in the moisture control section. It is assumed in the definitions that the soil supports whatever vegetation it is capable of supporting, i.e., crops, grass or native vegetation, and that the amount of stored moisture is not being increased by irrigation or fallowing. These cultural practices affect the soil moisture conditions as long as they continued.

AQ – Aquic moisture regime The aquic (Latin aqua, water) moisture regime is a reducing regime in a soil that is virtually free of dissolved oxygen because it is saturated by water. Some soils are saturated with water at times while dissolved oxygen is present, either because the water is moving or because the environment is unfavourable for micro-organisms (e.g. where the temperature is less than 1 °C); such a regime is not considered aquic. It is not known how long a soil must be saturated before it is said to have an aquic moisture regime. However, the duration must be at least a few days, because it is implicit in the concept that dissolved oxygen is virtually absent.

PQ – Peraquic moisture regime Very commonly, the level of groundwater fluctuates with the seasons. However, there are soils in which the groundwater is always at or very close to the surface. Examples are soils in tidal marshes or in closed, landlocked depressions fed by perennial streams. Such soils are considered to have a peraquic moisture regime. 94 Guidelines for soil description

AR – Aridic and TO – torric (Latin aridus, dry, and torridus, hot and dry) moisture regimes These terms are used for the same moisture regime but in different categories of the taxonomy. In the aridic (torric) moisture regime, the moisture control section in normal years is both: ¾dry in all parts for more than half on the cumulative days per year when the soil temperature at a depth of 50 cm from the soil surface is above 5 °C; ¾moist in some or all parts for less than 90 consecutive days when the soil temperature at a depth of 50 cm is above 8 °C. Soils that have an aridic (torric) moisture regime normally occur in areas of arid climates. A few are in areas of semi-arid climates and either have physical properties that keep them dry, such as a crusty surface that virtually precludes the infiltration of water, or are on steep slopes where runoff is high. There is little or no leaching in this moisture regime, and soluble salts accumulate in the soils if there is a source.

UD – Udic moisture regime The udic (Latin udus, humid) moisture regime is one in which the soil moisture control section is not dry in any part for as long as 90 cumulative days in normal years. Where the mean annual soil temperature is lower than 22 °C and where the mean winter and mean summer soil temperatures at depth of 50 cm from the soil surface differ by 6 °C or more, the soil moisture control section in normal years is dry in all parts for less than 45 consecutive in the 4 months following the summer solstice. In addition, the udic moisture regime requires, except for short periods, a three-phase system, solid–liquid–gas, in part or all of the soil moisture control section when the soil temperature is above 5 °C. The udic moisture regime is common to the soils of humid climates that: have well-distributed rainfall; have enough rain in summer so that the amount of stored moisture plus rainfall is approximately equal to, or exceeds, the amount of evapotranspiration; or have adequate winter rains to recharge the soils and cool, foggy summers, as in coastal areas. Water moves downwards though the soils at some time in normal years.

PU – Perudic moisture regime (Latin per, throughout in time, and udus, humid) In climates where precipitation exceeds evapotranspiration in all months of normal years, the moisture tension rarely reaches 100 kPa (pF 3) in the soil moisture control section, although there are occasional brief periods when some stored moisture is used. The water moves through the soil in all months when it is not frozen. Such an extremely wet moisture regime is called perudic.

US – Ustic moisture regime The ustic (Latin ustus, burnt; implying dryness) moisture regime is intermediate between the aridic regime and the udic regime. Its concept is one of moisture Annex 2 – Explanation of soil moisture regimes 95

that is limited but is present at a time when conditions are suitable for plant growth. The concept of the ustic moisture regime is not applied to soils that have permafrost or a cryic soil temperature regime (defined above). If the mean annual soil temperature is 22 °C or higher or if the mean summer and winter soil temperatures differ by less than 6 °C at a depth of 50 cm below the soil surface, the soil moisture control section in areas of the ustic moisture regime is dry in some or all parts for 90 or more cumulative days in normal years. However, it is moist in some part either for more than 180 cumulative days per year or for 90 or more consecutive days. If the mean annual soil temperature is lower than 22 °C and if the mean summer and winter soil temperatures differ by 6 °C or more at a depth of 50 cm from the soil surface, the soil moisture control section in areas of the ustic moisture regime is dry in some or all parts for 90 or more cumulative days in normal years, but it is not dry in all parts for more than half of the cumulative days when the soil temperature at a depth of 50 cm is higher than 5 °C. If in normal years the moisture control section is moist in all parts for 45 or more consecutive days in the 4 months following the winter solstice, the moisture control section is dry in all parts for less than 45 consecutive days in the 4 months following the summer solstice. In tropical and subtropical regions that have a climate with either one or two dry seasons, summer and winter seasons have little meaning. In such regions, the moisture regime is ustic if there is at least one rainy season of three months or more. In temperate regions of subhumid or semi-arid climates, the rainy seasons are usually spring and summer or spring and autumn, but never winter. Native plants are mainly annuals or plants that have a dormant period while the soil is dry.

XE – Xeric moisture regime The xeric (Greek xeros, dry) moisture regime is the typical moisture regime in areas of Mediterranean climates, where winters are moist and cool and summers are warm and dry. The moisture, which falls during the winter, when potential evapotranspiration is at a minimum, is particularly effective for leaching. In areas of a xeric moisture regime, the soil moisture control section, in normal years, is dry in all parts for 45 or more consecutive days in the 4 months following the summer solstice and moist in all parts for 45 or more consecutive days in the 4 months following the winter solstice. Moreover, in normal years, the moisture control section is moist in some part for more than half of the cumulative days per year when the soil temperature at a depth of 50 cm from the soil surface is higher than 6 °C or for 90 or more consecutive days when the soil temperature at a depth of 50 cm is higher than 8 °C. The mean annual soil temperature is lower than 22 °C, and the mean summer and mean winter soil temperatures differ by 6 °C or more either at a depth of 50 cm from the soil surface or at a densic, lithic or paralithic contact if shallower.

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Annex 3 Equipment necessary for field work

0 cm Map of topography (at least 1:25 000), (, land use, 1 vegetation where available) Global positioning system unit, compass 2 Guideline for soil description Guideline for soil classification 3

Field book, reading form 4 Munsell soil color charts

5 Shovel, spade, pick-axe, auger and hammer Field pH-/conductometer, standard solutions Box with: 6 ¾ pocket rule ¾ knife, palette-knife 7 ¾ hand lens (×10) ¾ platinum electrodes (redox measurement) 8 ¾ bottle with tap water ¾ bottle with aqua dest 9 ¾ bottle with 1 M KCl or 0.01 M CaCl2 solution (25 ml per pH measurement) ¾ five transparent plastic cups with marks for 8 cm3 soil (~ 10 g) and 25 ml water, per pH or EC measurement 10 ¾ drop flask with 10 percent HCl (~ 50 ml) ¾ drop flask with phenolphthalein pH indicator (8.2...9.8) solution (~ 30 ml) 11 ¾ drop flask with 1 M NaF adjusted to pH 7.5 (~ 30 ml) ¾ drop flask with 0.2 percent (M/V) _,_-dipyridyl solution in 10 percent 12 (V/V) acetic acid solution (~ 50 ml).

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A0510E-Cover-A4-Pant 4725 [ConvePage 1 30-05-2006 12:03:10 ISSN 0532-0488

WORLD SOIL 103 RESOURCES 103 REPORTS WORLD SOIL RESOURCES REPORTS 103

World reference base W orld reference base for soil resources 2006 for soil resources 2006 A framework for international classification, World reference base correlation and communication for soil resources 2006 A framework for international classification, correlation and communication

This publication is a revised and updated version of World Soil Resources Reports No. 84, a technical manual for soil

C

scientists and correlators, designed to facilitate the – A framework for international classification, correlation and communication

M exchange of information and experience related to soil

Y resources, their use and management. The document

CM provides a framework for international soil classification and

MY an agreed common scientific language to enhance communication across disciplines using soil information. It CY contains definitions and diagnostic criteria to recognize soil CMY horizons, properties and materials and gives rules and K guidelines for classifying and subdividing soil reference groups.

ISBN 92-5-105511-4 ISSN 0532-0488 F 9 7 8 9 2 5 1 0 5 5 1 1 3 AO TC/M/A0510E/1/05.06/5200 International Union of Soil Sciences Cover photograph: Ferralsol (), Cryosol (Russian Federation), (Hungary), Podzol (Austria), (United States of America), (United Republic of Tanzania), Luvisol (Hungary). Compiled by Erika Micheli.

Copies of FAO publications can be requested from: SALES AND MARKETING GROUP Information Division Food and Agriculture Organization of the United Nations Viale delle Terme di Caracalla 00100 Rome, Italy

E-mail: [email protected] Fax: (+39) 06 57053360 Web site: http://www.fao.org WORLD SOIL World reference base RESOURCES REPORTS for soil resources 2006 103 A framework for international classification, correlation and communication

2006 edition

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 2006 The designations employed and the presentation of material in this information product do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations concerning the legal or development status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.

IUSS Working Group WRB. 2006. World reference base for soil resources 2006. World Soil Resources Reports No. 103. FAO, Rome.

ISBN 92-5-105511-4

All rights reserved. Reproduction and dissemination of material in this information product for educational or other non-commercial purposes are authorized without any prior written permission from the copyright holders provided the source is fully acknowledged. Reproduction of material in this information product for resale or other commercial purposes is prohibited without written permission of the copyright holders. Applications for such permission should be addressed to: Chief Publishing Management Service Information Division FAO Viale delle Terme di Caracalla, 00100 Rome, Italy or by e-mail to: [email protected]

© FAO 2006 iii

Contents

Foreword vii Acknowledgements viii List of acronyms and abbreviations ix 1. Background to the world reference base for soil resources 1 History 1 From its beginnings to the first edition in 1998 1 From the first edition in 1998 to the second edition in 2006 2 Basic principles 3 Architecture 4 Key to the Reference Soil Groups 4 The qualifier level 6 Principles and use of the qualifiers in the WRB 6 The geographical dimension of WRB qualifiers – match to mapping scale 7 The object classified in the WRB 7 Rules for classification 8 Step one 8 Step two 8 Step three 8 Example of WRB soil classification 9

2. Diagnostic horizons, properties and materials 11 Diagnostic horizons 11 Albic horizon 11 Anthraquic horizon 12 Anthric horizon 12 Argic horizon 13 Calcic horizon 15 Cambic horizon 16 Cryic horizon 17 Duric horizon 17 Ferralic horizon 18 Ferric horizon 19 Folic horizon 20 Fragic horizon 20 Fulvic horizon 21 Gypsic horizon 22 Histic horizon 23 Hortic horizon 23 Hydragric horizon 24 Irragric horizon 24 Melanic horizon 25 Mollic horizon 25 Natric horizon 26 Nitic horizon 28 iv

Petrocalcic horizon 29 Petroduric horizon 30 Petrogypsic horizon 30 Petroplinthic horizon 31 Pisoplinthic horizon 32 Plaggic horizon 32 Plinthic horizon 33 Salic horizon 34 Sombric horizon 35 Spodic horizon 35 Takyric horizon 36 Terric horizon 37 Thionic horizon 38 Umbric horizon 38 Vertic horizon 39 Voronic horizon 40 Yermic horizon 41 Diagnostic properties 41 Abrupt textural change 41 Albeluvic tonguing 41 Andic properties 42 Aridic properties 43 Continuous rock 44 Ferralic properties 44 Geric properties 45 Gleyic colour pattern 45 Lithological discontinuity 46 Reducing conditions 46 Secondary carbonates 47 Stagnic colour pattern 47 Vertic properties 47 Vitric properties 47 Diagnostic materials 48 Artefacts 48 Calcaric material 48 Colluvic material 49 Fluvic material 49 Gypsiric material 49 Limnic material 49 Mineral material 50 Organic material 50 Ornithogenic material 50 Sulphidic material 51 Technic hard rock 51 Tephric material 51

3. Key to the reference soil groups of the WRB with lists of prefix and suffix qualifiers 53

4. Description, distribution, use and management of reference soil groups 67 Acrisols 67 68 v

Alisols 69 70 Anthrosols 71 Arenosols 72 Calcisols 74 Cambisols 75 Chernozems 76 Cryosols 76 77 Ferralsols 78 Fluvisols 79 Gleysols 80 Gypsisols 81 Histosols 82 83 Leptosols 84 Lixisols 85 Luvisols 86 Nitisols 87 88 Planosols 88 89 91 92 92 Solonetz 94 Stagnosols 95 Technosols 95 96 Vertisols 97

5. Definitions of formative elements for second-level units of the WRB 101

References 121

Annexes 1. Summary of analytical procedures for soil characterization 123 2. Recommended codes for the reference soil groups, qualifiers and specifiers 127 vi

List of tables

1. Rationalized Key to the WRB Reference Soil Groups 5 2. Prefix and suffix qualifiers in the WRB – case of Cryosols 6 vii

Foreword

The first official version of the World Reference Base for Soil Resources (WRB) was released at the 16th World Congress of Soil Science at Montpellier in 1998. At the same event, it was also endorsed and adopted as the system for soil correlation and international communication of the International Union of Soil Sciences (IUSS). After eight years of intensive worldwide testing and data collection, the current state- of-the-art of the WRB is presented. This publication reflects the valuable work of the authors of the earlier drafts and the first version of the WRB, as well as the experiences and contributions of many soil scientists who participated in the work of the IUSS Working Group on the WRB. Globalization and global environmental issues necessitate harmonization and correlation of technical languages, such as the one used in soil science. It is hoped that this publication will contribute to the understanding of soil science in the public debate and in the scientific community. The publication has been made possible by the sustained efforts of a large group of expert authors, and the cooperation and logistic support of the IUSS, the International Soil Reference and Information Centre (ISRIC) and the Food and Agriculture Organization of the United Nations (FAO).

Erika Michéli (Chair), Peter Schad (Vice-Chair) and Otto Spaargaren (Secretary) IUSS Working Group WRB

David Dent ISRIC – World Soil Information

Freddy Nachtergaele FAO Land and Water Development Division viii

Acknowledgements

The text of this publication is based on numerous valuable contributions from hundreds of soil scientists worldwide. They participated in field tours, workshops and conferences; they sent comments, and they tested the World Reference Base for Soil Resources (WRB) approach. This publication would not have been possible without the support of a number of international institutes and organizations, notably the Food and Agriculture Organization of the United Nations (FAO), the National Resources Conservation Service of the United States of America, the European Soils Bureau hosted by the Joint Research Centre of the European Commission, the West and Central African Union of Soil Scientists Association, and the International Soil Reference and Information Centre (ISRIC) – World Soil Information, to name only the major ones. Last but not least, the Working Group World Reference Base of the International Union of Soil Sciences (IUSS) was supported by other IUSS working groups, in particular the Soils in Urban, Industrial, Traffic and Mining Areas Working Group and the Cryosol Working Group of the IUSS/International Permafrost Association. National soil institutes in many countries assisted in WRB field tours, and organized conferences and WRB summer schools (listed in Chapter 1). This edition has been edited by Erika Michéli (Szent István University, Hungary), Peter Schad (Technische Universität München, Germany) and Otto Spaargaren (ISRIC – World Soil Information, Netherlands). Particular mention should go to Richard Arnold (United States of America), Hans-Peter Blume (Germany) and Rudi Dudal (Belgium). They were involved from the inception of the International Reference Base, more than 25 years ago, and have provided invaluable institutional memory for the objectives and approach. The Working Group wishes to express its gratitude to FAO for its support and for making possible the printing and distribution of this publication. ix

List of acronyms and abbreviations

Al Aluminium Ca Calcium

CaCO3 Calcium carbonate CEC Cation exchange capacity COLE Coefficient of linear extensibility EC Electrical conductivity

ECe Electrical conductivity of saturation extract ECEC Effective CEC ESP Exchangeable sodium percentage FAO Food and Agriculture Organization of the United Nations Fe Iron HCl Hydrochloric acid IRB International Reference Base for Soil Classification ISRIC International Soil Reference and Information Centre ISSS International Society of Soil Science IUSS International Union of Soil Sciences K KOH Potassium Mg Mn Manganese N Nitrogen Na Sodium NaOH Sodium hydroxide ODOE Optical density of the oxalate extract P Phosphorus RSG Reference Soil Group S Sulphur SAR Sodium adsorption ratio

SiO2 Silica SUITMA Soils in Urban, Industrial, Traffic and Mining Areas (special working group) Ti TRB Total reserve of bases UNEP United Nations Environment Programme UNESCO United Nations Educational, Scientific, and Cultural Organization USDA United States Department of Agriculture WRB World Reference Base for Soil Resources Zn Zinc

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Chapter 1 Background to the world reference base for soil resources

HISTORY

From its beginnings to the first edition in 1998 In the early 1980s, countries became increasingly interdependent for their supplies of food and agricultural products. Problems of land degradation, disparity of production potentials and of population-carrying capacities became international concerns that required harmonized soil information. Against this background, the Food and Agriculture Organization of the United Nations (FAO) felt that a framework should be created through which existing soil classification systems could be correlated and harmonized. Concurrently, it would serve as an international means of communication and for exchange of experience. The elaboration of such a framework required a more active involvement of the entire soils community. At the initiative of FAO, the United Nations Educational, Scientific, and Cultural Organization (UNESCO), the United Nations Environment Programme (UNEP), and the International Society of Soil Science (ISSS), a group of soil scientists representing a broad range of soil institutions met in Sofia, Bulgaria, in 1980 and 1981 to enhance international involvement in a follow-up to the Soil Map of the World (FAO–UNESCO, 1971–1981). The meeting was hosted by the Poushkarov Institute of Soil Science and Yield Programming. The meeting decided to launch a programme to develop an International Reference Base for Soil Classification (IRB) with the aim to reach agreement on the major soil groupings to be recognized at a global scale, as well as on the criteria to define and separate them. It was expected that such an agreement would facilitate the exchange of information and experience, provide a common scientific language, strengthen the applications of soil science, and enhance communication with other disciplines. The group met in 1981 for a second time at Sofia and laid down the general principles of a joint programme towards the development of an IRB. In 1982, the 12th Congress of the ISSS, in New Delhi, , endorsed and adopted this programme. The work was conducted by a newly created IRB working group, chaired by E. Schlichting with R. Dudal serving as secretary. At the 13th Congress of the ISSS, in Hamburg, Germany, in 1986, the IRB programme was entrusted to Commission V, with A. Ruellan as chair and R. Dudal as secretary. These charges were continued through the 14th Congress of the ISSS, in Kyoto, Japan, in 1990. In 1992, the IRB was renamed the World Reference Base for Soil Resources (WRB). Hence, a WRB working group was established at the 15th Congress of the ISSS, in Acapulco, , in 1994, with J. Deckers, F. Nachtergaele and O. Spaargaren as chair, vice-chair and secretary, respectively, through the 16th Congress of the ISSS, in Montpellier, France, in 1998. At the 17th World Congress of Soil Science, in Bangkok, Thailand, in 2002, the leadership for the WRB programme was entrusted to E. Michéli, P. Schad and O. Spaargaren as chair, vice-chair and secretary, respectively. At a meeting of the IRB Working Group in Montpellier in 1992, it was decided that the revised FAO–UNESCO legend would form the basis for the further development of the IRB and that efforts were to be merged. It would be the task of the IRB to apply its general principles to the further refinement of the FAO–UNESCO units and to provide them with the necessary depth and validation. 2 World reference base for soil resources 2006

Progress in the preparation of the WRB was reported to the 15th Congress of the ISSS at Acapulco in 1994 (FAO, 1994). Numerous contributions were received from soil scientists; the WRB was discussed and tested in meetings and excursions at Leuven, Belgium (1995), Kiel, Germany (1995), Moscow, Russian Federation (1996), (1996), (1997) and Vienna, Austria (1997). The first official text of the WRB was presented at the 16th World Congress of Soil Science in Montpellier in 1998 in three volumes: 1. World Reference Base for Soil Resources. An introduction. 2. World Reference Base for Soil Resources. Atlas. 3. World Reference Base for Soil Resources. The WRB text was then adopted by the ISSS Council as the officially recommended terminology to name and classify soils. By general agreement, it was then decided that the text would remain unchanged for at least eight years, but that it would be tested extensively during this period and a revision proposed at the 18th World Congress of Soil Science in 2006.

From the first edition in 1998 to the second edition in 2006 In the period 1998–2006, the WRB became the official reference soil nomenclature and soil classification for the European Commission and was adopted by the West and Central African Soil Science Association as the preferred tool to harmonize and exchange soil information in the region. The main text was translated in 13 languages (Chinese, French, German, Hungarian, Italian, Japanese, Latvian, Lithuanian, Polish, Rumanian, Russian, Spanish and Vietnamese) and adopted as a higher level of the national soil classification system in a number of countries (e.g. Italy, Mexico, Norway, Poland and Viet Nam). The text was further illustrated by lecture notes and a CD-ROM on the major soils of the world (FAO, 2001a and 2001b) and a World Soil Resources Map at a scale 1:25 000 000 by the Joint Research Centre, FAO and the International Soil Reference and Information Centre (ISRIC) in 2002. A Web site was established (http://www.fao.org/landandwater/agll/wrb/default.stm) and a newsletter was distributed to hundreds of soil scientists. Specific attention was paid to land-use and soil management issues for tropical and dryland soils using WRB information (FAO, 2003 and 2005). Numerous articles appeared in peer-reviewed soil science journals and books, suggesting improvements to the system. Two conferences were held together with field trips: in 2001 in Velence (Hungary, organized by the Szent István University in Gödöllö); and in 2004 in Petrozavodsk (Russian Federation, organized by the Institute of Biology, Karelian Research Centre). At the same time, a number of field excursions were organized to test and refine the WRB approach in the field: Burkina Faso and Côte d’Ivoire (1998); Viet Nam and (1998); Italy (1999); Georgia (2000); Ghana and Burkina Faso (2001); Hungary (2001); South Africa and Namibia (2003); Poland (2004); Italy (2004); Russian Federation (2004); Mexico (2005); Kenya and the United Republic of Tanzania (2005); and Ghana (2005). Summer schools, coordinated by E. Michéli (Hungary), were organized under the auspices of the EU Joint Research Centre in Ispra, Italy (2003 and 2004), and in Gödöllö, Hungary (2005), to teach the system to soil science students and practitioners. In the same period, the European Commission issued the Soil Atlas of Europe based on the WRB (European Soil Bureau Network/European Commission, 2005). A major effort was undertaken to harmonize nomenclature with the soil taxonomy of the United States Department of Agriculture (USDA) and other major national soil classification systems. Some national classifications took up elements of the WRB, e.g. the Chinese soil taxonomy (CRGCST, 2001), the Czech soil classification (Nmeek et al., 2001), the Lithuanian soil classification (Buivydaité et al., 2001), and the Russian soil classification system (Shishov et al., 2001). A WRB e-mail forum was organized in 2005 to enable finalization of suggestions for each Soil Reference Soil Group. Independently, Chapter 1 – Background to the world reference base for soil resources 3

special working groups of the International Union of Soil Sciences (IUSS) (formerly the ISSS), such as the ones on Cryosols and on Soils in Urban, Industrial, Traffic and Mining Areas (SUITMA) proposed changes to the system, some of which have been adopted in the present text. The second edition of the WRB has undergone a major revision. Technosols and Stagnosols have been introduced, leading to 32 Reference Soil Groups (RSGs) instead of 30. The Technosols are soils with a certain amount of artefacts, a constructed geomembrane or technic hard rock. The Stagnosols unify the former Epistagnic subunits of many other RSGs. Some re-arrangement has taken place in the order of the key, with Anthrosols, Solonetz, Nitisols and Arenosols moving upwards. The definitions of many diagnostic soil horizons, soil properties, and materials have been adjusted. The qualifiers are now subdivided into prefix and suffix ones. Prefix qualifiers comprise those that are typically associated with the RSG (in order of their importance) and the intergrades to other RSGs (in order of the key). All other qualifiers are listed as suffix qualifiers.

BASIC PRINCIPLES

The general principles on which the WRB is based were laid down during the early Sofia meetings in 1980 and 1981, and further elaborated upon by the working groups entrusted with its development. These general principles can be summarized as follows: ÿ The classification of soils is based on soil properties defined in terms of diagnostic horizons, properties and materials, which to the greatest extent possible should be measurable and observable in the field. ÿ The selection of diagnostic characteristics takes into account their relationship with soil forming processes. It is recognized that an understanding of soil-forming processes contributes to a better characterization of soils but that they should not, as such, be used as differentiating criteria. ÿ To the extent possible at a high level of generalization, diagnostic features are selected that are of significance for soil management. ÿ Climate parameters are not applied in the classification of soils. It is fully realized that they should be used for interpretation purposes, in dynamic combination with soil properties, but they should not form part of soil definitions. ÿ The WRB is a comprehensive classification system that enables people to accommodate their national classification system. It comprises two tiers of categorical detail: • the Reference Base, limited to the first level only and having 32 RSGs; • the WRB Classification System, consisting of combinations of a set of prefix and suffix qualifiers that are uniquely defined and added to the name of the RSG, allowing very precise characterization and classification of individual soil profiles. ÿ Many RSGs in the WRB are representative of major soil regions so as to provide a comprehensive overview of the world’s soil cover. ÿ The Reference Base is not meant to substitute for national soil classification systems but rather to serve as a common denominator for communication at an international level. This implies that lower-level categories, possibly a third category of the WRB, could accommodate local diversity at country level. Concurrently, the lower levels emphasize soil features that are important for land use and management. ÿ The Revised Legend of the FAO/UNESCO Soil Map of the World (FAO, 1988) has been used as a basis for the development of the WRB in order to take advantage of international soil correlation that has already been conducted through this project and elsewhere. 4 World reference base for soil resources 2006

ÿ The first edition of the WRB, published in 1998, comprised 30 RSGs; the second edition, published in 2006, has 32 RSGs. ÿ Definitions and descriptions of soil units reflect variations in soil characteristics both vertically and laterally so as to account for spatial linkages within the landscape. ÿ The term Reference Base is connotative of the common denominator function that the WRB assumes. Its units have sufficient width to stimulate harmonization and correlation of existing national systems. ÿ In addition to serving as a link between existing classification systems, the WRB also serves as a consistent communication tool for compiling global soil databases and for the inventory and monitoring of the world’s soil resources. ÿ The nomenclature used to distinguish soil groups retains terms that have been used traditionally or that can be introduced easily in current language. They are defined precisely in order to avoid the confusion that occurs where names are used with different connotations. Although the basic framework of the FAO Legend (with its two categorical levels and guidelines for developing classes at a third level) was adopted, it has been decided to merge the lower levels. Each RSG of the WRB is provided with a listing of possible prefix and suffix qualifiers in a priority sequence, from which the user can construct the second-level units. The broad principles that govern the WRB class differentiation are: ÿ At the higher categorical level, classes are differentiated mainly according to the primary pedogenetic process that has produced the characteristic soil features, except where special soil parent materials are of overriding importance. ÿ At the second level, soil units are differentiated according to any secondary soil- forming process that has affected the primary soil features significantly. In certain cases, soil characteristics that have a significant effect on use may be taken into account. It is recognized that a number of RSGs may occur under different climate conditions. However, it was decided not to introduce separations on account of climate characteristics so that the classification of soils is not subordinated to the availability of climate data.

ARCHITECTURE

Currently, the WRB comprises two tiers of categorical detail: 1. Tier 1: The RSGs, comprising 32 RSGs; 2. Tier 2: The combination of RSGs with qualifiers, detailing the properties of the RSGs by adding a set of uniquely defined qualifiers.

Key to the Reference Soil Groups The Key to the RSGs in the WRB stems from the Legend of the Soil Map of the World. The history behind the Key to the Major Soil Units of the Legend of the Soil Map of the World reveals that it is mainly based on functionality; the Key was conceived to derive the correct classification as efficiently as possible. The sequence of the Major Soil Units was such that the central concept of the major soils would come out almost automatically by specifying briefly a limited number of diagnostic horizons, properties or materials. Table 1 provides an overview and logic for the sequence of the RSGs in the WRB Key. The RSGs are allocated to sets on the basis of dominant identifiers, i.e. the soil- forming factors or processes that most clearly condition the soil formation. The sequencing of the groups is done according to the following principles: 1. First, organic soils key out to separate them from mineral soils (Histosols). Chapter 1 – Background to the world reference base for soil resources 5

2. The second major distinction in the WRB is to recognize human activity as a soil-forming factor, hence the position of the Anthrosols and Technosols after the Histosols; it also appears logical to key out the newly-introduced Technosols close to the beginning of the Key, for the following reasons: ÿ one can almost immediately key out soils that should not be touched (toxic soils that should be handled by experts); ÿ a homogeneous group of soils in strange materials is obtained; ÿ politicians and decision-makers who consult the Key will immediately encounter these problematic soils. 3. Next are the soils with a severe limitation to rooting (Cryosols and Leptosols). 4. Then comes a group of RSGs that are or have been strongly influenced by water: Vertisols, Fluvisols, Solonetz, Solonchaks, and Gleysols. 5. The following set of soil groups are the RSGs in which iron (Fe) and/or aluminium (Al) chemistry plays a major role in their formation: Andosols, Podzols, Plinthosols, Nitisols and Ferralsols.

TABLE 1 Rationalized Key to the WRB Reference Soil Groups 1. Soils with thick organic layers: Histosols 2. Soils with strong human influence Soils with long and intensive agricultural use: Anthrosols Soils containing many artefacts: Technosols 3. Soils with limited rooting due to shallow permafrost or stoniness Ice-affected soils: Cryosols Shallow or extremely gravelly soils: Leptosols 4. Soils influenced by water Alternating wet-dry conditions, rich in swelling clays: Vertisols Floodplains, tidal marshes: Fluvisols Alkaline soils: Solonetz Salt enrichment upon evaporation: Solonchaks Groundwater affected soils: Gleysols 5. Soils set by Fe/Al chemistry Allophanes or Al-humus complexes: Andosols Cheluviation and chilluviation: Podzols Accumulation of Fe under hydromorphic conditions: Plinthosols Low-activity clay, P fixation, strongly structured: Nitisols Dominance of and sesquioxides: Ferralsols 6. Soils with stagnating water Abrupt textural discontinuity: Planosols Structural or moderate textural discontinuity: Stagnosols 7. Accumulation of organic matter, high base status Typically mollic: Chernozems Transition to drier climate: Kastanozems Transition to more humid climate: Phaeozems 8. Accumulation of less soluble salts or non-saline substances Gypsum: Gypsisols Silica: Durisols Calcium carbonate: Calcisols 9. Soils with a clay-enriched subsoil Albeluvic tonguing: Albeluvisols Low base status, high-activity clay: Alisols Low base status, low-activity clay: Acrisols High base status, high-activity clay: Luvisols High base status, low-activity clay: Lixisols 10. Relatively young soils or soils with little or no profile development With an acidic dark topsoil: Umbrisols Sandy soils: Arenosols Moderately developed soils: Cambisols Soils with no significant profile development: Regosols 6 World reference base for soil resources 2006

6. Next comes a set of soils with perched water: Planosols and Stagnosols. 7. The next grouping comprises soils that occur predominantly in steppe regions and have humus-rich and a high base saturation: Chernozems, Kastanozems and Phaeozems. 8. The next set comprises soils from the drier regions with accumulation of gypsum (Gypsisols), silica (Durisols) or calcium carbonate (Calcisols). 9. Then comes a set of soils with a clay-rich subsoil: Albeluvisols, Alisols, Acrisols, Luvisols and Lixisols. 10. Finally, relatively young soils or soils with very little or no profile development, or very homogenous sands, are grouped together: Umbrisols, Arenosols, Cambisols and Regosols.

The qualifier level In the WRB, a distinction is made between typically associated qualifiers, intergrades and other qualifiers. Typically associated qualifiers are referred to in the Key to the particular RSGs, e.g. Hydragric or Plaggic for the Anthrosols. Intergrade qualifiers are those that reflect important diagnostic criteria of another RSG. The WRB Key will, in that case, dictate the choice of the RSG and the intergrade qualifier will provide the bridge to the other RSG. Other qualifiers are those not typically associated with an RSG and that do not link to other RSGs, e.g. Geric or Posic for Ferralsols. This group reflects characteristics such as colour, base status, and other chemical and physical properties provided that they are not used as a typically associated qualifier in that particular group.

Principles and use of the qualifiers in the WRB A two-tier system is used for the qualifier level, comprising: ÿ Prefix qualifiers: typically associated qualifiers and intergrade qualifiers; the sequence of the intergrade qualifiers follows that of the RSGs in the WRB Key, with the exception of Arenosols; this intergrade is ranked with the textural suffix qualifiers (see below). Haplic closes the prefix qualifier list indicating that neither typically associated nor intergrade qualifiers apply. ÿ Suffix qualifiers: other qualifiers, TABLE 2 sequenced as follows: (1) qualifiers related Prefix and suffix qualifiers in the WRB – case of to diagnostic horizons, properties or Cryosols materials; (2) qualifiers related to chemical Prefix qualifiers Suffix qualifiers Glacic Gypsiric characteristics; (3) qualifiers related to Turbic Calcaric physical characteristics; (4) qualifiers Folic Ornithic* related to mineralogical characteristics; (5) Histic Dystric qualifiers related to surface characteristics; Technic Eutric (6) qualifiers related to textural Hyperskeletic Reductaquic* characteristics, including coarse fragments; Leptic Oxyaquic Natric Thixotropic (7) qualifiers related to colour; and (8) Salic Aridic remaining qualifiers. Vitric Skeletic ÿ Table 2 provides an example of the listing Spodic Arenic of prefix and suffix qualifiers. Mollic Siltic Prefix qualifier names are always put Calcic Clayic* before the RSG; suffix qualifier names are Umbric Drainic* always placed between brackets following Cambic Novic* Haplic the RSG name. Combinations of qualifiers * = newly introduced qualifiers that indicate a similar status or duplicate each Examples: other are not permitted, such as combinations 1. Histic Turbic Cryosol (Reductaquic, Dystric). 2. Haplic Cryosol (Aridic, Skeletic). of Thionic and Dystric, Calcaric and Eutric, or Rhodic and Chromic. Chapter 1 – Background to the world reference base for soil resources 7

Specifiers such as Epi-, Endo-, Hyper-, Hypo-, Thapto-, Bathy-, Para-, Proto-, Cumuli- and Ortho- are used to indicate a certain expression of the qualifier. When classifying a soil profile, all applying qualifiers of the listing must be recorded. For mapping purposes, the scale will determine the number of qualifiers used. In that case, prefix qualifiers have priority over the suffix qualifiers. The qualifier listing for each RSG accommodates most cases. Where not listed qualifiers are needed, the cases should be documented and reported to the WRB Working Group.

The geographical dimension of WRB qualifiers – match to mapping scale The WRB was not designed originally for mapping soils but its roots are in the Legend of the Soil Map of the World. Before the WRB came into existence, the FAO Legend was used for soil mapping at various scales, and rather successfully (e.g. soil mapping in , Botswana, Ethiopia, the European Union, Kenya, and the United Republic of Tanzania). Whether desirable or not, people are using the WRB as a tool for soil mapping (e.g. 1:1 000 000 scale Soil Map of Europe; 1:250 000 Soil Map of the Central Highlands of Viet Nam). A basic principle in soil mapping is that the soil surveyor designs the legend of the map so as to best suit the purpose of the survey. If the WRB is designed to support small-scale mapping of the global soil landscapes, it would be advantageous to have a structure that lends itself to support such overview maps. Hence, the discussion on the qualifier listings should not be held in isolation of the overview maps of the soils of the world or the continents in the WRB. Therefore, it is suggested that the WRB qualifiers be linked to small-scale soil maps as follows: ÿ prefix qualifiers for mapping between 1/5*106 and 1/106 scale; ÿ suffix qualifiers for mapping between 1/106 and 1/250*103 scale. For larger mapping scales, it is suggested that, in addition, national or local soil classification systems be used. They are designed to accommodate local soil variability, which can never be accounted for in a world reference base.

THE OBJECT CLASSIFIED IN THE WRB

Like many common words, the word soil has several meanings. In its traditional meaning, soil is the natural medium for the growth of plants, whether or not it has discernible soil horizons (Soil Survey Staff, 1999). In the 1998 WRB, soil was defined as: “… a continuous natural body which has three spatial and one temporal dimension. The three main features governing soil are: ÿ It is formed by mineral and organic constituents and includes solid, liquid and gaseous phases. ÿ The constituents are organized in structures, specific for the pedological medium. These structures form the morphological aspect of the soil cover, equivalent to the anatomy of a living being. They result from the history of the soil cover and from its actual dynamics and properties. Study of the structures of the soil cover facilitates perception of the physical, chemical and biological properties; it permits understanding the past and present of the soil, and predicting its future. ÿ The soil is in constant evolution, thus giving the soil its fourth dimension, time.” Although there are good arguments to limit soil survey and mapping to identifiable stable soil areas with a certain thickness, the WRB has taken the more comprehensive approach to name any object forming part of the epiderm of the earth (Nachtergaele, 2005). This approach has a number of advantages, notably that it allows tackling environmental problems in a systematic and holistic way and avoids sterile discussions 8 World reference base for soil resources 2006

on a universally agreed definition of soil and its required thickness and stability. Therefore, the object classified in the WRB is: any material within 2 m from the Earth s surface that is in contact with the atmosphere, with the exclusion of living organisms, areas with continuous ice not covered by other material, and water bodies deeper than 2 m1. The definition includes continuous rock, paved urban soils, soils of industrial areas, cave soils as well as subaqueous soils. Soils under continuous rock, except those that occur in caves, are generally not considered for classification. In special cases, the WRB may be used to classify soils under rock, e.g. for palaeopedological reconstruction of the environment. The lateral dimension of the object classified should be large enough to represent the nature of any horizon and variability that may be present. The minimum horizontal area may range from 1 to 10 m2 depending on the variability of the soil cover.

RULES FOR CLASSIFICATION

Classification consists of three steps.

Step one The expression, thickness and depth of layers are checked against the requirements of WRB diagnostic horizons, properties and materials, which are defined in terms of morphology and/or analytical criteria (Chapter 2). Where a layer fulfils the criteria of more than one diagnostic horizon, property or material, they are regarded as overlapping or coinciding.

Step two The described combination of diagnostic horizons, properties and materials is compared with the WRB Key (Chapter 3) in order to find the RSG, which is the first level of WRB classification. The user should go through the Key systematically, starting at the beginning and excluding one by one all RSGs for which the specified requirements are not met. The soil belongs to the first RSG for which it meets all specified requirements.

Step three For the second level of WRB classification, qualifiers are used. The qualifiers are listed in the Key with each RSG as prefix and suffix qualifiers. Prefix qualifiers comprise those that are typically associated to the RSG and the intergrades to other RSGs. All other qualifiers are listed as suffix qualifiers. For classification at the second level, all applying qualifiers have to be added to the name of the RSG. Redundant qualifiers (the characteristics of which are included in a previously set qualifier) are not added. Specifiers can be used to indicate the degree of expression of qualifiers. Buried layers can be indicated by the Thapto- specifier, which can be used with any qualifier, listed in Chapter 5. Where a soil is buried under new material, the following rules apply: 1. The overlying new material and the buried soil are classified as one soil if both together qualify as Histosol, Technosol, Cryosol, Leptosol, Vertisol, Fluvisol, Gleysol, Andosol, Planosol, Stagnosol or Arenosol. 2. Otherwise, the new material is classified at the first level if the new material is 50 cm or more thick or if the new material, if it stood alone, fits the requirements of a RSG other than a Regosol. 3. In all other cases, the buried soil is classified at the first level.

1 In tidal areas, the depth of 2 m is to be applied at low tide. Chapter 1 – Background to the world reference base for soil resources 9

4. If the overlying soil is classified at the first level, the buried soil is recognized with the Thapto- specifier and -ic added to the RSG name of the buried soil. The whole is placed in brackets after the name of the overlying soil, e.g. Technic Umbrisol (Greyic) (Thapto-Podzolic). If the buried soil is classified at the first level, the overlying material is indicated with the Novic qualifier. It is recommended that the Guidelines for Soil Description (FAO, 2006) be used to describe the soil and its features. It is useful to list the occurrence and depth of diagnostic horizons, properties and materials identified. The field classification provides a preliminary assessment using all observable or easily measurable properties and features of the soil and associated terrain. The final classification is made when analytical data are available. It is recommended that Procedures for Soil Analysis (Van Reeuwijk, 2006) is followed in determining chemical and physical characteristics. A summary of these is included in Annex 1.

Example of WRB soil classification A soil has a ferralic horizon; texture in the upper part of the ferralic horizon changes from sandy loam to sandy clay within 15 cm. The pH is between 5.5 and 6, indicating moderate to high base saturation. The B horizon is dark red; below 50 cm, mottling occurs. The field classification of this soil is: Lixic Ferralsol (Ferric, Rhodic). If subsequent laboratory analysis reveals that the cation exchange capacity (CEC) of the -1 ferralic horizon is less than 4 cmolc kg clay, the soil finally classifies as Lixic Vetic Ferralsol (Ferric, Rhodic).

11

Chapter 2 Diagnostic horizons, properties and materials

Diagnostic horizons and properties are characterized by a combination of attributes that reflect widespread, common results of the processes of soil formation (Bridges, 1997) or indicate specific conditions of soil formation. Their features can be observed or measured, either in the field or in the laboratory, and require a minimum or maximum expression to qualify as diagnostic. In addition, diagnostic horizons require a certain thickness, thus forming a recognizable layer in the soil. Diagnostic materials are materials that influence pedogenetic processes significantly.

DIAGNOSTIC HORIZONS

Albic horizon General description The albic horizon (from Latin albus, white) is a light-coloured subsurface horizon from which clay and free iron oxides have been removed, or in which the oxides have been segregated to the extent that the colour of the horizon is determined by the colour of the sand and silt particles rather than by coatings on these particles. It generally has a weakly expressed soil structure or lacks structural development altogether. The upper and lower boundaries are normally abrupt or clear. The morphology of the boundaries is variable and sometimes associated with albeluvic tonguing. Albic horizons usually have coarser textures than the overlying or underlying horizons. However, with respect to an underlying spodic horizon, this difference may only be slight. Many albic horizons are associated with wetness and contain evidence of reducing conditions.

Diagnostic criteria An albic horizon has: 1. a Munsell colour (dry) with either: a. a value of 7 or 8 and a chroma of 3 or less; or b. a value of 5 or 6 and a chroma of 2 or less; and 2. a Munsell colour (moist) with either: a. a value of 6, 7 or 8 and a chroma of 4 or less; or b. a value of 5 and a chroma of 3 or less; or c. a value of 4 and a chroma of 2 or less1. A chroma of 3 is permitted if the parent materials have a hue of 5 YR or redder, and the chroma is due to the colour of uncoated silt or sand grains; and 3. a thickness of 1 cm or more.

Field identification Identification in the field depends on soil colours. In addition, a ×10 hand-lens may be used to ascertain that sand and silt grains are free of coatings.

1 Colour requirements have been changed slightly with respect to those defined by FAO–UNESCO–ISRIC (FAO, 1988) and Soil Survey Staff (1999) in order to accommodate albic horizons with a considerable shift in chroma when wetted. Such albic horizons occur frequently in, for example, southern Africa. 12 World reference base for soil resources 2006

Additional characteristics The presence of coatings around sand and silt grains can be determined using an optical microscope for analysing thin sections. Uncoated grains usually show a very thin rim at their surface. Coatings may be of an organic nature, consist of iron oxides, or both, and are dark coloured under translucent light. Iron coatings become reddish in colour under reflected light, while organic coatings remain brownish-black.

Relationships with some other diagnostic horizons Albic horizons are normally overlain by humus-enriched surface layers but may be at the surface as a result of erosion or artificial removal of the surface layer. They can be considered an extreme type of eluvial horizon, and usually occur in association with illuvial horizons such as an argic, natric or spodic horizon, which they overlie. In sandy materials, albic horizons can reach considerable thickness, up to several metres, especially in humid tropical regions, and associated diagnostic horizons may be hard to establish.

Anthraquic horizon General description An anthraquic horizon (from Greek anthropos, human, and Latin aqua, water) is a human-induced surface horizon that comprises a puddled layer and a plough pan.

Diagnostic criteria An anthraquic horizon is a surface horizon and has: 1. a puddled layer with both: a. a Munsell hue of 7.5 YR or yellower, or GY, B or BG hues; value (moist) of 4 or less; chroma (moist) of 2 or less1; and b. sorted soil aggregates and vesicular pores; and 2. a plough pan underlying the puddled layer with all of the following: a. a platy structure; and b. a bulk density higher by 20 percent or more (relative) than that of the puddled layer; and c. yellowish-brown, brown or reddish-brown iron–manganese mottles or coatings; and 3. a thickness of 20 cm or more.

Field identification An anthraquic horizon shows evidence of reduction and oxidation owing to flooding for part of the year. When not flooded, it is very dispersible and has a loose packing of sorted small aggregates. The plough pan is compact, with platy structure and very slow infiltration. It has yellowish-brown, brown or reddish-brown rust mottles along cracks and root holes.

Anthric horizon General description An anthric horizon (from Greek anthropos, human) is a moderately thick, dark- coloured surface horizon that is the result of long-term cultivation (ploughing, , fertilization, etc.).

Diagnostic criteria An anthric horizon2 is a mineral surface horizon and: 1. meets all colour, structure and organic matter requirements of a mollic or umbric horizon; and 1 Colour requirements taken from the Chinese soil taxonomy (CRGCST, 2001). 2 Modified after Krogh and Greve (1999). Chapter 2 – Diagnostic horizons, properties and materials 13

2. shows evidence of human disturbance by having one or more of the following: a. an abrupt lower boundary at ploughing depth, a plough pan; or b. lumps of applied lime; or c. mixing of soil layers by cultivation; or -1 d. 1.5 g kg or more P2O5 soluble in 1-percent citric acid; and 3. has less than 5 percent (by volume) of animal pores, coprolites or other traces of soil animal activity below tillage depth; and 4. has a thickness of 20 cm or more.

Field identification Anthric horizons are associated with old arable lands that have been cultivated for centuries. Signs of mixing or cultivation, evidence of liming (e.g. remnants of applied lime chunks) and their dark colour are the main criteria for recognition.

Relationships with other horizons Anthric horizons can resemble or overlap with mollic or umbric horizons. Anthric horizons may have developed from umbric horizons through human intervention. As they have been limed for a considerable period of time, their base saturation is high. This sets them apart from umbric horizons. The usually low biological activity below tillage depth is uncommon in soils with mollic horizons.

Argic horizon General description The argic horizon (from Latin argilla, white clay) is a subsurface horizon with distinct higher clay content than the overlying horizon. The textural differentiation may be caused by: ÿ an illuvial accumulation of clay; ÿ predominant pedogenetic formation of clay in the subsoil; ÿ destruction of clay in the surface horizon; ÿ selective surface erosion of clay; ÿ upward movement of coarser particles due to swelling and shrinking; ÿ biological activity; ÿ a combination of two or more of these different processes. Sedimentation of surface materials that are coarser than the subsurface horizon may enhance a pedogenetic textural differentiation. However, a mere lithological discontinuity, such as may occur in alluvial deposits, does not qualify as an argic horizon. Soils with argic horizons often have a specific set of morphological, physico-chemical and mineralogical properties other than a mere clay increase. These properties allow various types of argic horizons to be distinguished and their pathways of development to be traced (Sombroek, 1986).

Diagnostic criteria An argic horizon: 1. has a texture of loamy sand or finer and 8 percent or more clay in the fine earth fraction; and 2. one or both of the following: a. has, if an overlying coarser textured horizon is present that is not ploughed and not separated from the argic horizon by a lithological discontinuity, more total clay than this overlying horizon such that: i. if the overlying horizon has less than 15 percent clay in the fine earth fraction, the argic horizon must contain at least 3 percent more clay; or 14 World reference base for soil resources 2006

ii. if the overlying horizon has 15 percent or more but less than 40 percent clay in the fine earth fraction, the ratio of clay in the argic horizon to that of the overlying horizon must be 1.2 or more; or iii. if the overlying horizon has 40 percent or more total clay in the fine earth fraction, the argic horizon must contain at least 8 percent more clay; or b. has evidence of clay illuviation in one or more of the following forms: i. oriented clay bridging the sand grains; or ii. clay films lining pores; or iii. clay films on both vertical and horizontal surfaces of soil aggregates; or iv. in thin section, oriented clay bodies that constitute 1 percent or more of the section; or v. a coefficient of linear extensibility (COLE) of 0.04 or higher, and a ratio of fine clay1 to total clay in the argic horizon greater by 1.2 times or more than the ratio in the overlying coarser textured horizon; and 3. has, if an overlying coarser textured horizon is present that is not ploughed and not separated from the argic horizon by a lithological discontinuity, an increase in clay content within a vertical distance of one of the following: a. 30 cm, if there is evidence of clay illuviation; or b. 15 cm; and 4. does not form part of a natric horizon; and 5. has a thickness of one-tenth or more of the sum of the thicknesses of all overlying horizons, if present, and one of the following: a. 7.5 cm or more, if it is not entirely composed of lamellae (that are 0.5 cm or more thick) and the texture is finer than loamy sand; or b. 15 cm or more (combined thickness, if composed entirely of lamellae that are 0.5 cm or more thick).

Field identification Textural differentiation is the main feature for recognition of argic horizons. The illuvial nature may be established using an ×10 hand-lens if clay skins occur on ped surfaces, in fissures, in pores and in channels – illuvial argic horizon should show clay skins on at least 5 percent of both horizontal and vertical ped faces and in the pores. Clay skins are often difficult to detect in shrink–swell soils. The presence of clay skins in protected positions, e.g. in pores, meets the requirements for an illuvial argic horizon.

Additional characteristics The illuvial character of an argic horizon can best be established using thin sections. Diagnostic illuvial argic horizons must show areas with oriented clays that constitute on average at least 1 percent of the entire cross-section. Other tests involved are particle-size distribution analysis, to determine the increase in clay content over a specified depth, and the fine clay/total clay analysis. In illuvial argic horizons, the fine clay to total clay ratio is larger than in the overlying horizons, caused by preferential eluviation of fine clay particles. If the soil shows a lithological discontinuity over or within the argic horizon, or if the surface horizon has been removed by erosion, or if only a plough layer overlies the argic horizon, the illuvial nature must be clearly established.

Relationships with some other diagnostic horizons Argic horizons are normally associated with and situated below eluvial horizons, i.e. horizons from which clay and Fe have been removed. Although initially formed as a

1 Fine clay: < 0.2 μm equivalent diameter. Chapter 2 – Diagnostic horizons, properties and materials 15

subsurface horizon, argic horizons may occur at the surface as a result of erosion or removal of the overlying horizons. Some clay-increase horizons may have the set of properties that characterize the ferralic horizon, i.e. a low CEC and effective CEC (ECEC), a low content of water- dispersible clay and a low content of weatherable minerals, all over a depth of 50 cm. In such cases, a ferralic horizon has preference over an argic horizon for classification purposes. However, an argic horizon prevails if it overlies a ferralic horizon and it has, in its upper part over a depth of 30 cm, 10 percent or more water-dispersible clay, unless the soil material has geric properties or more than 1.4 percent organic carbon. Argic horizons lack the sodium saturation characteristics of the natric horizon. Argic horizons in cool and moist, freely drained soils of high and mountains in tropical and subtropical regions may occur in association with sombric horizons.

Calcic horizon General description The calcic horizon (from Latin calx, lime) is a horizon in which secondary calcium carbonate (CaCO3) has accumulated in a diffuse form (calcium carbonate present only in the form of fine particles of less than 1 mm, dispersed in the matrix) or as discontinuous concentrations (pseudomycelia, , soft and hard nodules, or veins). The accumulation may be in the parent material or in subsurface horizons, but it can also occur in surface horizons. If the accumulation of soft carbonates becomes such that all or most of the pedological and/or lithological structures disappear and continuous concentrations of calcium carbonate prevail, a hypercalcic qualifier is used.

Diagnostic criteria A calcic horizon has: 1. a calcium carbonate equivalent in the fine earth fraction of 15 percent or more; and 2. 5 percent or more (by volume) secondary carbonates or a calcium carbonate equivalent of 5 percent or more higher (absolute, by mass) than that of an underlying layer; and 3. a thickness of 15 cm or more.

Field identification Calcium carbonate can be identified in the field using a 10-percent hydrochloric acid (HCl) solution. The degree of effervescence (audible only, visible as individual bubbles, or foam-like) is an indication of the amount of lime present. This test is important if only diffuse distributions are present. When foam develops after adding 1 M HCl, it indicates a calcium carbonate equivalent near or more than 15 percent. Other indications for the presence of a calcic horizon are: ÿ white, pinkish to reddish, or grey colours (if not overlapping horizons rich in organic carbon); ÿ a low porosity (interaggregate porosity is usually less than that in the horizon immediately above and, possibly, also less than in the horizon directly underneath). Calcium carbonate content may decrease with depth, but this is difficult to establish in some places, particularly where the calcic horizon occurs in the deeper subsoil. Therefore, accumulation of secondary lime is sufficient to diagnose a calcic horizon.

Additional characteristics Determination of the amount of calcium carbonate (by mass) and the changes within the soil profile of the calcium carbonate content are the main analytical criteria for establishing the presence of a calcic horizon. Determination of the pH (H2O) enables 16 World reference base for soil resources 2006

distinction between accumulations with a basic (calcic) character (pH 8.0–8.7) due to

the dominance of CaCO3, and those with an ultrabasic (non-calcic) character (pH > 8.7)

because of the presence of MgCO3 or Na2CO3. In addition, microscopical analysis of thin sections may reveal the presence of dissolution forms in horizons above or below a calcic horizon, evidence of silicate epigenesis (calcite pseudomorphs after quartz), or the presence of other calcium carbonate accumulation structures, while clay mineralogical analyses of calcic horizons often show clays characteristic of confined environments, such as smectite, palygorskite and sepiolite.

Relationships with some other diagnostic horizons When calcic horizons become indurated, transition takes place to the petrocalcic horizon, the expression of which may be massive or platy. In dry regions and in the presence of sulphate-bearing soil or groundwater solutions, calcic horizons occur associated with gypsic horizons. Calcic and gypsic horizons typically (but not everywhere) occupy different positions in the soil profile because of the difference in of calcium carbonate and gypsum, and they can normally be distinguished clearly from each other by the difference in morphology. Gypsum crystals tend to be needle-shaped, often visible to the naked eye, whereas pedogenetic calcium carbonate crystals are much finer in size.

Cambic horizon General description The cambic horizon (from Italian cambiare, to change) is a subsurface horizon showing evidence of alteration relative to the underlying horizons. Diagnostic criteria A cambic horizon: 1. has a texture in the fine earth fraction of very fine sand, loamy very fine sand1, or finer; and 2. has soil structure or absence of rock structure2 in half or more of the volume of the fine earth; and 3. shows evidence of alteration in one or more of the following: a. higher Munsell chroma (moist), higher value (moist), redder hue, or higher clay content than the underlying or an overlying layer; or b. evidence of removal of carbonates3 or gypsum; or c. presence of soil structure and absence of rock structure in the entire fine earth, if carbonates and gypsum are absent in the parent material and in the dust that falls on the soil; and 4. does not form part of a plough layer, does not consist of organic material and does not form part of an anthraquic, argic, calcic, duric, ferralic, fragic, gypsic, hortic, hydragric, irragric, mollic, natric, nitic, petrocalcic, petroduric, petrogypsic, petroplinthic, pisolithic, plaggic, plinthic, salic, sombric, spodic, umbric, terric or vertic horizon; and 5. has a thickness of 15 cm or more.

1 Very fine sand and loamy very fine sand: 50 percent or more of the fraction between 63 and 125 μm. 2 The term rock structure also applies to unconsolidated sediments in which stratification s still visible. 3 A cambic horizon always has less carbonate than an underlying horizon with calcium carbonate accumulation. However, not all primary carbonates have to be leached from a horizon in order for it to qualify as a cambic horizon. If all coarse fragments in the underlying horizon are completely coated with lime, some of these fragments in the cambic horizon are partly free of coatings. If the coarse fragments in the horizon showing calcium carbonate accumulation are coated only on the underside, those in the cambic horizon are free of coatings. Chapter 2 – Diagnostic horizons, properties and materials 17

Relationships with some other diagnostic horizons The cambic horizon can be considered the predecessor of many other diagnostic horizons. All these horizons have specific properties, such as illuvial or residual accumulations, removal of substances other than carbonates or gypsum, accumulation of soluble components, or development of specific soil structure, that are not recognized in the cambic horizon. Cambic horizons in cool and moist, freely drained soils of high plateaus and mountains in tropical and subtropical regions may occur in association with sombric horizons.

Cryic horizon General description The cryic horizon (from Greek kryos, cold, ice) is a perennially frozen soil horizon in mineral or organic materials.

Diagnostic criteria A cryic horizon has: 1. continuously for two or more consecutive years one of the following: a. massive ice, cementation by ice or readily visible ice crystals; or b. a soil temperature of 0 °C or less and insufficient water to form readily visible ice crystals; and 2. a thickness of 5 cm or more.

Field identification Cryic horizons occur in areas with permafrost1 and show evidence of perennial ice segregation, often associated with evidence of cryogenic processes (mixed soil material, disrupted soil horizons, involutions, organic intrusions, frost heave, separation of coarse from fine soil materials, cracks, patterned surface features, such as earth hummocks, frost mounds, stone circles, stripes, nets and polygons) above the cryic horizon or at the soil surface. Soils that contain saline water do not freeze at 0 °C. In order to develop a cryic horizon, such soils must be cold enough to freeze. To identify features of cryoturbation, sorting or thermal contraction, a soil profile should intersect different elements of patterned ground, if any, or be wider than 2 m. Engineers distinguish between warm and cold permafrost. Warm permafrost has a temperature higher than -2 °C and has to be considered unstable. Cold permafrost has a temperature of -2 °C or lower and can be used more safely for construction purposes provided the temperature remains under control.

Relationships with some other diagnostic horizons Cryic horizons may bear characteristics of histic, andic or spodic horizons, and may occur in association with salic, calcic, mollic or umbric horizons. In cold arid regions, yermic horizons may be found in association with cryic horizons.

Duric horizon General description The duric horizon (from Latin durus, hard) is a subsurface horizon showing weakly cemented to indurated nodules or concretions cemented by silica (SiO2), presumably in the form of opal and microcrystalline forms of silica (durinodes). Durinodes often have

1 Permafrost: layer of soil or rock, at some depth beneath the surface, in which the temperature has been continuously below 0 °C for at least some years. It exists where summer heating fails to reach the base of the layer of frozen ground. Arctic Climatology and Meteorology Glossary, National Snow and Ice Data Center, Boulder, USA (http://nsidc.org). 18 World reference base for soil resources 2006

carbonate coatings that have to be removed with HCl before slaking the durinodes with potassium hydroxide (KOH).

Diagnostic criteria A duric horizon has: 1. 10 percent or more (by volume) of weakly cemented to indurated, silica- enriched nodules (durinodes) or fragments of a broken-up petroduric horizon that show all of the following: a. when air-dry, less than 50 percent slake in 1 M HCl even after prolonged soaking, but 50 percent or more slake in concentrated KOH, concentrated NaOH or in alternating acid and alkali; and b. are firm or very firm and brittle when wet, both before and after treatment with acid; and c. have a diameter of 1 cm or more; and 2. a thickness of 10 cm or more.

Additional characteristics Dry durinodes do not slake appreciably in water, but prolonged soaking can result in the breaking-off of very thin platelets and in some slaking. In cross-section, most durinodes are roughly concentric, and concentric stringers of opal may be visible under a hand-lens.

Relationships with some other diagnostic horizons In arid regions, duric horizons occur associated with gypsic, petrogypsic, calcic and petrocalcic horizons. In more humid climates, the duric horizon may grade into fragic horizons.

Ferralic horizon General description The ferralic horizon (from Latin ferrum, iron, and alumen, alum) is a subsurface horizon resulting from long and intense weathering in which the clay fraction is dominated by low-activity clays and the silt and sand fractions by highly resistant minerals, such as (hydr)oxides of Fe, Al, Mn and titanium (Ti).

Diagnostic criteria A ferralic horizon: 1. has a sandy loam or finer particle size and less than 80 percent (by volume) gravel, stones, pisoplinthic nodules or petroplinthic gravel; and -1 1 2. has a CEC (by 1 M NH4OAc) of less than 16 cmolc kg clay and an ECEC (sum of exchangeable bases plus exchangeable acidity in 1 M KCl) of less than -1 12 cmolc kg clay; and 3. has less than 10 percent water-dispersible clay, unless it has one or both of the following: a. geric properties; or b. 1.4 percent or more organic carbon; and 4. has less than 10 percent (by grain count) weatherable minerals2 in the 0.05– 0.2 mm fraction; and 1 See Annex 1. 2 Examples of minerals that are included in the meaning of weatherable minerals are all 2:1 phyllosilicates, chlorite, sepiolites, palygorskite, allophane, 1:1 trioctahedral phyllosilicates (serpentines), feldspars, feldspathoids, ferromagnesian minerals, glass, zeolites, dolomite and apatite. The intent of the term weatherable minerals is to include those minerals that are unstable in humid climates compared with other minerals, such as quartz and 1:1 lattice clays, but that are more resistant to weathering than calcite (Soil Survey Staff 2003). Chapter 2 – Diagnostic horizons, properties and materials 19

5. does not have andic or vitric properties; and 6. has a thickness of 30 cm or more.

Field identification Ferralic horizons are associated with old and stable landforms. The macrostructure seems to be moderate to weak at first sight but typical ferralic horizons have a strong microaggregation. The consistence is usually friable; the disrupted, dry soil material flows like flour between the fingers. Lumps of ferralic horizons are usually relatively light in mass because of the low bulk density; many ferralic horizons give a hollow sound when tapped, indicating high porosity. Illuviation and stress features such as clay skins and pressure faces are generally lacking. Boundaries of a ferralic horizon are normally diffuse and little differentiation in colour or particle-size distribution within the horizon can be detected. Texture is sandy loam or finer in the fine earth fraction; gravel, stones, pisoplinthic nodules or petroplinthic gravel comprise less than 80 percent (by volume).

Additional characteristics As an alternative to the weatherable minerals requirement, a total reserve of bases (TRB = exchangeable plus mineral calcium [Ca], magnesium [Mg], potassium [K] and sodium -1 [Na]) of less than 25 cmolc kg soil may be indicative.

Relationships with some other diagnostic horizons Ferralic horizons may meet the clay increase requirements that characterize the argic horizon. If the upper 30 cm of the horizon showing a clay increase contains 10 percent or more water-dispersible clay, an argic horizon has preference over a ferralic horizon for classification purposes, unless the soil material has geric properties or more than 1.4 percent organic carbon.

Acid ammonium oxalate (pH 3) extractable Fe, Al and silicon (Alox, Feox, Siox) in ferralic horizons are very low, which sets them apart from the nitic horizons and layers with andic or vitric properties. Nitic horizons have a significant amount of active iron oxides: more than 0.2 percent acid oxalate (pH 3) extractable Fe from the fine earth fraction which, in addition, is more than 5 percent of the citrate-dithionite extractable

Fe. Vitric properties require an Alox + ½Feox content of at least 0.4 percent, and andic properties at least 2 percent. The interface with the cambic horizon is formed by the CEC/ECEC/weatherable mineral requirements. Some cambic horizons have a low CEC; however, the amount of weatherable minerals (or, alternatively, the TRB) is too high for a ferralic horizon. Such horizons represent an advanced stage of weathering and form the transition between the cambic and the ferralic horizon. Ferralic horizons in cool and moist, freely drained soils of high plateaus and mountains in tropical and subtropical regions may occur in association with sombric horizons.

Ferric horizon General description The ferric horizon (from Latin ferrum, iron) is one in which segregation of Fe, or Fe and manganese (Mn), has taken place to such an extent that large mottles or discrete nodules have formed and the intermottle/internodular matrix is largely depleted of Fe. Generally, such segregation to poor aggregation of the soil particles in Fe- depleted zones and compaction of the horizon.

Diagnostic criteria A ferric horizon has: 20 World reference base for soil resources 2006

1. one or both of the following: a. 15 percent or more of the exposed area occupied by coarse mottles with a Munsell hue redder than 7.5 YR and a chroma of more than 5, moist; or b. 5 percent or more of the volume consisting of discrete reddish to blackish nodules with a diameter of 2 mm or more, with at least the exteriors of the nodules being at least weakly cemented or indurated and the exteriors having redder hue or stronger chroma than the interiors; and 2. less than 40 percent of the volume consisting of strongly cemented or indurated nodules and an absence of continuous, fractured or broken sheets; and 3. less than 15 percent consisting of firm to weakly cemented nodules or mottles that change irreversibly to strongly cemented or indurated nodules or mottles on exposure to repeated wetting and drying with free access of oxygen; and 4. a thickness of 15 cm or more.

Relationships with some other diagnostic horizons If the amount of weakly-cemented nodules or mottles reaches 15 percent or more (by volume) and these harden irreversibly to hard nodules or a hardpan or to irregular aggregates on exposure to repeated wetting and drying with free access of oxygen, the horizon is considered to be a plinthic horizon. Therefore, ferric horizons may, in tropical or subtropical regions, grade laterally into plinthic horizons. If the amount of hard nodules reaches 40 percent or more, it is a pisoplinthic horizon.

Folic horizon General description The folic horizon (from Latin folium, leaf) is a surface horizon, or a subsurface horizon occurring at shallow depth, that consists of well-aerated organic material.

Diagnostic criteria A folic horizon consists of organic material that: a. is saturated with water for less than 30 consecutive days in most years; and b. has a thickness of 10 cm or more.

Relationships with some other diagnostic horizons Histic horizons have similar characteristics to the folic horizon; however, these are saturated with water for one month or more in most years. Moreover, the composition of the histic horizon is generally different from that of the folic horizon as the vegetative cover is often different.

Fragic horizon General description The fragic horizon (from Latin frangere, to break) is a natural non-cemented subsurface horizon with pedality and a porosity pattern such that roots and percolating water penetrate the soil only along interped faces and streaks. The natural character excludes plough pans and surface traffic pans.

Diagnostic criteria A fragic horizon: 1. shows evidence of alteration1, at least on the faces of structural units; separations between these units, which allow roots to enter, have an average horizontal spacing of 10 cm or more; and 2. contains less than 0.5 percent (by mass) organic carbon; and

1 As defined in the cambic horizon. Chapter 2 – Diagnostic horizons, properties and materials 21

3. shows in 50 percent or more of the volume slaking or fracturing of air-dry clods, 5–10 cm in diameter, within 10 minutes when placed in water; and 4. does not cement upon repeated wetting and drying; and 5. has a penetration resistance at field capacity of 50 kPa or more in 90 percent or more of the volume; and 6. does not show effervescence after adding a 10-percent HCl solution; and 7. has a thickness of 15 cm or more.

Field identification A fragic horizon has a prismatic and/or blocky structure. The inner parts of the prisms may have a relatively high total porosity (including pores larger than 200 mm) but, as a result of a dense outer rim, there is no continuity between the intraped pores and the interped pores and fissures. The result is a closed box system with 90 percent or more of the soil volume that cannot be explored by roots and is isolated from percolating water. It is essential that the required soil volume be measured from both vertical and horizontal sections; horizontal sections often reveal polygonal structures. Three or four such polygons (or a cut up to 1 m2) are sufficient to test the volumetric basis for the definition of the fragic horizon. The ped interface can have the colour, mineralogical and chemical characteristics of an eluvial or albic horizon, or meet the requirements of albeluvic tonguing. In the presence of a fluctuating water table, this part of the soil is depleted of Fe and Mn. A concomitant Fe accumulation is observed at the level of the ped surface and Mn accumulations will occur further inside the peds (stagnic colour pattern). Fragic horizons are commonly loamy, but loamy sand and clay textures are not excluded. In the latter case, the clay mineralogy is dominantly kaolinitic. Dry clods are hard to extremely hard; moist clods are firm to extremely firm; moist consistence may be brittle. A ped or clod from a fragic horizon tends to rupture suddenly under pressure rather than to undergo slow deformation. The fragic horizon has little active faunal activity except, occasionally, between the polygons.

Relationships with some other diagnostic horizons A fragic horizon may underlie, although not necessarily directly, an albic, cambic, spodic or argic horizon, unless the soil has been truncated. It can overlap partly or completely with an argic horizon. Laterally, fragic horizons may grade into (petro-) duric horizons in dry regions. Moreover, fragic horizons can have reducing conditions and a stagnic colour pattern.

Fulvic horizon General description The fulvic horizon (from Latin fulvus, dark yellow) is a thick, dark-brown horizon at or near to the surface that is typically associated with short-range-order minerals (commonly allophane) or with organo-aluminium complexes. It has a low bulk density and contains highly humified organic matter that shows a lower ratio of humic acids to fulvic acids compared with the melanic horizon.

Diagnostic criteria A fulvic horizon has: 1. andic properties; and 2. one or both of the following: a. Munsell colour value or chroma (moist) of more than 2; or 22 World reference base for soil resources 2006

b. melanic index1 of 1.70 or more; and 3. a weighted average of 6 percent or more organic carbon, and 4 percent or more organic carbon in all parts; and 4. a cumulative thickness of 30 cm or more with less than 10 cm non-fulvic material in between.

Field identification When dark brown, the fulvic horizon is easily identifiable by its colour, thickness, as well as its typical, although not exclusive2, association with pyroclastic deposits. Distinction between the blackish coloured fulvic and melanic horizons is made after determining the melanic index, which requires laboratory analyses.

Gypsic horizon General description The gypsic horizon (from Greek gypsos) is a commonly non-cemented horizon

containing secondary accumulations of gypsum (CaSO4.2H2O) in various forms. If the accumulation of gypsum becomes such that all or most of the pedological and/or lithological structures disappear and continuous concentrations of gypsum prevail, a hypergypsic qualifier is used.

Diagnostic criteria A gypsic horizon has: 1. 5 percent3 or more gypsum and 1 percent or more (by volume) of visible secondary gypsum; and 2. a product of thickness (in centimetres) times gypsum content (percentage) of 150 or more; and 3. a thickness of 15 cm or more.

Field identification Gypsum occurs as pseudomycelia, as coarse crystals, as nests, beards or coatings, as elongated groupings of fibrous crystals, or as powdery accumulations. The last form gives the gypsic horizon a massive structure. The distinction between compact powdery accumulations and the others is important in terms of soil capability. Gypsum crystals may be mistaken for quartz. Gypsum is soft and can easily be broken between thumbnail and forefinger. Quartz is hard and cannot be broken except by hammering. Gypsic horizons may be associated with calcic horizons but usually occur in separate positions within the soil profile, because of the higher solubility of gypsum compared with lime.

Additional characteristics Determination of the amount of gypsum in the soil to verify the required content and increase, as well as thin section analysis, is helpful to establish the presence of a gypsic horizon and the distribution of the gypsum in the soil mass.

Relationships with some other diagnostic horizons When gypsic horizons become indurated, transition takes place to the petrogypsic horizon, the expression of which may be as massive or platy structures.

1 See Annex 1. 2 Fulvic horizons may also be found in aluandic-type of soils derived from other material than pyroclastics. 3 -1 The percentage gypsum is calculated as the product of gypsum content, expressed as cmolc kg soil, and the equivalent mass of gypsum (86) expressed as a percentage. Chapter 2 – Diagnostic horizons, properties and materials 23

In dry regions, gypsic horizons are associated with calcic or salic horizons. Calcic and gypsic horizons usually occupy distinct positions in the soil profile as the solubility of calcium carbonate is different from that of gypsum. They normally can be distinguished clearly from each other by the morphology (see calcic horizon). Salic and gypsic horizons also occupy different positions for the same reasons.

Histic horizon General description The histic horizon (from Greek histos, tissue) is a surface horizon, or a subsurface horizon occurring at shallow depth, that consists of poorly aerated organic material.

Diagnostic criteria A histic horizon consists of organic material that: 1. is saturated with water for 30 consecutive days or more in most years (unless drained); and 2. has a thickness of 10 cm or more. If the layer with organic material is less than 20 cm thick, the upper 20 cm of the soil after mixing, or if continuous rock is present within 20 cm depth, the entire soil above after mixing, must contain 20 percent or more organic carbon.

Relationships with some other diagnostic horizons The folic horizon has similar characteristics to the histic horizon; however, the folic horizon is saturated with water for less than one month in most years. Moreover, the composition of the histic horizon is generally different from that of the folic horizon as the vegetative cover is often different. The lower limit of organic carbon content, varying from 12 percent (20 percent organic matter) to 18 percent organic carbon (30 percent organic matter), sets the histic horizon apart from mollic or umbric horizons, which have these contents as upper limits. Histic horizons with less than 25 percent organic carbon may have andic or vitric properties.

Hortic horizon General description A hortic horizon (from Latin hortus, garden) is a human-induced mineral surface horizon that results from deep cultivation, intensive fertilization and/or long- continued application of human and animal wastes and other organic residues (e.g. manures, kitchen refuse, compost and night soil).

Diagnostic criteria A hortic horizon is a mineral surface horizon and has: 1. a Munsell colour value and chroma (moist) of 3 or less; and 2. a weighted average organic carbon content of 1 percent or more; and 1 -1 3. a 0.5 M NaHCO3 extractable P2O5 content of 100 mg kg fine earth or more in the upper 25 cm2; and

4. a base saturation (by 1 M NH4OAc) of 50 percent or more; and 5. 25 percent (by volume) or more of animal pores, coprolites or other traces of soil animal activity; and 6. a thickness of 20 cm or more.

1 Known as the Olsen routine method (Olsen et al., 1954). 2 Gong et al., 1997. 24 World reference base for soil resources 2006

Field identification The hortic horizon is thoroughly mixed. Potsherds and other artefacts are common although often abraded. Tillage marks or evidence of mixing of the soil can be present.

Relationships with some other diagnostic horizons Hortic horizons closely resemble mollic horizons. Therefore, the human influence must be clearly established in order to separate the two diagnostic horizons.

Hydragric horizon General description A hydragric horizon (from Greek hydor, water, and Latin ager, field) is a human- induced subsurface horizon associated with wet cultivation.

Diagnostic criteria A hydragric horizon is associated with wet cultivation and has: 1. one or more of the following: a. Fe or Mn coatings or Fe or Mn concretions; or b. dithionite-citrate extractable Fe 2 times or more, or dithionite-citrate extractable Mn 4 times or more that of the surface horizon; or c. redox depleted zones with a Munsell colour value 4 or more and a chroma of 2 or less (both moist) in macropores; and 2. a thickness of 10 cm or more.

Field identification The hydragric horizon occurs below the puddled layer and the plough pan. It has either reduction features in pores, such as coatings or halos with a colour hue of 2.5 Y or yellower and a chroma (moist) of 2 or less, or segregations of Fe and/or Mn in the matrix as a result of the oxidative environment. It usually shows grey clay-fine silt and clay-silt-humus cutans on ped faces.

Irragric horizon General description The irragric horizon (from Latin irrigare, to irrigate, and ager, field) is a human- induced mineral surface horizon that builds up gradually through continuous application of irrigation water with substantial amounts of sediments, and which may include fertilizers, soluble salts, organic matter, etc.

Diagnostic criteria An irragric horizon is a mineral surface horizon and has: 1. a uniformly structured surface layer; and 2. a higher clay content, particularly fine clay, than the underlying original soil; and 3. relative differences among medium, fine and very fine sand, clay and carbonates less than 20 percent among parts within the horizon; and 4. a weighted average organic carbon content of 0.5 percent or more, decreasing with depth but remaining at 0.3 percent or more at the lower limit of the irragric horizon; and 5. 25 percent (by volume) or more of animal pores, coprolites or other traces of soil animal activity; and 6. a thickness of 20 cm or more. Chapter 2 – Diagnostic horizons, properties and materials 25

Field identification Soils with an irragric horizon show evidence of surface raising, which may be inferred either from field observation or from historical records. The irragric horizon shows evidence of considerable biological activity. The lower boundary is clear and irrigation deposits or buried soils may be present below.

Relationships with some other diagnostic horizons Irragric horizons differ from fluvic materials in lacking evidence of stratification owing to continuous ploughing.

Melanic horizon General description The melanic horizon (from Greek melas, black) is a thick, black horizon at or near the surface, which is typically associated with short-range-order minerals (commonly allophane) or with organo-aluminium complexes. It has a low bulk density and contains highly humified organic matter that shows a lower ratio of fulvic acids to humic acids compared with the fulvic horizon.

Diagnostic criteria A melanic horizon has: 1. andic properties; and 2. a Munsell colour value and chroma (both moist) of 2 or less, and 3. a melanic index1 of less than 1.70; and 4. a weighted average of 6 percent or more organic carbon, and 4 percent or more organic carbon in all parts; and 5. a cumulative thickness of 30 cm or more with less than 10 cm non-melanic material in between.

Field identification The intense dark colour, its thickness, as well as its common association with pyroclastic deposits help to recognize the melanic horizon in the field. However, laboratory analyses to determine the type of organic matter may be necessary to identify the melanic horizon unambiguously.

Mollic horizon General description The mollic horizon (from Latin mollis, soft) is a well-structured, dark-coloured surface horizon with a high base saturation and a moderate to high content of organic matter.

Diagnostic criteria A mollic horizon, after mixing either the upper 20 cm of the mineral soil or, if continuous rock, a cryic, petrocalcic, petroduric, petrogypsic or petroplinthic horizon is present within 20 cm of the mineral soil surface, the entire mineral soil above, has: 1. a soil structure sufficiently strong that the horizon is not both massive and hard or very hard when dry in both the mixed part and the underlying unmixed part if the minimum thickness is larger than 20 cm (prisms larger than 30 cm in diameter are included in the meaning of massive if there is no secondary structure within the prisms); and 2. Munsell colours with a chroma of 3 or less when moist, a value of 3 or less when moist and 5 or less when dry on broken samples in both the mixed part and the underlying unmixed part if the minimum thickness is greater than 20 cm. If

1 See Annex 1. 26 World reference base for soil resources 2006

there is 40 percent or more finely divided lime, the limits of dry colour value are waived; the colour value, moist, is 5 or less. The colour value is one unit or more darker than that of the parent material (both moist and dry), unless the parent material has a colour value of 4 or less, moist, in which case the colour contrast requirement is waived. If a parent material is not present, comparison must be made with the layer immediately underlying the surface layer; and 3. an organic carbon content of 0.6 percent or more in both the mixed part and the underlying unmixed part if the minimum thickness is larger than 20 cm. The organic carbon content is 2.5 percent or more if the colour requirements are waived because of finely divided lime, or 0.6 percent more than in the parent material if the colour requirements are waived because of dark coloured parent materials; and

4. a base saturation (by 1 M NH4OAc) of 50 percent or more on a weighted average throughout the depth of the horizon; and 5. a thickness of one of the following: a. 10 cm or more if directly overlying continuous rock, or a cryic, petrocalcic, petroduric, petrogypsic or petroplinthic horizon; or b. 20 cm or more and one-third or more of the thickness between the soil surface and the upper boundary of continuous rock, or a calcic, cryic, gypsic, petrocalcic, petroduric, petrogypsic, petroplinthic or salic horizon or calcaric, fluvic or gypsyric material within 75 cm; or c. 20 cm or more and one-third or more of the thickness between the soil surface and the lower boundary of the lowest diagnostic horizon within 75 cm and, if present, above any of the diagnostic horizons listed under b.; or d. 25 cm or more.

Field identification A mollic horizon may easily be identified by its dark colour, caused by the accumulation of organic matter, well-developed structure (usually a granular or fine

subangular blocky structure), an indication of high base saturation (e.g. pHwater > 6), and its thickness.

Relationships with some other diagnostic horizons The base saturation of 50 percent separates the mollic horizon from the umbric horizon, which is otherwise similar. The upper limit of organic carbon content varies from 12 percent (20 percent organic matter) to 18 percent organic carbon (30 percent organic matter), which is the lower limit for the histic horizon, or 20 percent, the lower limit for a folic horizon. A special type of mollic horizon is the voronic horizon. It has a higher organic carbon content (1.5 percent or more), a specific structure (granular or fine subangular blocky), a very dark colour in its upper part, a high biological activity, and a minimum thickness of 35 cm.

Natric horizon General description The natric horizon (from Arabic natroon, salt) is a dense subsurface horizon with distinct higher clay content than the overlying horizon or horizons. It has a high content in exchangeable Na and/or Mg.

Diagnostic criteria A natric horizon: 1. has a texture of loamy sand or finer and 8 percent or more clay in the fine earth fraction; and Chapter 2 – Diagnostic horizons, properties and materials 27

2. one or both of the following: a. has, if an overlying coarser textured horizon is present that is not ploughed and not separated from the natric horizon by a lithological discontinuity, more clay than this overlying horizon such that: i. if the overlying horizon has less than 15 percent clay in the fine earth fraction, the natric horizon must contain at least 3 percent more clay; or ii. if the overlying horizon has 15 percent or more and less than 40 percent clay in the fine earth fraction, the ratio of clay in the natric horizon to that of the overlying horizon must be 1.2 or more; or iii. if the overlying horizon has 40 percent or more clay in the fine earth fraction, the natric horizon must contain at least 8 percent more clay; or b. has evidence of clay illuviation in one or more of the following forms: i. oriented clay bridging the sand grains; or ii. clay films lining pores; or iii. clay films on both vertical and horizontal surfaces of soil aggregates; or iv. in thin sections, oriented clay bodies that constitute 1 percent or more of the section; or v. a COLE of 0.04 or higher, and a ratio of fine clay1 to total clay in the natric horizon greater by 1.2 times or more than the ratio in the overlying coarser textured horizon; and 3. has, if an overlying coarser textured horizon is present that is not ploughed and not separated from the natric horizon by a lithological discontinuity, an increase in clay content within a vertical distance of 30 cm; and 4. has one or more of the following: a. a columnar or prismatic structure in some part of the horizon; or b. a blocky structure with tongues of an overlying coarser textured horizon in which there are uncoated silt or sand grains, extending 2.5 cm or more into the natric horizon; or c. a massive appearance; and 5. has an exchangeable Na percentage (ESP2) of 15 or more within the upper 40 cm, or more exchangeable Mg plus Na than Ca plus exchange acidity (at pH 8.2) within the same depth if the saturation with exchangeable Na is 15 percent or more in some subhorizon within 200 cm of the soil surface; and 6. has a thickness of one-tenth or more of the sum of the thicknesses of all overlying horizons, if present, and one of the following: a. 7.5 cm or more, if it is not entirely composed of lamellae (that are 0.5 cm or more thick) and the texture is finer than loamy sand; or b. 15 cm or more (combined thickness, if composed entirely of lamellae that are 0.5 cm or more thick).

Field identification The colour of the natric horizon ranges from brown to black, especially in the upper part. The structure is coarse columnar or prismatic, sometimes blocky or massive. Rounded and often whitish tops of the structural elements are characteristic. Both colour and structural characteristics depend on the composition of the exchangeable cations and the soluble salt content in the underlying layers. Often, thick and dark-coloured clay coatings occur, especially in the upper part of the horizon. Natric horizons have a poor aggregate stability and very low permeability under wet conditions. When dry, the natric horizon becomes hard to extremely hard. Soil reaction is strongly alkaline; pH (H2O) is more than 8.5.

1 Fine clay: < 0.2 µm equivalent diameter. 2 ESP = exchangeable Na × 100/CEC (at pH 7). 28 World reference base for soil resources 2006

Additional characteristics

Natric horizons are characterized by a high pH (H2O), which is frequently more than 9.0. Another measure to characterize the natric horizon is the sodium adsorption ratio (SAR), which has to be 13 or more. The SAR is calculated from soil solution data (Na+, 2+ 2+ + 2+ 2+ 0.5 Ca , Mg given in mmolc/litre): SAR = Na /[(Ca + Mg )/2] . Micromorphologically, natric horizons show a specific fabric. The peptized plasma shows a strong orientation in a mosaic or parallel-striated pattern. The plasma separations also show a high content in associated humus. Microcrusts, cutans, papules and infillings appear when the natric horizon is impermeable.

Relationships with some other diagnostic horizons A surface horizon usually rich in organic matter overlies the natric horizon. This horizon of humus accumulation varies in thickness from a few centimetres to more than 25 cm, and may be a mollic horizon. An albic horizon may be present between the surface and the natric horizon. Frequently, a salt-affected layer occurs below the natric horizon. The salt influence may extend into the natric horizon, which besides being sodic then also becomes saline. Salts present may be chlorides, sulphates or carbonates/bicarbonates.

The humus-illuvial part of natric horizons has a base saturation (by 1 M NH4OAc) of more than 50 percent, which separates it from the sombric horizon.

Nitic horizon General description The nitic horizon (from Latin nitidus, shiny) is a clay-rich subsurface horizon. It has moderately to strongly developed polyhedric or nutty structure with many shiny ped faces, which cannot or can only partially be attributed to clay illuviation.

Diagnostic criteria A nitic horizon has: 1. less than 20 percent change (relative) in clay content over 12 cm to layers immediately above and below; and 2. all of the following: a. 30 percent or more clay; and b. a water-dispersible clay to total clay ratio less than 0.10; and c. a silt to clay ratio less than 0.40; and 3. moderate to strong, angular blocky structure breaking to flat-edged or nut- shaped elements with shiny ped faces. The shiny faces are not, or are only partially, associated with clay coatings; and 4. all of the following: a. 4.0 percent or more citrate-dithionite extractable Fe (free iron) in the fine earth fraction; and b. 0.20 percent or more acid oxalate (pH 3) extractable Fe (active iron) in the fine earth fraction; and c. a ratio between active and free iron of 0.05 or more; and 5. a thickness of 30 cm or more.

Field identification A nitic horizon has a clay loam or finer texture but feels loamy. The change in clay content with the overlying and underlying horizons is gradual. Similarly, there is no abrupt colour change with the horizons above and below. The colours are of low value and chroma with hues often 2.5 YR, but sometimes redder or yellower. The structure is moderate to strong angular blocky, breaking to flat-edged or nut-shaped elements showing shiny faces. Chapter 2 – Diagnostic horizons, properties and materials 29

Additional characteristics -1 In many nitic horizons, the CEC (by 1 M NH4OAc) is less than 36 cmolc kg clay, -1 1 or even less than 24 cmolc kg clay . The ECEC (sum of exchangeable bases plus exchangeable acidity in 1 M KCl) is about half of the CEC. The moderate to low CEC and ECEC reflect the dominance of 1:1 lattice clays (either kaolinite and/or [meta]).

Relationships with some other diagnostic horizons The nitic horizon may be considered as a special type of argic horizon, or a strongly expressed cambic horizon, with specific properties such as a low amount of water- dispersible clay and a high amount of active iron. As such, the nitic horizon has preference over both for classification purposes. Its mineralogy (kaolinitic/ [meta]halloysitic) sets it apart from most vertic horizons, which have dominantly a smectitic mineralogy. However, nitic horizons may grade laterally into vertic horizons in lower landscape positions. The well-expressed soil structure, the high amount of active iron, and the frequently intermediate CEC in nitic horizons set them apart from ferralic horizons. Nitic horizons in cool and moist, freely drained soils of high plateaus and mountains in tropical and subtropical regions may occur in association with sombric horizons.

Petrocalcic horizon General description A petrocalcic horizon (from Greek petros, rock, and Latin calx, lime) is an indurated calcic horizon that is cemented by calcium carbonate and, in places, by calcium and some magnesium carbonate. It is either massive or platy in nature, and extremely hard.

Diagnostic criteria A petrocalcic horizon has: 1. very strong effervescence after adding a 10-percent HCl solution; and 2. induration or cementation, at least partially by secondary carbonates, to the extent that air-dry fragments do not slake in water and roots cannot enter except along vertical fractures (which have an average horizontal spacing of 10 cm or more and which occupy less than 20 percent [by volume] of the layer); and 3. extremely hard consistence when dry, so that it cannot be penetrated by spade or auger; and 4. a thickness of 10 cm or more, or 1 cm or more if it is laminar and rests directly on continuous rock.

Field identification Petrocalcic horizons occur as non-platy calcrete (either massive or nodular) or as platy calcrete, of which the following types are the most common: ÿ Lamellar calcrete: superimposed, separate, petrified layers varying in thickness from a few millimetres to several centimetres. The colour is generally white or pink. ÿ Petrified lamellar calcrete: one or several extremely hard layers, grey or pink in colour. They are generally more cemented than the lamellar calcrete and very massive (no fine lamellar structures, but coarse lamellar structures may be present). Non-capillary pores in petrocalcic horizons are filled, and the hydraulic conductivity is moderately slow to very slow.

1 See Annex 1. 30 World reference base for soil resources 2006

Relationships with some other diagnostic horizons In arid regions, petrocalcic horizons may occur in association with (petro-) duric horizons, into which they may grade laterally. The cementing agent differentiates petrocalcic and duric horizons. In petrocalcic horizons, calcium and some magnesium carbonate constitute the main cementing agent while some accessory silica may be present. In duric horizons, silica is the main cementing agent, with or without calcium carbonate. Petrocalcic horizons also occur in association with gypsic or petrogypsic horizons.

Petroduric horizon General description A petroduric horizon (from Greek petros, rock, and Latin durus, hard), also known as duripan or dorbank (South Africa), is a subsurface horizon, usually reddish or reddish

brown in colour, that is cemented mainly by secondary silica (SiO2, presumably opal and microcrystalline forms of silica). Air-dry fragments of petroduric horizons do not slake in water, even after prolonged wetting. Calcium carbonate may be present as accessory cementing agent.

Diagnostic criteria A petroduric horizon has: 1. induration or cementation in 50 percent or more of some subhorizon; and 2. evidence of silica accumulation (opal or other forms of silica), e.g. as coatings in some pores, on some structural faces or as bridges between sand grains; and 3. when air-dry, less than 50 percent slakes in 1 M HCl even after prolonged soaking, but 50 percent or more slake in concentrated KOH, concentrated NaOH or in alternating acid and alkali; and 4. a lateral continuity such that roots cannot penetrate except along vertical fractures (which have an average horizontal spacing of 10 cm or more and which occupy less than 20 percent [by volume] of the layer); and 5. a thickness of 1 cm or more.

Field identification A petroduric horizon has a very to extremely firm consistence when moist, and is very or extremely hard when dry. Effervescence after applying 1 M HCl may take place, but is probably not as vigorous as in petrocalcic horizons, which appear similar. However, it may occur in conjunction with a petrocalcic horizon.

Relationships with some other diagnostic horizons In dry and arid climates, petroduric horizons may grade laterally into petrocalcic horizons, and/or occur in conjunction with calcic or gypsic horizons, which they normally overlie. In more humid climates, petroduric horizons may grade laterally into fragic horizons.

Petrogypsic horizon General description A petrogypsic horizon (from Greek petros, rock, and gypsos) is a cemented horizon

containing secondary accumulations of gypsum (CaSO4.2H2O).

Diagnostic criteria A petrogypsic horizon has: 1. 5 percent1 or more gypsum and 1 percent or more (by volume) visible secondary gypsum; and

1 -1 The percentage gypsum is calculated as the product of gypsum content, expressed as cmolc kg soil, and the equivalent mass of gypsum (86) expressed as a percentage. Chapter 2 – Diagnostic horizons, properties and materials 31

2. induration or cementation by secondary gypsum, at least partially, to the extent that air-dry fragments do not slake in water and that it cannot be penetrated by roots except along vertical fractures (which have an average horizontal spacing of 10 cm or more and which occupy less than 20 percent [by volume] of the layer); and 3. a thickness of 10 cm or more.

Field identification Petrogypsic horizons are hard, whitish and composed predominantly of gypsum. They may be capped by a thin, laminar layer about 1 cm thick.

Additional characteristics Determinations of the amount of gypsum in the soil to verify the required content and increase, as well as thin section analysis, are helpful techniques to establish the presence of a petrogypsic horizon and the distribution of the gypsum in the soil mass. In thin sections, the petrogypsic horizon shows a compacted microstructure with only a few cavities. The matrix is composed of densely packed lenticular gypsum crystals mixed with small amounts of detrital material. The matrix has a faint yellow colour in plain light. Irregular nodules formed by colourless transparent zones consist of coherent crystal aggregates with a hypidiotopic or xenotopic fabric and are mostly associated with pores or former pores. Traces of biological activity (pedotubules) are sometimes visible.

Relationships with some other diagnostic horizons As the petrogypsic horizon develops from a gypsic horizon, the two are closely linked. Petrogypsic horizons frequently occur associated with calcic horizons. Calcic and gypsic accumulations usually occupy different positions in the soil profile because the solubility of calcium carbonate is different from that of gypsum. Normally, they can be distinguished clearly from each other by their morphology (see calcic horizon).

Petroplinthic horizon General description A petroplinthic horizon (from Greek petros, rock, and plinthos, brick) is a continuous, fractured or broken layer of indurated material, in which Fe (and in cases also Mn) is an important cement and in which organic matter is either absent or present only in traces.

Diagnostic criteria A petroplinthic horizon: 1. is a continuous, fractured or broken sheet of connected, strongly cemented to indurated a. reddish to blackish nodules; or b. reddish, yellowish to blackish mottles in platy, polygonal or reticulate patterns; and 2. has a penetration resistance1 of 4.5 MPa or more in 50 percent or more of the volume; and 3. has a ratio between acid oxalate (pH 3) extractable Fe and citrate-dithionite extractable Fe of less than 0.102; and 4. has a thickness of 10 cm or more.

1 Asiamah (2000). From this point onwards, the horizon will start hardening irreversibly. 2 Estimated from data given by Varghese and Byju (1993). 32 World reference base for soil resources 2006

Field identification Petroplinthic horizons are extremely hard; typically rusty brown to yellowish brown; either massive, or show an interconnected nodular, or a reticulate, platy or columnar pattern that encloses non-indurated material. They may be fractured or broken.

Relationships with some other diagnostic horizons Petroplinthic horizons are closely associated with plinthic horizons from which they develop. In some places, plinthic horizons can be traced by following petroplinthic layers, which have formed, for example, in road cuts. The low ratio between acid oxalate (pH 3) extractable Fe and citrate-dithionite extractable Fe separates the petroplinthic horizon from thin iron pans, bog iron and indurated spodic horizons as occurring in, for example, Podzols, which in addition contain a fair amount of organic matter.

Pisoplinthic horizon General description A pisoplinthic horizon (from Latin pisum, pea, and Greek plinthos, brick) contains nodules that are strongly cemented to indurated with Fe (and in some cases also with Mn).

Diagnostic criteria A pisoplinthic horizon has: 1. 40 percent or more of the volume occupied by discrete, strongly cemented to indurated, reddish to blackish nodules with a diameter of 2 mm or more; and 2. a thickness of 15 cm or more.

Relationships with some other diagnostic horizons A pisoplinthic horizon results if a plinthic horizon hardens in the form of discrete nodules. The hardness and the amount of the nodules separate it also from the ferric horizon.

Plaggic horizon General description A plaggic horizon (from Dutch plag, sod) is a black or brown human-induced mineral surface horizon that has been produced by long-continued manuring. In medieval times, sod and other materials were commonly used for bedding livestock and the manure was spread on fields being cultivated. The mineral materials brought in by this kind of manuring eventually produced an appreciably thickened horizon (in places as much as 100 cm or more thick) that is rich in organic carbon. Base saturation is typically low.

Diagnostic criteria A plaggic horizon is a mineral surface horizon and: 1. has a texture of sand, loamy sand, sandy loam or loam, or a combination of them; and 2. contains artefacts, but less than 20 percent, or has spade marks below 30 cm depth; and 3. has Munsell colours with a value of 4 or less, moist, 5 or less, dry, and a chroma of 2 or less, moist; and 4. has an organic carbon content of 0.6 percent or more; and 5. occurs in locally raised land surfaces; and 6. has a thickness of 20 cm or more. Chapter 2 – Diagnostic horizons, properties and materials 33

Field identification The plaggic horizon has brownish or blackish colours, related to the origin of source materials. Its reaction is slightly to strongly acid. It shows evidence of agricultural operations such as spade marks as well as old cultivation layers. Plaggic horizons commonly overlie buried soils although the original surface layers may be mixed. The lower boundary is typically clear.

Additional characteristics The texture is in most cases sand or loamy sand. Sandy loam and silt loam are rare.

The P2O5 content (extractable in 1-percent citric acid) in plaggic horizons may be high, often more than 0.25 percent within 20 cm of the surface, but frequently more than 1 percent. Owing to the abandonment of the practice, phosphate contents may have lowered considerably, and can no longer be seen as diagnostic for the plaggic horizon.

Relationships with some other diagnostic horizons Few soil characteristics differentiate the terric and plaggic horizons from each other. Terric horizons usually show a high biological activity, have a neutral to slightly alkaline soil reaction (pH [H2O] is normally more than 7.0), and may contain free lime. The colour is strongly related to the source material or the underlying substrate. Buried soils may be observed at the base of the horizon although mixing can obscure the contact. The plaggic horizon has many characteristics in common with umbric horizons, and evidence of human influences, such as the spade marks or surface raising, is often required to distinguish between the two.

Plinthic horizon General description A plinthic horizon (from Greek plinthos, brick) is a subsurface horizon that consists of an Fe-rich (in some cases also Mn-rich), humus-poor mixture of kaolinitic clay (and other products of strong weathering, such as gibbsite) with quartz and other constituents, and which changes irreversibly to a layer with hard nodules, a hardpan or irregular aggregates on exposure to repeated wetting and drying with free access of oxygen.

Diagnostic criteria A plinthic horizon has: 1. within 15 percent or more of the volume single or in combination: a. discrete nodules that are firm to weakly cemented, with a redder hue or stronger chroma than the surrounding material, and which change irreversibly to strongly cemented or indurated nodules on exposure to repeated wetting and drying with free access of oxygen; or b. mottles in platy, polygonal or reticulate patterns that are firm to weakly cemented, with a redder hue or stronger chroma than the surrounding material, and which change irreversibly to strongly cemented or indurated mottles on exposure to repeated wetting and drying with free access of oxygen; and 2. less than 40 percent of the volume strongly cemented or indurated nodules and no continuous, fractured or broken sheets; and 3. both: a. 2.5 percent (by mass) or more citrate-dithionite extractable Fe in the fine earth fraction or 10 percent or more in the nodules or mottles; and b. a ratio between acid oxalate (pH 3) extractable Fe and citrate-dithionite extractable Fe of less than 0.101; and 4. a thickness of 15 cm or more.

1 Estimated from data given by Varghese and Byju (1993). 34 World reference base for soil resources 2006

Field identification A plinthic horizon shows red nodules or mottles in platy, polygonal, vesicular or reticulate patterns. In a perennially moist soil, many nodules or mottles are not hard but firm or very firm and can be cut with a spade. They do not harden irreversibly as a result of a single cycle of drying and rewetting but repeated wetting and drying will change them irreversibly to hard nodules or a hardpan (ironstone) or irregular aggregates, especially if also exposed to heat from the sun.

Additional characteristics Micromorphological studies may reveal the extent of impregnation of the soil mass by Fe. The plinthic horizon with nodules has developed under redoximorphic conditions caused by temporally stagnating water and shows a stagnic colour pattern. The plinthic horizon with mottles in platy, polygonal or reticulate patterns has developed under oximorphic conditions in the capillary fringe of groundwater. In this case, the plinthic horizon shows a gleyic colour pattern with oximorphic colours and is in many cases underlain by a whitish horizon. In many plinthic horizons, there are no prolonged reducing conditions.

Relationships with some other diagnostic horizons If the plinthic horizon hardens to a continuous sheet (which later may be broken or fractured), it becomes a petroplinthic horizon. If nodules reach 40 percent or more of the volume and harden separately, it becomes a pisoplinthic horizon. If the nodules or mottles that harden on exposure to repeated wetting and drying do not reach 15 percent of the volume, it may be a ferric horizon if it has 5 percent or more nodules or 15 percent or more mottles fulfilling certain additional requirements.

Salic horizon General description The salic horizon (from Latin sal, salt) is a surface or shallow subsurface horizon that contains a secondary enrichment of readily soluble salts, i.e. salts more soluble than

gypsum (CaSO4.2H2O; log Ks = -4.85 at 25 °C).

Diagnostic criteria A salic horizon has:

1. averaged over its depth an electrical conductivity of the saturation extract (ECe) -1 -1 of 15 dS m or more at 25 °C at some time of the year, or an ECe of 8 dS m or

more at 25 °C if the pH (H2O) of the saturation extract is 8.5 or more; and - 2. averaged over its depth a product of thickness (in centimetres) and ECe (in dS m 1) of 450 or more; and 3. a thickness of 15 cm or more.

Field identification Salicornica or halophyte vegetation such as Tamarix and salt-tolerant crops are first indicators. Salt-affected layers are often puffy. Salts precipitate only after evaporation of the soil moisture; if the soil is moist, salt may not be visible. Salts may precipitate at the surface (external Solonchaks) or at depth (internal Solonchaks). A salt crust is part of the salic horizon.

Additional characteristics -1 An ECe of 8 dS m or more at 25 °C if the pH (H2O) of the saturation extract is 8.5 or more is very common in alkaline carbonate soils. Chapter 2 – Diagnostic horizons, properties and materials 35

Sombric horizon General description A sombric horizon (from French sombre, dark) is a dark-coloured subsurface horizon containing illuvial humus that is neither associated with Al nor dispersed by Na.

Diagnostic criteria A sombric horizon: 1. has a lower Munsell colour value or chroma than the overlying horizon; and

2. has a base saturation (by 1 M NH4OAc) less than 50 percent; and 3. shows evidence of humus accumulation, by a higher organic carbon content with respect to the overlying horizon, or through illuvial humus on ped surfaces or in pores visible in thin sections; and 4. does not underlie an albic horizon; and 5. has a thickness of 15 cm or more.

Field identification Dark-coloured subsoils, associated with cool and moist, well-drained soils of high plateaus and mountains in tropical and subtropical regions. They resemble buried horizons but, in contrast to many of these, sombric horizons more or less follow the shape of the surface.

Relationships with some other diagnostic horizons Sombric horizons may form or have been formed in argic, cambic, ferralic or nitic horizons. Umbric horizons, as well as the dark-coloured melanic and fulvic horizons of Andosols, form at the surface and, as such, are different from sombric horizons. Spodic horizons are differentiated from sombric horizons by their much higher CEC of the clay fraction. The humus-illuvial part of natric horizons has a base saturation

(by 1 M NH4OAc) of more than 50 percent, which separates it from the sombric horizon.

Spodic horizon General description The spodic horizon (from Greek spodos, wood ash) is a subsurface horizon that contains illuvial amorphous substances composed of organic matter and Al, or of illuvial Fe. The illuvial materials are characterized by a high pH-dependent charge, a relatively large surface area and high water retention.

Diagnostic criteria A spodic horizon: 1. has a pH (1:1 in water) of less than 5.9 in 85 percent or more of the horizon, unless the soil is cultivated; and 2. has an organic carbon content of 0.5 percent or more or an optical density of the oxalate extract (ODOE) value of 0.25 or more, at least in some part of the horizon; and 3. has one or both of the following: a. an albic horizon directly overlying the spodic horizon and has, directly under the albic horizon, one of the following Munsell colours, when moist (crushed and smoothed sample): i. a hue of 5 YR or redder; or ii. a hue of 7.5 YR with a value of 5 or less and a chroma of 4 or less; or iii. a hue of 10 YR or neutral and a value and a chroma of 2 or less; or iv. a colour of 10 YR 3/1; or 36 World reference base for soil resources 2006

b. with or without an albic horizon, one of the colours listed above, or a hue of 7.5 YR, a value of 5 or less and chroma of 5 or 6, both when moist (crushed and smoothed sample), and one or more of the following: i. cementation by organic matter and Al with or without Fe, in 50 percent or more of the volume and a very firm or firmer consistency in the cemented part; or ii. cracked coatings on sand grains covering 10 percent or more of the surface of the horizon; or 1 iii. 0.50 percent or more Alox + ½Feox and a value less than one-half that amount in an overlying mineral horizon; or iv. an ODOE value of 0.25 or more, and a value less than one-half that amount in an overlying mineral horizon; or v. 10 percent or more (by volume) Fe lamellae2 in a layer 25 cm or more thick. c. does not form part of a natric horizon; and d. has a thickness of 2.5 cm or more.

Field identification A spodic horizon normally underlies an albic horizon and has brownish-black to reddish-brown colours. Spodic horizons can also be characterized by the presence of a thin iron pan, when weakly developed by the presence of organic pellets, or by the accumulation of Fe in lamellar form.

Relationships with some other diagnostic horizons Spodic horizons are usually associated with albic horizons, which they underlie; there may be an anthric, hortic, plaggic, terric or umbric horizons at the surface. Spodic horizons may exhibit andic properties owing to the alumino-organic

complexes. Spodic horizons have at least twice as much the Alox + ½Feox percentages as overlying layers, such as an albic, anthric, hortic, plaggic, terric or umbric horizon. This criterion does not normally apply to non-spodic layers with andic properties in which the alumino-organic complexes are hardly mobile. Similar to many spodic horizons, sombric horizons also contain more organic matter than an overlying layer. They can be differentiated from each other by the clay mineralogy (kaolinite usually dominating in sombric horizons, whereas the clay fraction of spodic horizons commonly contains significant amounts of vermiculite and Al-interlayered chlorite) and the much higher CEC of the clay fraction in spodic horizons. Similarly, plinthic horizons, which contain large amounts of illuvial Fe, are dominated by kaolinitic clay minerals and, therefore, have a much lower CEC of the clay fraction than that of spodic horizons.

Takyric horizon General description A takyric horizon (from Turkic languages takyr, barren land) is a heavy-textured surface horizon comprising a surface crust and a platy structured lower part. It occurs under arid conditions in periodically flooded soils.

Diagnostic criteria A takyric horizon has: 1. aridic properties; and

1 Alox and Feox: acid oxalate-extractable aluminium and iron, respectively (Blakemore, Searle and Daly, 1981), expressed as percent of the fine earth (0–2 mm) fraction on an oven-dried (105 °C) basis. 2 Iron lamellae are non-cemented bands of illuvial iron less than 2.5 cm thick. Chapter 2 – Diagnostic horizons, properties and materials 37

2. a platy or massive structure; and 3. a surface crust that has all of the following: a. thickness enough that it does not curl entirely upon drying; and b. polygonal cracks extending 2 cm or more deep when the soil is dry; and c. sandy clay loam, clay loam, silty clay loam or finer texture; and d. very hard consistence when dry, and very plastic and sticky consistence when wet; and -1 e. an electrical conductivity (ECe) of the saturated extract of less than 4 dS m , or less than that of the layer immediately below the takyric horizon.

Field identification Takyric horizons occur in depressions in arid regions, where surface water, rich in clay and silt but relatively low in soluble salts, accumulates and leaches the upper soil horizons. Periodic leaching of salt disperses clay and forms a thick, compact, fine- textured crust with prominent polygonal cracks when dry. The crust often contains more than 80 percent clay and silt.

Relationships with some other diagnostic horizons Takyric horizons occur in association with many diagnostic horizons, the most important ones being the salic, gypsic, calcic and cambic horizons. The low EC and low soluble-salt content of takyric horizons set them apart from the salic horizon.

Terric horizon General description A terric horizon (from Latin terra, earth) is a human-induced mineral surface horizon that develops through addition of earthy manures, compost, beach sands or mud over a long period of time. It builds up gradually and may contain stones, randomly sorted and distributed.

Diagnostic criteria A terric horizon is a mineral surface horizon and: 1. has a colour related to the source material; and 2. contains less than 20 percent artefacts (by volume); and

3. has a base saturation (by 1 M NH4OAc) of 50 percent or more; and 4. occurs in locally raised land surfaces; and 5. does not show stratification but has an irregular textural differentiation; and 6. has a lithological discontinuity at its base; and 7. has a thickness of 20 cm or more.

Field identification Soils with a terric horizon show a raised surface that may be inferred either from field observation or from historical records. The terric horizon is not homogeneous, but subhorizons are thoroughly mixed. It commonly contains artefacts such as pottery fragments, cultural debris and refuse, which are typically very small (less than 1 cm in diameter) and much abraded.

Relationships with some other diagnostic horizons Few soil characteristics differentiate the terric and plaggic horizons from each other. Terric horizons commonly show a high biological activity, have a neutral to slightly alkaline soil reaction (pH [H2O] is normally more than 7.0), and may contain free lime, whereas plaggic horizons have an acid soil reaction. The colour of the terric horizon is strongly related to the source material. Buried soils may be observed at the base of the horizon although mixing can obscure the contact. 38 World reference base for soil resources 2006

Thionic horizon General description The thionic horizon (from Greek theion, sulphur) is an extremely acid subsurface horizon in which sulphuric acid is formed through oxidation of sulphides.

Diagnostic criteria A thionic horizon has: 1. a pH (1:1 in water) of less than 4.0; and 2. one or more of the following: a. yellow jarosite or yellowish-brown schwertmannite mottles or coatings; or b. concentrations with a Munsell hue of 2.5 Y or yellower and a chroma of 6 or more, moist; or c. direct superposition on sulphidic material; or d. 0.05 percent (by mass) or more water-soluble sulphate; and 2. a thickness of 15 cm or more.

Field identification Thionic horizons generally exhibit pale yellow jarosite or yellowish-brown

schwertmannite mottles or coatings. Soil reaction is extremely acid; pH (H2O) of 3.5 is not uncommon. While mostly associated with recent sulphidic coastal sediments, thionic horizons also develop inland in sulphidic materials exposed by excavation or erosion.

Relationships with some other diagnostic horizons The thionic horizon often underlies a strongly mottled horizon with pronounced redoximorphic features (reddish to reddish-brown iron hydroxide mottles and a light- coloured, Fe-depleted matrix).

Umbric horizon General characteristics The umbric horizon (from Latin umbra, shade) is a thick, dark-coloured, base-depleted surface horizon rich in organic matter.

Diagnostic criteria An umbric horizon, after mixing either the upper 20 cm of the mineral soil or, if continuous rock, a cryic, petroduric or petroplinthic horizon is present within 20 cm of the mineral soil surface, the entire mineral soil above, has: 1. a soil structure sufficiently strong that the horizon is not both massive and hard or very hard when dry in both the mixed part and the underlying unmixed part, if the minimum thickness is larger than 20 cm (prisms larger than 30 cm in diameter are included in the meaning of massive if there is no secondary structure within the prisms); and 2. Munsell colours with a chroma of 3 or less when moist, a value of 3 or less when moist and 5 or less when dry, both on broken samples in both the mixed part and the underlying unmixed part, if the minimum thickness is greater than 20 cm. The colour value is one unit or more darker than that of the parent material unless the parent material has a colour value of 4 or less, moist, in which case the colour contrast requirement is waived. If a parent material is absent, comparison must be made with the layer immediately underlying the surface layer; and 3. an organic carbon content of 0.6 percent or more, in both the mixed part and the underlying unmixed part, if the minimum thickness is larger than 20 cm. The organic carbon content is at least 0.6 percent more than in the parent material if Chapter 2 – Diagnostic horizons, properties and materials 39

the colour requirements are waived because of dark coloured parent materials; and

4. a base saturation (by 1 M NH4OAc) of less than 50 percent on a weighted average throughout the depth of the horizon; and 5. a thickness of one of the following: a. 10 cm or more if directly overlying continuous rock, a cryic, petroplinthic or petroduric horizon; or b. 20 cm or more and one-third or more of the thickness between the soil surface and the upper boundary of continuous rock, or a cryic, gypsic, petroduric, petrogypsic, petroplinthic or salic horizon or calcaric, fluvic or gypsyric material within 75 cm; or c. 20 cm or more and one-third or more of the thickness between the soil surface and the lower boundary of the lowest diagnostic horizon within 75 cm and, if present, above any of the diagnostic horizons listed under b.; or d. 25 cm or more.

Field identification The main field characteristics of an umbric horizon are its dark colour and its structure. In general, umbric horizons tend to have a lesser grade of soil structure than mollic horizons.

Most umbric horizons have an acid reaction (pH [H2O, 1:2.5] of less than about 5.5), which represents a base saturation of less than 50 percent. An additional indication for the acidity is a shallow, horizontal rooting pattern in the absence of a physical barrier.

Relationships with some other diagnostic horizons The base saturation requirement sets the umbric horizon apart from the mollic horizon, which is otherwise very similar. The upper limit of organic carbon content varies from 12 percent (20 percent organic matter) to 18 percent (30 percent organic matter), which is the lower limit for the histic horizon, or 20 percent, the lower limit of a folic horizon. Some thick, dark-coloured, organic-rich, base-desaturated surface horizons occur, which are formed as a result of human activities, such as deep cultivation and manuring, the addition of organic manures, the presence of ancient settlements, and kitchen middens (anthragric, hortic, plaggic or terric horizons). These horizons can usually be recognized in the field by the presence of artefacts, spade marks, contrasting mineral inclusions or stratification indicating the intermittent addition of manurial material, a relative higher position in the landscape, or by checking the agricultural history of the area.

Vertic horizon General description The vertic horizon (from Latin vertere, to turn) is a clayey subsurface horizon that, as a result of shrinking and swelling, has slickensides and wedge-shaped structural aggregates.

Diagnostic criteria A vertic horizon: 1. contains 30 percent or more clay throughout; and 2. has wedge-shaped structural aggregates with a longitudinal axis tilted between 10 ° and 60 ° from the horizontal; and 3. has slickensides1; and

1 Slickensides are polished and grooved ped surfaces that are produced by aggregates sliding one past another. 40 World reference base for soil resources 2006

4. has a thickness of 25 cm or more.

Field identification Vertic horizons are clayey, with a hard to very hard consistency. When dry, vertic horizons show cracks of 1 cm or more wide. Polished, shiny ped surfaces (slickensides), often at sharp angles, are distinctive.

Additional characteristics The COLE is a measure for the shrink–swell potential and is defined as the ratio of the difference between the moist length and the dry length of a clod to its dry length:

(Lm - Ld)/Ld, in which Lm is the length at 33 kPa tension and Ld the length when dry. In vertic horizons, the COLE is more than 0.06.

Relationships with some other diagnostic horizons Several other diagnostic horizons may also have high clay content, viz. the argic, natric and nitic horizons. These horizons lack the characteristic typical for the vertic horizon; however, they may be laterally linked in the landscape with the vertic horizon usually taking up the lowest position.

Voronic horizon General description The voronic horizon (from Russian voronoj, black) is a special type of mollic horizon. It is a deep, well-structured, blackish surface horizon with a high base saturation, a high content of organic matter and a high biological activity.

Diagnostic criteria A voronic horizon is a mineral surface horizon and has: 1. granular or fine subangular blocky soil structure; and 2. Munsell colours with a chroma of less than 2.0 when moist, a value less than 2.0 when moist and 3.0 when dry on broken samples. If there is 40 percent or more finely divided lime, or if the texture of the horizon is loamy sand or coarser, the limits of colour value when dry are waived; the colour value when moist is 3.0 or less. The colour value is one unit or more darker than that of the parent material (both moist and dry), unless the parent material has a colour value less than 4.0, moist. If a parent material is not present, comparison must be made with the layer immediately underlying the surface layer. The above colour requirements apply to the upper 15 cm of the voronic horizon, or immediately below any plough layer; and 3. 50 percent or more (by volume) of the horizon consisting of worm burrows, worm casts, and filled burrows; and 4. an organic carbon content of 1.5 percent or more. The organic carbon content is 6 percent or more if the colour requirements are waived because of finely divided lime, or 1.5 percent more than in the parent material if the colour requirements are waived because of dark coloured parent materials; and

5. a base saturation (by 1 M NH4OAc) of 80 percent or more; and 6. a thickness of 35 cm or more.

Field identification The voronic horizon is identified by its blackish colour, well-developed structure (usually granular), high activity of worms and other burrowing animals, and its thickness. Chapter 2 – Diagnostic horizons, properties and materials 41

Relationships with some other diagnostic horizons Its higher organic carbon content, the darker colours required, the high biological contribution to the soil structure, and its greater minimum depth express the special character of the voronic horizon with respect to the mollic horizon.

Yermic horizon General description The yermic horizon (from Spanish yermo, desert) is a surface horizon that usually, but not always, consists of surface accumulations of rock fragments (desert pavement) embedded in a loamy vesicular layer that may be covered by a thin aeolian sand or loess layer.

Diagnostic criteria A yermic horizon has: 1. aridic properties; and 2. one or more of the following: a. a pavement that is varnished or includes wind-shaped gravel or stones (ventifacts); or b. a pavement associated with a vesicular layer; or c. a vesicular layer below a platy surface layer.

Field identification A yermic horizon comprises a pavement and/or a vesicular layer that has a loamy texture. The vesicular layer shows a polygonal network of desiccation cracks, often filled with in-blown material, that extend into the underlying layers. The surface layers have a weak to moderate platy structure.

Relationships with some other diagnostic horizons Yermic horizons often occur in association with other diagnostic horizons characteristic for desert environments (salic, gypsic, duric, calcic and cambic horizons). In very cold deserts (e.g. Antarctica), they may occur associated with cryic horizons. Under these conditions, coarse cryoclastic material dominates and there is little dust to be deflated and deposited by wind. Here, a dense pavement with varnish, ventifacts, aeolian sand layers and soluble mineral accumulations may occur directly on loose deposits, without a vesicular layer.

DIAGNOSTIC PROPERTIES

Abrupt textural change General description An abrupt textural change (from Latin abruptus) is a very sharp increase in clay content within a limited depth range.

Diagnostic criteria An abrupt textural change requires 8 percent or more clay in the underlying layer and: 1. doubling of the clay content within 7.5 cm if the overlying layer has less than 20 percent clay; or 2. 20 percent (absolute) increase in clay content within 7.5 cm if the overlying layer has 20 percent or more clay.

Albeluvic tonguing General description The term albeluvic tonguing (from Latin albus, white, and eluere, to wash out) is connotative of penetrations of clay- and Fe-depleted material into an argic horizon. 42 World reference base for soil resources 2006

When peds are present, albeluvic tongues occur along ped surfaces.

Diagnostic criteria Albeluvic tongues: 1. have the colour of an albic horizon; and 2. have greater depth than width, with the following horizontal dimensions: a. 5 mm or more in clayey argic horizons; or b. 10 mm or more in clay loam and silty argic horizons; or c. 15 mm or more in coarser (silt loam, loam or sandy loam) argic horizons; and 3. occupy 10 percent or more of the volume in the first 10 cm of the argic horizon, measured on both vertical and horizontal sections; and 4. have a particle-size distribution matching that of the coarser textured horizon overlying the argic horizon.

Andic properties General description Andic properties (from Japanese an, dark, and do, soil) result from moderate weathering of mainly pyroclastic deposits. However, some soils develop andic properties from non- volcanic materials (e.g. loess, argillite and ferralitic weathering products). The presence of short-range-order minerals and/or organo-metallic complexes is characteristic for andic properties. These minerals and complexes are commonly part of the weathering sequence in pyroclastic deposits (tephric soil material à vitric properties à andic properties). Andic properties may be found at the soil surface or in the subsurface, commonly occurring as layers. Many surface layers with andic properties contain a high amount of organic matter (more than 5 percent), are commonly very dark coloured (Munsell value and chroma, moist, are 3 or less), have a fluffy macrostructure and, in some places, a smeary consistence. They have a low bulk density and commonly have a silt loam or finer texture. Andic surface layers rich in organic matter may be very thick, having a thickness of 50 cm or more (pachic characteristic) in some soils. Andic subsurface layers are generally somewhat lighter coloured. Andic layers may have different characteristics, depending on the type of the dominant weathering process acting upon the soil material. They may exhibit thixotropy, i.e. the soil material changes, under pressure or by rubbing, from a plastic solid into a liquefied stage and back into the solid condition. In perhumid climates, humus-rich andic layers may contain more than twice the water content of samples that have been oven-dried and rewetted (hydric characteristic). Two major types of andic properties are recognized: one in which allophane and similar minerals are predominant (the sil-andic type); and one in which Al complexed by organic acids prevails (the alu-andic type). The sil-andic property typically gives a strongly acid to neutral soil reaction, while the alu-andic property gives an extremely acid to acid reaction.

Diagnostic criteria Andic properties1 require: 2 1. an Alox + ½Feox value of 2.0 percent or more; and 2. a bulk density3 of 0.90 kg dm-3 or less; and 3. a phosphate retention of 85 percent or more; and

1 Shoji et al., 1996; Takahashi, Nanzyo and Shoji, 2004. 2 Alox and Feox are acid oxalate-extractable aluminium and iron, respectively (Blakemore, Searle and Daly, 1981), expressed as percent of the fine earth (0–2 mm) fraction on an oven-dried (105 °C) basis. 3 For bulk density, the volume is determined after an undried soil sample has been desorbed at 33 kPa (no prior drying) and afterwards weighed oven-dried (see Annex 1). Chapter 2 – Diagnostic horizons, properties and materials 43

4. if occurring under tephric material that meets the requirements of an albic 1 4 horizon, a Cpy/OC or a Cf/Cpy of less than 0.5; and 5. less than 25 percent (by mass) organic carbon. Andic properties may be divided into sil-andic and alu-andic properties. Sil-andic properties show an acid-oxalate (pH 3) extractable silica (Siox) content of 0.6 percent or 2 more or an Alpy /Alox of less than 0.5; alu-andic properties show a Siox content of less than 0.6 percent and an Alpy/Alox of 0.5 or more. Transitional alu-sil-andic properties that show a Siox content between 0.6 and 0.9 percent and an Alpy/Alox between 0.3 and 0.5 may occur (Poulenard and Herbillon, 2000).

Field identification Andic properties may be identified using the sodium fluoride field test of Fieldes and Perrott (1966). A pH in NaF of more than 9.5 indicates allophane and/or organo- aluminium complexes. The test is indicative for most layers with andic properties, except for those very rich in organic matter. However, the same reaction occurs in spodic horizons and in certain acid clays that are rich in Al-interlayered clay minerals. Uncultivated, organic matter-rich surface layers with sil-andic properties typically have a pH (H2O) of 4.5 or higher, while uncultivated surface layers with alu-andic properties and rich in organic matter typically have a pH (H2O) of less than 4.5.

Generally, pH (H2O) in sil-andic subsoil layers is more than 5.0.

Relationships with some diagnostic horizons and properties Vitric properties are distinguished from andic properties by a lesser degree of weathering. This is evidenced by a lower amount of non-crystalline or paracrystalline pedogenetic minerals, as characterized by the moderate amount of acid oxalate (pH 3) extractable Al and Fe in layers with vitric properties (Alox + ½Feox = 0.4–2.0 percent), by a higher bulk density (BD > 0.9 kg dm-3), or by a lower phosphate retention (25 – <85 percent). Histic or folic horizons with less than 25 percent organic carbon may have andic properties. In organic layers with 25 percent or more organic carbon, andic properties are not considered. Spodic horizons, which also contain complexes of sesquioxides and organic substances, can have similar characteristics to those of layers with andic properties rich in alumino-organic complexes. Many spodic horizons have at least twice as much Alox + ½Feox as an overlying layer. This normally does not apply to layers with andic properties in which the alumino-organic complexes are virtually immobile. However, particularly in Podzols to which the Entic qualifier applies and which have a spodic horizon without the requirement of at least twice as much Alox + ½Feox as an overlying layer, other diagnostic criteria such as the bulk density are needed in order to discriminate between layers with andic properties and spodic horizons. Some layers with andic properties are covered by relatively young, light-coloured volcanic ejecta that are difficult to distinguish from an albic horizon. Therefore, in a number of cases, analytical tests are needed in order to verify the difference between layers with andic properties and spodic horizons, in particular the Cpy to OC or Cf to

Cpy ratio tests.

Aridic properties General description The term aridic properties (from Latin aridus, dry) combines a number of properties that are common in surface horizons of soils occurring under arid conditions and

1 Cpy, Cf and OC are pyrophosphate-extractable C, fulvic acid C and organic C, respectively (Ito et al., 1991), expressed as percent of the fine earth (0–2 mm) fraction on an oven-dried (105 °C) basis. 2 Alpy: pyrophosphate-extractable aluminium, expressed as percent of the fine earth (0–2 mm) fraction on an oven-dried (105 °C) basis. 44 World reference base for soil resources 2006

where pedogenesis exceeds new accumulation at the soil surface by aeolian or alluvial activity.

Diagnostic criteria Aridic properties require all of the following: 1. an organic carbon content of less than 0.6 percent1 if the texture is sandy loam or finer, or less than 0.2 percent if the texture is coarser than sandy loam, as a weighted average in the upper 20 cm of the soil or down to the top of a diagnostic subsurface horizon, a cemented layer, or to continuous rock, whichever is shallower; and 2. evidence of aeolian activity in one or more of the following forms: a. the sand fraction in some layer or in in-blown material filling cracks contains rounded or subangular sand particles showing a matt surface (use a ×10 hand- lens). These particles make up 10 percent or more of the medium and coarser quartz sand fraction; or b. wind-shaped rock fragments (ventifacts) at the surface; or c. aeroturbation (e.g. cross-bedding); or d. evidence of wind erosion or deposition; and 3. both broken and crushed samples with a Munsell colour value of 3 or more when moist and 4.5 or more when dry, and a chroma of 2 or more when moist; and

4. base saturation (by 1 M NH4OAc) of 75 percent or more.

Additional remarks The presence of acicular (needle-shaped) clay minerals (e.g. sepiolite and palygorskite) in soils is considered connotative of a desert environment, but it has not been reported in all desert soils. This may be due either to the fact that, under arid conditions, acicular clays are not produced but only preserved, provided they exist in the parent material or in the dust that falls on the soil, or that, in some desert environments, there has not been sufficient weathering to produce detectable quantities of secondary clay minerals.

Continuous rock Definition Continuous rock is consolidated material underlying the soil, exclusive of cemented pedogenetic horizons such as petrocalcic, petroduric, petrogypsic and petroplinthic horizons. Continuous rock is sufficiently consolidated to remain intact when an air- dried specimen 25–30 mm on a side is submerged in water for 1 hour. The material is considered continuous only if cracks into which roots can enter are on average 10 cm or more apart and occupy less than 20 percent (by volume) of the continuous rock, with no significant displacement of the rock having taken place.

Ferralic properties General description Ferralic properties (from Latin ferrum, iron, and alumen, alum) refer to mineral soil material that has a relatively low CEC. It also includes soil materials that fulfil the requirements of a ferralic horizon except texture.

Diagnostic criteria Ferralic properties require in some subsurface layer: -1 2 1. a CEC (by 1 M NH4OAc) of less than 24 cmolc kg clay ; or

1 -1 The organic carbon content may be higher if the soil is periodically flooded, or if it has an ECe of 4 dS m or more somewhere within 100 cm of the soil surface. 2 See Annex 1. Chapter 2 – Diagnostic horizons, properties and materials 45

-1 2. a CEC (by 1 M NH4OAc) of less than 4 cmolc kg soil and a Munsell chroma of 5 or more, moist.

Geric properties General description Geric properties (from Greek geraios, old) refer to mineral soil material that has a very low ECEC or even acts as an anion exchanger.

Diagnostic criteria Geric properties require: 1. an ECEC (sum of exchangeable bases plus exchangeable acidity in 1 M KCl) of -1 1 less than 1.5 cmolc kg clay ; or

2. a delta pH (pHKCl minus pHwater) of +0.1 or more.

Gleyic colour pattern General description Soil materials develop a gleyic colour pattern (from Russian gley, mucky soil mass) if they are saturated with groundwater, unless drained, for a period that allows reducing conditions to occur (this may range from a few days in the tropics to a few weeks in other areas), and show a gleyic colour pattern.

Diagnostic criteria A gleyic colour pattern shows one or both of the following: 1. 90 percent or more reductimorphic colours, which comprise neutral white to black (Munsell N1/ to N8/) or bluish to greenish (Munsell 2.5 Y, 5 Y, 5 G, 5 B); or 2. 5 percent or more mottles of oximorphic colours, which comprise any colour, excluding reductimorphic colours.

Field identification A gleyic colour pattern results from a redox gradient between groundwater and capillary fringe causing an uneven distribution of iron and manganese (hydr)oxides. In the lower part of the soil and/or inside the peds, the oxides are either transformed into insoluble Fe/Mn(II) compounds or they are translocated; both processes lead to the absence of colours with a hue redder than 2.5 Y. Translocated Fe and Mn compounds can be concentrated in the oxidized form (Fe[III], Mn[IV]) on ped surfaces or in biopores (rusty root channels), and towards the surface even in the matrix. Manganese concentrations can be recognized by strong effervescence using a 10-percent H2O2 solution. Reductimorphic colours reflect permanently wet conditions. In loamy and clayey material, blue-green colours dominate owing to Fe (II, III) hydroxy salts (green rust). If the material is rich in sulphur (S), blackish colours prevail owing to colloidal iron sulphides such as greigite or mackinawite (easily recognized by smell after applying 1 M HCl). In calcareous material, whitish colours are dominant owing to calcite and/or siderite. Sands are usually light grey to white in colour and often also impoverished in Fe and Mn. Bluish-green and black colours are unstable and often oxidize to a reddish brown within a few hours of exposure to air. The upper part of a reductimorphic layer may show up to 10 percent rusty colours, mainly around channels of burrowing animals or plant roots. Oximorphic colours reflect alternating reducing and oxidizing conditions, as is the case in the capillary fringe and in the surface horizons of soils with fluctuating

1 See Annex 1. 46 World reference base for soil resources 2006

groundwater levels. Specific colours indicate ferrihydrite (reddish brown), (bright yellowish brown), lepidocrocite (orange), and jarosite (pale yellow). In loamy and clayey soils, the iron oxides/ are concentrated on aggregate surfaces and the walls of larger pores (e.g. old root channels).

Additional characteristics If a layer has a gleyic colour pattern in 50 percent of its volume, the layer has in the other 50 percent a matrix of oximorphic colours, i.e. neither reductimorphic colours nor mottles of oximorphic colours.

Lithological discontinuity General description Lithological discontinuities (from Greek lithos, stone, and Latin continuare, to continue) are significant changes in particle-size distribution or mineralogy that represent differences in lithology within a soil. A lithological discontinuity can also denote an age difference.

Diagnostic criteria A lithological discontinuity requires one or more of the following: 1. an abrupt change in particle-size distribution that is not solely associated with a change in clay content resulting from pedogenesis; or 2. a relative change of 20 percent or more in the ratios between coarse sand, medium sand, and fine sand; or 3. rock fragments that do not have the same lithology as the underlying continuous rock; or 4. a layer containing rock fragments without weathering rinds overlying a layer containing rocks with weathering rinds; or 5. layers with angular rock fragments overlying or underlying layers with rounded rock fragments; or 6. abrupt changes in colour not resulting from pedogenesis; or 7. marked differences in size and shape of resistant minerals between superimposed layers (as shown by micromorphological or mineralogical methods).

Additional characteristics In cases, a horizontal line of rock fragments (stone line) overlying and underlying layers with smaller amounts or rock fragments or a decreasing percentage of rock fragments with increasing depth may also be suggestive of a lithological discontinuity, although the sorting action of small such as termites can produce similar effects in what would initially have been lithologically uniform parent material.

Reducing conditions Definition Reducing conditions (from Latin reducere) show one or more of the following: 1. a negative logarithm of the hydrogen partial pressure (rH) of less than 20; or 2. the presence of free Fe2+, as shown on a freshly broken and smoothed surface of a field-wet soil by the appearance of a strong red colour after wetting it with a 0.2-percent a,a, dipyridyl solution in 10-percent acetic acid1; or 3. the presence of iron sulphide; or 4. the presence of methane.

1 This test may not give the strong red colour in soil materials with a neutral or alkaline soil reaction. Chapter 2 – Diagnostic horizons, properties and materials 47

Secondary carbonates General description The term secondary carbonates (from Latin carbo, coal) refers to translocated lime, precipitated in place from the soil solution rather than inherited from a soil parent material. As a diagnostic property, it should be present in significant quantities.

Field identification Secondary carbonates either may disrupt the soil structure or fabric, forming masses, nodules, concretions or spheroidal aggregates (white eyes) that are soft and powdery when dry, or may be present as soft coatings in pores, on structural faces or on the undersides of rock or cemented fragments. If present as coatings, secondary carbonates cover 50 percent or more of the structural faces and are thick enough to be visible when moist. If present as soft nodules, they occupy 5 percent or more of the soil volume. Filaments (pseudomycelia) are only included in the definition of secondary carbonates if they are permanent and do not come and go with changing moisture conditions. This can be checked by spraying some water.

Stagnic colour pattern General description Soil material has a stagnic colour pattern (from Latin stagnare, to stagnate) if it is, at least temporarily, saturated with surface water, unless drained, for a period long enough to allow reducing conditions to occur (this may range from a few days in the tropics to a few weeks in other areas).

Diagnostic criteria A stagnic colour pattern shows mottling in such a way that the surfaces of the peds (or parts of the soil matrix) are lighter (at least one Munsell value unit more) and paler (at least one chroma unit less), and the interiors of the peds (or parts of the soil matrix) are more reddish (at least one hue unit) and brighter (at least one chroma unit more) than the non-redoximorphic parts of the layer, or than the mixed average of the interior and surface parts.

Additional characteristics If a layer has a stagnic colour pattern in 50 percent of its volume the other 50 percent of the layer are neither lighter and paler nor more reddish and brighter.

Vertic properties Diagnostic criteria Soil material with vertic properties (from Latin vertere, to turn) has one or both of the following: 1. 30 percent or more clay throughout a thickness of 15 cm or more and one or both of the following: a. slickensides or wedge-shaped aggregates; or b. cracks that open and close periodically and are 1 cm or more wide; or 2. a COLE of 0.06 or more averaged over depth of 100 cm from the soil surface.

Vitric properties General description Vitric properties (from Latin vitrum, glass) apply to layers with volcanic glass and other primary minerals derived from volcanic ejecta and which contain a limited amount of short-range-order minerals. 48 World reference base for soil resources 2006

Diagnostic criteria Vitric properties1 require: 1. 5 percent or more (by grain count) volcanic glass, glassy aggregates and other glass-coated primary minerals, in the fraction between 0.05 and 2 mm, or in the fraction between 0.02 and 0.25 mm; and 2 2. an Alox + ½Feox value of 0.4 percent or more; and 3. a phosphate retention of 25 percent or more; and 4. if occurring under tephric material that meets the requirements of an albic 3 4 horizon, a Cpy/OC or a Cf/Cpy of less than 0.5; and 5. less than 25 percent (by mass) organic carbon.

Field identification Vitric properties can occur in a surface layer. However, they can also occur under some tens of centimetres of recent pyroclastic deposits. Layers with vitric properties can have an appreciable amount of organic matter. The sand and coarse silt fractions of layers with vitric properties have a significant amount of unaltered or partially altered volcanic glass, glassy aggregates and other glass-coated primary minerals (coarser fractions may be checked by ×10 hand-lens; finer fractions may be checked by microscope).

Relationships with some diagnostic horizons, properties and materials Vitric properties are, on the one hand, closely linked with andic properties, into which they may eventually develop. On the other hand, layers with vitric properties develop from tephric materials. Mollic and umbric horizons may exhibit vitric properties as well.

DIAGNOSTIC MATERIALS

Artefacts Definition Artefacts (from Latin ars, art, and facere, to make) are solid or liquid substances that are: 1. one or both of the following: a created or substantially modified by humans as part of an industrial or artisanal manufacturing process; or b. brought to the surface by human activity from a depth where they were not influenced by surface processes, with properties substantially different from the environment where they are placed; and 2. have substantially the same properties as when first manufactured, modified or excavated. Examples of artefacts are bricks, pottery, glass, crushed or dressed stone, industrial waste, garbage, processed oil products, mine spoil and crude oil.

Calcaric material Definition Calcaric material (from Latin calcarius) effervescences strongly with 1 M HCl in most of the fine earth. It applies to material that contains 2 percent or more calcium carbonate equivalent.

1 Adapted after Takahashi, Nanzyo and Shoji (2004) and findings of the COST 622 Action. 2 Alox and Feox are acid oxalate-extractable aluminium and iron, respectively (Blakemore, Searle and Daly, 1987), expressed as percent of the fine earth (0–2 mm) fraction on an oven-dried (105 °C) basis. 3 Cpy, Cf and OC are pyrophosphate-extractable C, fulvic acid C and organic C, respectively (Ito et al., 1991), expressed as percent of the fine earth (0–2 mm) fraction on an oven-dried (105 °C) basis. Chapter 2 – Diagnostic horizons, properties and materials 49

Colluvic material General description Colluvic (from Latin colluere, to wash) material is formed by sedimentation through human-induced erosion. It normally accumulates in foot slope positions, in depressions or above hedge walls. The erosion may have taken place since Neolithic times.

Field identification The upper part of the colluvic material shows characteristics (texture, colour, pH and organic carbon content) similar to the surface layer of the source in the neighbourhood. Many colluvic materials have artefacts such as pieces of bricks, ceramics and glass. Stratification is common although not always easily detectable, and many colluvic materials have a lithological discontinuity at their base.

Fluvic material General description Fluvic material (from Latin fluvius, river) refers to fluviatile, marine and lacustrine sediments that receive fresh material at regular intervals or have received it in the recent past1.

Diagnostic criteria Fluvic material is of fluviatile, marine or lacustrine origin that shows stratification in at least 25 percent of the soil volume over a specified depth; stratification may also be evident from an organic carbon content decreasing irregularly with depth, or remaining above 0.2 percent to a depth of 100 cm from the mineral soil surface. Thin strata of sand may have less organic carbon if the finer sediments below meet the latter requirement.

Field identification Stratification, taking such forms as alternating darker coloured soil layers, reflects an irregular decrease in organic carbon content with depth. Fluvic material is always associated with organized water bodies and should be distinguished from colluvial deposits (sheet colluvia, splays and colluvial cones), even though they look very much the same.

Gypsiric material Definition Gypsiric material (from Greek gypsos) is mineral material that contains 5 percent or more gypsum (by volume).

Limnic material Diagnostic criteria Limnic material (from Greek limnae, pool) includes both organic and mineral materials that are: 1. deposited in water by precipitation or through action of aquatic organisms, such as diatoms and other algae; or 2. derived from underwater and floating aquatic plants and subsequently modified by aquatic animals.

Field identification Limnic material occurs as subaquatic deposits (or at the surface after drainage). Four types of limnic material are distinguished:

1 Recent past covers the period during which the soil has been protected from flooding, e.g. by empoldering, embanking, canalization or artificial drainage, and during which time soil formation has not resulted in the development of any diagnostic subsurface horizon apart from a salic or sulphuric horizon. 50 World reference base for soil resources 2006

1. Coprogenous earth or sedimentary peat: dominantly organic, identifiable through many faecal pellets, Munsell colour value (moist) 4 or less, slightly viscous water suspension, non- or slightly plastic and non-sticky consistence, shrinking upon drying, difficult to rewet after drying, and cracking along horizontal planes. 2. Diatomaceous earth: mainly diatoms (siliceous), identifiable by irreversible changing of the matrix colour (Munsell value 3, 4 or 5 in field moist or wet condition) as a result of the irreversibly shrinkage of the organic coatings on diatoms (use 440× microscope). 3. Marl: strongly calcareous, identifiable by a Munsell colour value, moist, of 5 or more, and a reaction with 10-percent HCl. The colour of marl does not usually change upon drying. 4. Gyttja: small coprogenic aggregates of strongly humified organic matter and minerals of predominantly clay to silt size, 0.5 percent or more organic carbon, a Munsell colour hue of 5 Y, GY or G, strong shrinkage after drainage and an rH value of 13 or more.

Mineral material General description In mineral material (from Celtic mine, mineral), the soil properties are dominated by mineral components.

Diagnostic criteria Mineral material has one or both of the following: 1. less than 20 percent organic carbon in the fine earth (by mass) if saturated with water for less than 30 consecutive days in most years without being drained; or 2. one or both of the following: a. less than (12 + [clay percentage of the mineral fraction × 0.1]) percent organic carbon in the fine earth (by mass), or b. less than 18 percent organic carbon in the fine earth (by mass), if the mineral fraction has 60 percent or more clay.

Organic material General description Organic material (from Greek organon, tool) consists of a large amount of organic debris that accumulates at the surface under either wet or dry conditions and in which the mineral component does not significantly influence the soil properties.

Diagnostic criteria Organic material has one or both of the following: 1. 20 percent or more organic carbon in the fine earth (by mass); or 2. if saturated with water for 30 consecutive days or more in most years (unless drained), one or both of the following: a. (12 + [clay percentage of the mineral fraction × 0.1]) percent or more organic carbon in the fine earth (by mass), or b. 18 percent or more organic carbon in the fine earth (by mass).

Ornithogenic material General description Ornithogenic material (from Greek ornithos, bird, and genesis, origin) is material with strong influence of bird excrement. It often has a high content of gravel that has been transported by birds. Chapter 2 – Diagnostic horizons, properties and materials 51

Diagnostic criteria Ornithogenic material has: 1. remnants of birds or bird activity (bones, feathers, and sorted gravel of similar size); and

2. a P2O5 content of 0.25 percent or more in 1-percent citric acid.

Sulphidic material General description Sulphidic material (from English sulphide) is a waterlogged deposit containing S, mostly in the form of sulphides, and only moderate amounts of calcium carbonate.

Diagnostic criteria Sulphidic material has: 1. 0.75 percent or more S (dry mass) and less than three times as much calcium carbonate equivalent as S; and 2. pH (1:1 in water) of 4.0 or more.

Field identification In moist or wet conditions, deposits containing sulphides often show a golden shine, the colour of pyrite. Forced oxidation with a 30-percent hydrogen peroxide solution lowers the pH to 2.5 or less, the reaction may be vigorous in sunlight or on heating. Munsell colours range: hues of N, 5 Y, 5 GY, 5 BG, or 5 G; values of 2, 3 or 4; chroma always 1. The colour is usually unstable, and blackens upon exposure. Sulphidic clay is usually practically unripe. If the soil is disturbed, a whiff of rotten eggs may be noticed. This is accentuated by application of 1 M HCl.

Technic hard rock Definition Technic hard rock (from Greek technikos, skilfully made or constructed) is consolidated material resulting from an industrial process, with properties substantially different from those of natural materials.

Tephric material General description Tephric material1 (from Greek tephra, pile ash) consists either of tephra, i.e. unconsolidated, non- or only slightly weathered primary pyroclastic products of volcanic eruptions (including ash, cinders, lapilli, pumice, pumice-like vesicular pyroclastics, blocks and volcanic bombs), or of tephric deposits, i.e. tephra that has been reworked and mixed with material from other sources. This includes tephric loess, tephric blown sand and volcanogenic alluvium.

Diagnostic criteria Tephric material has: 1. 30 percent or more (by grain count) volcanic glass, glass-coated primary minerals, glassy materials, and glassy aggregates in the 0.02–2 mm particle-size fraction; and 2. no andic or vitric properties.

Relationships with some diagnostic horizons The low amount of acid oxalate extractable Al and Fe sets tephric material apart from layers with vitric or andic properties.

1 Description and diagnostic criteria are adapted from Hewitt (1992).

53

Chapter 3 Key to the reference soil groups of the WRB with lists of prefix and suffix qualifiers

Key to the reference soil groups Prefix qualifiers Suffix qualifiers Soils having organic material, either Folic Thionic 1. 10 cm or more thick starting at the soil surface and immediately Limnic Ornithic overlying ice, continuous rock, or fragmental materials, the Lignic Calcaric interstices of which are filled with organic material; or Fibric Sodic 2. cumulatively within 100 cm of the soil surface either 60 cm or more thick if 75 percent (by volume) or more of the material consists of Hemic Alcalic moss fibres or 40 cm or more thick in other materials and starting within 40 cm of the soil surface. Sapric Toxic HISTOSOLS Floatic Dystric Subaquatic Eutric Glacic Turbic Ombric Gelic Rheic Petrogleyic Technic Placic Cryic Drainic Leptic Transportic Vitric Novic Andic Salic Calcic Other soils having Hydragric Sodic 1. either a hortic, irragric, plaggic or terric horizon 50 cm or more Irragric Alcalic thick; or Terric Dystric 2. an anthraquic horizon and an underlying hydragric horizon with a Plaggic Eutric combined thickness of 50 cm or more. Hortic Oxyaquic ANTHROSOLS Escalic Arenic Technic Siltic Fluvic Clayic Salic Novic Gleyic Stagnic Spodic Ferralic Regic 54 World reference base for soil resources 2006

Key to the reference soil groups Prefix qualifiers Suffix qualifiers Other soils having Ekranic Calcaric 1. 20 percent or more (by volume, by weighted average) artefacts in Linic Ruptic the upper 100 cm from the soil surface or to continuous rock or a Urbic Toxic cemented or indurated layer, whichever is shallower; or Spolic Reductic 2. a continuous, very slowly permeable to impermeable, constructed geomembrane of any thickness starting within 100 cm of the soil Garbic Humic surface; or Folic Densic 3. technic hard rock starting within 5 cm of the soil surface and Histic Oxyaquic covering 95 percent or more of the horizontal extent of the soil. Cryic Skeletic TECHNOSOLS1 Leptic Arenic Fluvic Siltic Gleyic Clayic Vitric Drainic Stagnic Novic Mollic Alic Acric Luvic Lixic Umbric Other soils having Glacic Gypsiric 1. a cryic horizon starting within 100 cm of the soil surface; or Turbic Calcaric 2. a cryic horizon starting within 200 cm of the soil surface and Folic Ornithic evidence of cryoturbation2 in some layer within 100 cm of the soil Histic Dystric surface. Technic Eutric CRYOSOLS Hyperskeletic Reductaquic Leptic Oxyaquic Natric Thixotropic Salic Aridic Vitric Skeletic Spodic Arenic Mollic Siltic Calcic Clayic Umbric Drainic Cambic Novic Haplic Other soils having Nudilithic Brunic 1. one of the following: Lithic Gypsiric a. limitation of depth by continuous rock within 25 cm of the soil Hyperskeletic Calcaric surface; or Rendzic Ornithic b. less than 20 percent (by volume) fine earth averaged over a depth Folic Tephric of 75 cm from the soil surface or to continuous rock, whichever is shallower; and Histic Humic 2. no calcic, gypsic or spodic horizon. Technic Sodic LEPTOSOLS Vertic Dystric Salic Eutric Gleyic Oxyaquic Vitric Gelic Andic Placic Stagnic Greyic Mollic Yermic Umbric Aridic Cambic Skeletic Haplic Drainic Novic 1 Buried layers occur frequently in this RSG and can be indicated with the specifier thapto- followed by a qualifier or a RSG. 2 Evidence of cryoturbation includes frost heave, cryogenic sorting, thermal cracking, ice segregation, patterned ground, etc. Chapter 3 – Key to the reference soil groups of the WRB with lists of prefix and suffix qualifiers 55

Key to the reference soil groups Prefix qualifiers Suffix qualifiers Other soils having Grumic Thionic 1. a vertic horizon starting within 100 cm of the soil surface; and Mazic Albic 2. after the upper 20 cm have been mixed, 30 percent or more clay Technic Manganesic between the soil surface and the vertic horizon throughout; and Endoleptic Ferric 3. cracks1 that open and close periodically. Salic Gypsiric VERTISOLS Gleyic Calcaric Sodic Humic Stagnic Hyposalic Mollic Hyposodic Gypsic Mesotrophic Duric Eutric Calcic Pellic Haplic Chromic Novic Other soils having Subaquatic Thionic 1. fluvic material starting within 25 cm of the soil surface or starting Tidalic Anthric immediately below a plough layer of any depth and continuing to a Limnic Gypsiric depth of 50 cm or more; and Folic Calcaric 2. no layers with andic or vitric properties with a combined thickness of 30 cm or more within 100 cm of the soil surface and starting within Histic Tephric 25 cm of the soil surface. Technic Petrogleyic FLUVISOLS2 Salic Gelic Gleyic Oxyaquic Stagnic Humic Mollic Sodic Gypsic Dystric Calcic Eutric Umbric Greyic Haplic Takyric Yermic Aridic Skeletic Arenic Siltic Clayic Drainic Other soils having a natric horizon starting within 100 cm of the soil surface. Technic Glossalbic SOLONETZ Vertic Albic Gleyic Abruptic Salic Colluvic Stagnic Ruptic Mollic Magnesic Gypsic Humic Duric Oxyaquic Petrocalcic Takyric Calcic Yermic Haplic Aridic Arenic Siltic Clayic Novic 1 A crack is a separation between big blocks of soil. If the surface is self-mulching, or if the soil is cultivated while cracks are open, the cracks may be filled mainly by granular materials from the soil surface but they are open in the sense that the blocks are separated; it controls the infiltration and percolation of water. If the soil is irrigated, the upper 50 cm has a COLE of 0.06 or more. 2 Buried layers occur frequently in this RSG and can be indicated with the specifier thapto- followed by a qualifier or a RSG. 56 World reference base for soil resources 2006

Key to the reference soil groups Prefix qualifiers Suffix qualifiers Other soils having Petrosalic Sodic 1. a salic horizon starting within 50 cm of the soil surface; and Hypersalic Aceric 2. no thionic horizon starting within 50 cm of the soil surface. Puffic Chloridic SOLONCHAKS Folic Sulphatic Histic Carbonatic Technic Gelic Vertic Oxyaquic Gleyic Takyric Stagnic Yermic Mollic Aridic Gypsic Arenic Duric Siltic Calcic Clayic Haplic Drainic Novic Other soils having Folic Thionic 1. within 50 cm of the mineral soil surface in some parts reducing Histic Abruptic conditions and in half or more of the soil volume a gleyic colour Anthraquic Calcaric pattern; and Technic Tephric 2. no layers with andic or vitric properties with a combined thickness of either Endosalic Colluvic a. 30 cm or more within 100 cm of the soil surface and starting within Vitric Humic 25 cm of the soil surface; or Andic Sodic b. 60 percent or more of the entire thickness of the soil when continuous rock or a cemented or indurated layer is starting Spodic Alcalic between 25 and 50 cm from the soil surface. Plinthic Alumic GLEYSOLS Mollic Toxic Gypsic Dystric Calcic Eutric Alic Petrogleyic Acric Turbic Luvic Gelic Lixic Greyic Umbric Takyric Haplic Arenic Siltic Clayic Drainic Novic Chapter 3 – Key to the reference soil groups of the WRB with lists of prefix and suffix qualifiers 57

Key to the reference soil groups Prefix qualifiers Suffix qualifiers Other soils having Vitric Anthric 1. one or more layers with andic or vitric properties with a combined Aluandic Fragic thickness of either Eutrosilic Calcaric a. 30 cm or more within 100 cm of the soil surface and starting Silandic Colluvic within 25 cm of the soil surface; or Melanic Acroxic b. 60 percent or more of the entire thickness of the soil when continuous rock or a cemented or indurated layer is starting Fulvic Sodic between 25 and 50 cm from the soil surface; and Hydric Dystric 2. no argic, ferralic, petroplinthic, pisoplinthic, plinthic or spodic horizon (unless buried deeper than 50 cm). Folic Eutric ANDOSOLS1 Histic Turbic Technic Gelic Leptic Oxyaquic Gleyic Placic Mollic Greyic Gypsic Thixotropic Petroduric Skeletic Duric Arenic Calcic Siltic Umbric Clayic Drainic Novic Other soils having a spodic horizon starting within 200 cm of the mineral Placic Hortic soil surface. Ortsteinic Plaggic PODZOLS Carbic Terric Rustic Anthric Entic Ornithic Albic Fragic Folic Ruptic Histic Turbic Technic Gelic Hyperskeletic Oxyaquic Leptic Lamellic Gleyic Skeletic Vitric Drainic Andic Novic Stagnic Umbric Haplic

1 Buried layers occur frequently in this RSG and can be indicated with the specifier thapto- followed by a qualifier or a RSG. 58 World reference base for soil resources 2006

Key to the reference soil groups Prefix qualifiers Suffix qualifiers Other soils having either Petric Albic 1. a plinthic, petroplinthic or pisoplinthic horizon starting within 50 cm Fractipetric Manganiferric of the soil surface; or Pisolithic Ferric 2. a plinthic horizon starting within 100 cm of the soil surface and, Gibbsic Endoduric directly above, a layer 10 cm or more thick, that has in some parts reducing conditions for some time during the year and in half or Posic Abruptic more of the soil volume, single or in combination Geric Colluvic a. a stagnic colour pattern; or Vetic Ruptic b. an albic horizon. Folic Alumic Histic Humic PLINTHOSOLS Technic Dystric Stagnic Eutric Acric Oxyaquic Lixic Pachic Umbric Umbriglossic Haplic Arenic Siltic Clayic Drainic Novic Other soils having Vetic Humic 1. a nitic horizon starting within 100 cm of the soil surface; and Technic Alumic 2. gradual to diffuse1 horizon boundaries between the soil surface and Andic Dystric the nitic horizon; and Ferralic Eutric 3. no ferric, petroplinthic, pisoplinthic, plinthic or vertic horizon Mollic Oxyaquic starting within 100 cm of the soil surface; and Alic Colluvic 4. no gleyic or stagnic colour pattern starting within 100 cm of the soil surface. Acric Rhodic NITISOLS Luvic Novic Lixic Umbric Haplic Other soils having Gibbsic Sombric 1. a ferralic horizon starting within 150 cm of the soil surface; and Posic Manganiferric 2. no argic horizon that has, in the upper 30 cm, 10 percent or more Geric Ferric water-dispersible clay unless the upper 30 cm of the argic horizon Vetic Colluvic has one or both of the following: Folic Humic a. geric properties; or Technic Alumic b. 1.4 percent or more organic carbon. Andic Dystric FERRALSOLS Plinthic Eutric Mollic Ruptic Acric Oxyaquic Lixic Arenic Umbric Siltic Haplic Clayic Rhodic Xanthic Novic

1 As defined in FAO (2006). Chapter 3 – Key to the reference soil groups of the WRB with lists of prefix and suffix qualifiers 59

Key to the reference soil groups Prefix qualifiers Suffix qualifiers Other soils having Solodic Thionic 1. an abrupt textural change within 100 cm of the soil surface and, Folic Albic directly above or below, a layer 5 cm or more thick, that has in some Histic Manganiferric parts reducing conditions for some time during the year and in half or more of the soil volume, single or in combination Technic Ferric a. a stagnic colour pattern; or Vertic Geric b. an albic horizon; and Endosalic Ruptic 2. no albeluvic tonguing starting within 100 cm of the soil surface. Plinthic Calcaric PLANOSOLS Endogleyic Sodic Mollic Alcalic Gypsic Alumic Petrocalcic Dystric Calcic Eutric Alic Gelic Acric Greyic Luvic Arenic Lixic Siltic Umbric Clayic Haplic Chromic Drainic Novic Other soils having Folic Thionic 1. within 50 cm of the mineral soil surface in some parts reducing Histic Albic conditions for some time during the year and in half or more of the Technic Manganiferric soil volume, single or in combination, Vertic Ferric a. a stagnic colour pattern; or Endosalic Ruptic b. an albic horizon; and Plinthic Geric 2. no albeluvic tonguing starting within 100 cm of the soil surface. Endogleyic Calcaric STAGNOSOLS Mollic Ornithic Gypsic Sodic Petrocalcic Alcalic Calcic Alumic Alic Dystric Acric Eutric Luvic Gelic Lixic Greyic Umbric Placic Haplic Arenic Siltic Clayic Rhodic Chromic Drainic Novic 60 World reference base for soil resources 2006

Key to the reference soil groups Prefix qualifiers Suffix qualifiers Other soils having Voronic Anthric 1. a mollic horizon with a moist chroma of 2 or less to a depth of Vermic Glossic 20 cm or more, or having this chroma directly below any plough Technic Tephric layer that is 20 cm or more deep; and Leptic Sodic 2. a calcic horizon, or concentrations of secondary carbonates starting within 50 cm below the lower limit of the mollic horizon and, if Vertic Pachic present, above a cemented or indurated layer; and Endofluvic Oxyaquic 3. a base saturation (by 1 M NH4OAc) of 50 percent or more from the Endosalic Greyic soil surface to the calcic horizon or the concentrations of secondary carbonates throughout. Gleyic Skeletic CHERNOZEMS Vitric Arenic Andic Siltic Stagnic Clayic Petrogypsic Novic Gypsic Petroduric Duric Petrocalcic Calcic Luvic Haplic Other soils having Vermic Anthric 1. a mollic horizon; and Technic Glossic 2. a calcic horizon, or concentrations of secondary carbonates starting Leptic Tephric within 50 cm below the lower limit of the mollic horizon and, if Vertic Sodic present, above a cemented or indurated layer; and Endosalic Oxyaquic 3. a base saturation (by 1 M NH4OAc) of 50 percent or more from the soil surface to the calcic horizon or the concentrations of secondary Gleyic Greyic carbonates throughout. Vitric Skeletic KASTANOZEMS Andic Arenic Stagnic Siltic Petrogypsic Clayic Gypsic Chromic Petroduric Novic Duric Petrocalcic Calcic Luvic Haplic Other soils having Vermic Anthric 1. a mollic horizon; and Greyic Albic

2. a base saturation (by 1 M NH4OAc) of 50 percent or more Technic Abruptic throughout to a depth of 100 cm or more from the soil surface or Rendzic Glossic to continuous rock or a cemented or indurated layer, whichever is shallower. Leptic Calcaric PHAEOZEMS Vertic Tephric Endosalic Sodic Gleyic Pachic Vitric Oxyaquic Andic Skeletic Ferralic Arenic Stagnic Siltic Petrogypsic Clayic Petroduric Chromic Duric Novic Petrocalcic Calcic Luvic Haplic Chapter 3 – Key to the reference soil groups of the WRB with lists of prefix and suffix qualifiers 61

Key to the reference soil groups Prefix qualifiers Suffix qualifiers Other soils having Petric Ruptic 1. a petrogypsic horizon starting within 100 cm of the soil surface; or Hypergypsic Sodic 2. a gypsic horizon starting within 100 cm of the soil surface and no Hypogypsic Hyperochric argic horizon unless the argic horizon is permeated with gypsum or Arzic Takyric calcium carbonate. Technic Yermic GYPSISOLS Leptic Aridic Vertic Skeletic Endosalic Arenic Endogleyic Siltic Petroduric Clayic Duric Novic Petrocalcic Calcic Luvic Haplic Other soils having a petroduric or duric horizon starting within 100 cm of Petric Ruptic the soil surface. Fractipetric Sodic DURISOLS Technic Takyric Leptic Yermic Vertic Aridic Endogleyic Hyperochric Gypsic Arenic Petrocalcic Siltic Calcic Clayic Luvic Chromic Lixic Novic Haplic Other soils having Petric Ruptic 1. a petrocalcic horizon starting within 100 cm of the soil surface; or Hypercalcic Sodic 2. a calcic horizon starting within 100 cm of the soil surface and Hypocalcic Takyric a. a calcareous matrix between 50 cm from the soil surface and Technic Yermic the calcic horizon throughout if the calcic horizon starts below Leptic Aridic 50 cm; and Vertic Hyperochric b. no argic horizon unless the argic horizon is permeated with calcium carbonate. Endosalic Skeletic CALCISOLS Endogleyic Arenic Gypsic Siltic Luvic Clayic Lixic Chromic Haplic Novic Other soils having an argic horizon starting within 100 cm of the soil Fragic Anthric surface with albeluvic tonguing at its upper boundary. Cutanic Manganiferric ALBELUVISOLS Folic Ferric Histic Abruptic Technic Ruptic Gleyic Alumic Stagnic Dystric Umbric Eutric Haplic Gelic Oxyaquic Greyic Arenic Siltic Clayic Drainic Novic 62 World reference base for soil resources 2006

Key to the reference soil groups Prefix qualifiers Suffix qualifiers Other soils having Hyperalic Anthric

-1 1. an argic horizon, which has a CEC (by 1 M NH4OAc) of 24 cmolc kg Lamellic Fragic clay1 or more throughout or to a depth of 50 cm below its upper Cutanic Manganiferric limit, whichever is shallower, either starting within 100 cm of the soil surface, or within 200 cm of the soil surface if the argic horizon is Albic Ferric overlain by loamy sand or coarser textures throughout; and Technic Abruptic

2. a base saturation (by 1 M NH4OAc) of less than 50 percent in the major part between 50 and 100 cm. Leptic Ruptic ALISOLS Vertic Alumic Fractiplinthic Humic Petroplinthic Hyperdystric Pisoplinthic Epieutric Plinthic Turbic Gleyic Gelic Vitric Oxyaquic Andic Greyic Nitic Profondic Stagnic Hyperochric Umbric Skeletic Haplic Arenic Silltic Clayic Rhodic Chromic Novic Other soils having Vetic Anthric

1. an argic horizon that has a CEC (by 1 M NH4OAc) of less than Lamellic Albic 24 cmol kg-1 clay2 in some part to a maximum depth of 50 cm below c Cutanic Fragic its upper limit, either starting within 100 cm of the soil surface, or within 200 cm of the soil surface if the argic horizon is overlain by Technic Sombric loamy sand or coarser textures throughout, and Leptic Manganiferric

2. a base saturation (by 1 M NH4OAc) of less than 50 percent in the major part between 50 and 100 cm. Fractiplinthic Ferric ACRISOLS Petroplinthic Abruptic Pisoplinthic Ruptic Plinthic Alumic Gleyic Humic Vitric Hyperdystric Andic Epieutric Nitic Oxyaquic Stagnic Greyic Umbric Profondic Haplic Hyperochric Skeletic Arenic Siltic Clayic Rhodic Chromic Novic 1 See Annex 1. 2 See Annex 1. Chapter 3 – Key to the reference soil groups of the WRB with lists of prefix and suffix qualifiers 63

Key to the reference soil groups Prefix qualifiers Suffix qualifiers

Other soils having an argic horizon with a CEC (by 1 M NH4OAc) of Lamellic Anthric 24 cmol kg-1 clay1 or more throughout or to a depth of 50 cm below its c Cutanic Fragic upper limit, whichever is shallower, either starting within 100 cm of the soil surface or within 200 cm of the soil surface if the argic horizon is Albic Manganiferric overlain by loamy sand or coarser textures throughout. Escalic Ferric LUVISOLS Technic Abruptic Leptic Ruptic Vertic Humic Gleyic Sodic Vitric Epidystric Andic Hypereutric Nitic Turbic Stagnic Gelic Calcic Oxyaquic Haplic Greyic Profondic Hyperochric Skeletic Arenic Siltic Clayic Rhodic Chromic Novic Other soils having an argic horizon, either starting within 100 cm of the Vetic Anthric soil surface or within 200 cm of the soil surface if the argic horizon is Lamellic Albic overlain by loamy sand or coarser textures throughout. Cutanic Fragic LIXISOLS Technic Manganiferric Leptic Ferric Gleyic Abruptic Vitric Ruptic Andic Humic Fractiplinthic Epidystric Petroplinthic Hypereutric Pisoplinthic Oxyaquic Plinthic Greyic Nitic Profondic Stagnic Hyperochric Calcic Skeletic Haplic Arenic Siltic Clayic Rhodic Chromic Novic

1 See Annex 1. 64 World reference base for soil resources 2006

Key to the reference soil groups Prefix qualifiers Suffix qualifiers Other soils having an umbric or mollic horizon. Folic Anthric UMBRISOLS Histic Albic Technic Brunic Leptic Ornithic Vitric Thionic Andic Glossic Endogleyic Humic Ferralic Alumic Stagnic Hyperdystric Mollic Endoeutric Cambic Pachic Haplic Turbic Gelic Oxyaquic Greyic Laxic Placic Skeletic Arenic Siltic Clayic Chromic Drainic Novic Other soils having Lamellic Ornithic 1. a weighted average texture of loamy sand or coarser, if cumulative Hypoluvic Gypsiric layers of finer texture are less than 15 cm thick, either to a depth Hyperalbic Calcaric of 100 cm from the soil surface or to a petroplinthic, pisoplinthic, plinthic or salic horizon starting between 50 and 100 cm from the Albic Tephric soil surface; and Rubic Hyposalic 2. less than 40 percent (by volume) of or coarser fragments in all layers within 100 cm of the soil surface or to a petroplinthic, Brunic Dystric pisoplinthic, plinthic or salic horizon starting between 50 and 100 cm Hydrophobic Eutric from the soil surface; and Protic Petrogleyic 3. no fragic, irragric, hortic, plaggic or terric horizon; and Folic Turbic 4. no layers with andic or vitric properties with a combined thickness of 15 cm. Technic Gelic ARENOSOLS Endosalic Greyic Endogleyic Placic Fractiplinthic Hyperochric Petroplinthic Yermic Pisoplinthic Aridic Plinthic Transportic Ferralic Novic Haplic Chapter 3 – Key to the reference soil groups of the WRB with lists of prefix and suffix qualifiers 65

Key to the reference soil groups Prefix qualifiers Suffix qualifiers Other soils having Folic Fragic 1. a cambic horizon starting within 50 cm of the soil surface and having Anthraquic Manganiferric its base 25 cm or more below the soil surface or 15 cm or more Hortic Ferric below any plough layer; or Irragric Ornithic 2. an anthraquic, hortic, hydragric, irragric, plaggic or terric horizon; or Plaggic Ruptic 3. a fragic, petroplinthic, pisoplinthic, plinthic, salic or vertic horizon starting within 100 cm of the soil surface; or Terric Colluvic 4. one or more layers with andic or vitric properties with a combined Technic Gypsiric thickness of 15 cm or more within 100 cm of the soil surface. Leptic Calcaric CAMBISOLS Vertic Tephric Fluvic Alumic Endosalic Sodic Vitric Alcalic Andic Humic Endogleyic Dystric Fractiplinthic Eutric Petroplinthic Laxic Pisoplinthic Turbic Plinthic Gelic Ferralic Oxyaquic Gelistagnic Greyic Stagnic Hyperochric Haplic Takyric Yermic Aridic Skeletic Siltic Clayic Rhodic Chromic Escalic Novic Other soils. Aric Ornithic REGOSOLS Colluvic Gypsiric Technic Calcaric Leptic Tephric Endogleyic Humic Thaptovitric Hyposalic Thaptandic Sodic Gelistagnic Dystric Stagnic Eutric Haplic Turbic Gelic Oxyaquic Vermic Hyperochric Takyric Yermic Aridic Skeletic Arenic Siltic Clayic Escalic Transportic

67

Chapter 4 Description, distribution, use and management of reference soil groups

This chapter gives an overview of all the RSGs recognized in the WRB. A brief description is provided with corresponding names in other major soil classification systems, followed by the regional distribution of each group. Land use and management concludes each description. More detailed information on each RSG, including morphological, chemical and physical characteristics and genesis, is available in FAO (2001a) and a number of CD-ROMs (FAO, 2001b, 2003 and 2005). All these publications reflect the first edition of the WRB (FAO, 1998); new publications based on the current second edition are planned for the near future.

ACRISOLS

Acrisols are soils that have a higher clay content in the subsoil than in the topsoil as a result of pedogenetic processes (especially clay migration) leading to an argic subsoil horizon. Acrisols have in certain depths a low base saturation and low-activity clays. Many Acrisols correlate with Red Yellow Podzolic soils (e.g. ), Argissolos (), sols ferralitiques fortement ou moyennement désaturés (France), Red and Yellow , and with low-activity clays (United States of America).

Summary description of Acrisols Connotation: From Latin acer, very acid. Strongly weathered acid soils with low base saturation at some depth. Parent material: On a wide variety of parent materials, most extensive from weathering of acid rocks, and notably in strongly weathered clays that are undergoing further degradation. Environment: Mostly old land surfaces with hilly or undulating topography, in regions with a wet tropical/monsoonal, subtropical or warm . Forest is the natural vegetation type. Profile development: Pedogenetic differentiation of clay content with a lower content in the topsoil and a higher content in the subsoil; leaching of base cations owing to the humid environment and advanced degree of weathering.

Regional distribution of Acrisols Acrisols are found in humid tropical, humid subtropical and warm temperate regions and are most extensive in Southeast , the southern fringes of the , the southeast of the United States of America, and in both East and . There are about 1 000 million ha of Acrisols worldwide.

Management and use of Acrisols Preservation of the surface soil with its all-important organic matter and preventing erosion are preconditions for farming on Acrisols. Mechanical clearing of natural forest by extraction of root balls and filling of the holes with surrounding surface soil 68 World reference base for soil resources 2006

produces land that is largely sterile where Al concentrations of the former subsoil reach toxic levels. Adapted cropping systems with complete fertilization and careful management are required if sedentary farming is to be practised on Acrisols. The widely used slash-and- burn agriculture (shifting cultivation) may seem primitive but it is a well-adapted form of land use, developed over centuries of trial and error. If occupation periods are short (one or a few years only) and followed by a sufficiently long generation period (up to several decades), this system makes a good use of the limited resources of Acrisols. Agroforestry is recommended as a soil-protecting alternative to shifting cultivation to achieve higher yields without requiring expensive inputs. Low-input farming on Acrisols is not very rewarding. Undemanding, acidity- tolerant cash crops such as pineapple, cashew, tea and rubber can be grown with some success. Increasing areas of Acrisols are planted to oil-palm (e.g. in Malaysia and on Sumatra). Large areas of Acrisols are under forest, ranging from high, dense rain forest to open woodland. Most of the tree roots are concentrated in the humous surface horizon with only a few tap-roots extending down into the subsoil. In , Acrisols are also found under savannah. Acrisols are suitable for production of rainfed and irrigated crops only after liming and full fertilization. Rotation of annual crops with improved pasture maintains the organic matter content.

ALBELUVISOLS

Albeluvisols are soils that have, beginning within 1 m of the soil surface, a clay illuviation horizon with an irregular or broken upper boundary resulting in tonguing of bleached soil material into the illuviation horizon. Many Albeluvisols correlate with: Podzoluvisols (FAO); Sod-podzolic or Podzolic soils (Russian Federation); Fahlerden (Germany); and Glossaqualfs, Glossocryalfs and Glossudalfs (United States of America).

Summary description of Albeluvisols Connotation: From Latin albus, white, and Latin eluere, to wash out. Parent material: Mostly unconsolidated glacial till, materials of lacustrine or fluvial origin and aeolian deposits (loess). Environment: Flat to undulating plains under coniferous forest (including boreal ) or mixed forest. The climate is temperate to boreal with cold winters, short and cool summers, and an average annual precipitation sum of 500–1 000 mm. Precipitation is distributed evenly over the year or, in the continental part of the Albeluvisol belt, has a peak in early summer. Profile development: A thin, dark surface horizon over an albic subsurface horizon that tongues into an underlying brown argic horizon. Temporarily reducing conditions with a stagnic colour pattern are common in boreal Albeluvisols.

Regional distribution of Albeluvisols Albeluvisols cover an estimated 320 million ha in Europe, North Asia and , with minor occurrences in North America. Albeluvisols are concentrated in two regions, each having a particular set of climate conditions: ÿ the continental regions that had permafrost in the Pleistocene of northeast Europe, northwest Asia and southwest Canada, which constitute by far the largest areas of Albeluvisols; ÿ the loess and cover sand areas and old alluvial areas in moist temperate regions, such as France, central Belgium, the southeast of the Netherlands and the west of Germany. Chapter 4 – Description, distribution, use and management of reference soil groups 69

Management and use of Albeluvisols The agricultural suitability of Albeluvisols is limited because of their acidity, low nutrient levels, tillage and drainage problems and because of the climate, with its short growing season and severe frost during the long winter. The Albeluvisols of the northern taiga zone are almost exclusively under forest; small areas are used as pasture or hay fields. In the southern taiga zone, less than 10 percent of the non-forested area is used for agricultural production. Livestock farming is the main agricultural land use on Albeluvisols (dairy production and cattle rearing); arable cropping (cereals, potatoes, and forage maize) plays a minor role. In the Russian Federation, the share of arable farming increases in southern and western directions, especially on Albeluvisols with higher base saturations in the subsoil. With careful tillage, liming and application of fertilizers, Albeluvisols can produce 25–30 tonnes of potatoes per hectare, 2–5 tonnes of winter wheat or 5–10 tonnes of dry herbage.

ALISOLS

Alisols are soils that have a higher clay content in the subsoil than in the topsoil as a result of pedogenetic processes (especially clay migration) leading to an argic subsoil horizon. Alisols have a low base saturation at certain depths and high-activity clays throughout the argic horizon. They lack the albeluvic tonguing as in Albeluvisols. They occur predominantly in humid tropical, humid subtropical and warm temperate regions. Many Alisols correlate with: Alissolos (Brazil); Ultisols with high-activity clays (United States of America); Kurosols (); and Fersialsols and sols fersiallitiques très lessivés (France).

Summary description of Alisols Connotation: Soils with a low base saturation at some depths; from Latin alumen, alum. Parent material: In a wide variety of parent materials. Most occurrences of Alisols reported so far are on weathering products of basic rocks and unconsolidated materials. Environment: Most common in hilly or undulating topography, in humid tropical, humid subtropical and monsoon climates. Profile development: Pedogenetic differentiation of clay contents with a lower content in the topsoil and a higher content in the subsoil, leaching of base cations owing to the humid environment without advanced weathering of high-activity clays; highly leached Alisols might have an albic eluviation horizon between the surface horizon and the argic subsurface horizon but lack the albeluvic tonguing of Albeluvisols.

Regional distribution of Alisols Major occurrences of Alisols are found in Latin America (, Nicaragua, , , and Brazil), in the West Indies (Jamaica, Martinique and Saint Lucia), in West Africa, the highlands of , Madagascar, and in and . FAO (2001a) estimates that about 100 million ha of these soils are used for agriculture in the tropics. Alisols occur also in subtropical regions; they are found in China, Japan and the southeast of the United States of America, and minor occurrences have been reported from around the Mediterranean Sea (Italy, France and Greece). They also occur in humid temperate regions.

Management and use of Alisols Alisols occur predominantly on hilly or undulating topography. The generally unstable surface soil of cultivated Alisols makes them susceptible to erosion; truncated soils 70 World reference base for soil resources 2006

are quite common. Toxic levels of Al at shallow depth and poor natural are added constraints in many Alisols. As a consequence, many Alisols allow only cultivation of shallow-rooting crops and crops suffer from drought stress in the . A significant part of the Alisols is unproductive under a wide variety of crops. The use of acidity-tolerant crops or low-volume grazing is common. The productivity of Alisols in subsistence agriculture is generally low as these soils have a limited capacity to recover from chemical exhaustion. Where fully limed and fertilized, crops on Alisols may benefit from the considerable CEC and good water-holding capacity, and the Alisols may eventually grade into Luvisols. Alisols are increasingly planted to Al-tolerant estate crops such as tea and rubber but also to oil-palm and, in places, to coffee and sugar cane.

ANDOSOLS

Andosols accommodate the soils that develop in volcanic ejecta or glasses under almost any climate (except under hyperarid climate conditions). However, Andosols may also develop in other silicate-rich materials under acid weathering in humid and perhumid climates. Many Andosols belong to: Kuroboku (Japan); (United States of America); Andosols and Vitrisols (France); and volcanic ash soils.

Summary description of Andosols Connotation: Typically black soils of volcanic landscapes; from Japanese an, black, and do, soil. Parent material: Volcanic glasses and ejecta (mainly ash, but also tuff, pumice, cinders and others) or other silicate-rich material. Environment: Undulating to mountainous, humid, and arctic to tropical regions with a wide range of vegetation types. Profile development: Rapid weathering of porous volcanic ejecta or glasses results in accumulation of stable organo-mineral complexes or short-range-order minerals such as allophane, imogolite and ferrihydrite. Acid weathering of other silicate-rich material in humid and perhumid climates also leads to the formation of stable organo-mineral complexes.

Regional distribution of Andosols Andosols occur in volcanic regions all over the world. Important concentrations are found around the Pacific rim: on the west coast of South America, in Central America, Mexico, United States of America (the Rocky Mountains, Alaska), Japan, the Philippine Archipelago, Indonesia, , and . They are also prominent on many islands in the Pacific: Fiji, Vanuatu, New Caledonia, Samoa and . In Africa, major occurrences of Andosols are found along the Rift Valley, in Kenya, Rwanda and Ethiopia and in Madagascar. In Europe, Andosols occur in Italy, France, Germany and . The total Andosol area is estimated at some 110 million ha or less than 1 percent of the global land surface. More than half of this area is situated in the tropics. Andosols originating from parent materials other than volcanic ejecta or glasses occur in humid (often mountainous) regions.

Management and use of Andosols Andosols have a high potential for agricultural production, but many of them are not used up to their capacity. Andosols are generally fertile soils, particularly Andosols in intermediate or basic volcanic ash and not exposed to excessive leaching. The strong phosphate fixation of Andosols (caused by active Al and Fe) is a problem. Ameliorative measures to reduce this effect include application of lime, silica, organic material, and phosphate . Chapter 4 – Description, distribution, use and management of reference soil groups 71

Andosols are easy to cultivate and have good rootability and water storage properties. Strongly hydrated Andosols are difficult to till because of their low bearing capacity and their stickiness. Andosols are planted to a wide variety of crops including sugar cane, tobacco, sweet potato (tolerant of low phosphate levels), tea, vegetables, wheat and orchard crops. Andosols on steep slopes are perhaps best kept under forest. Paddy rice cultivation is a major land use on Andosols in lowlands with shallow groundwater.

ANTHROSOLS

Anthrosols comprise soils that have been modified profoundly through human activities, such as addition of organic materials or household wastes, irrigation and cultivation. The group includes soils otherwise known as: Plaggen soils, Paddy soils, Oasis soils, do Indio (Brazil), Agrozems (Russian Federation), Terrestrische anthropogene Böden (Germany), Anthroposols (Australia), and Anthrosols (China).

Summary description of Anthrosols Connotation: Soils with prominent characteristics that result from human activities; from Greek anthropos, human being. Parent material: Virtually any soil material, modified by long-continued cultivation or addition of material. Environment: In many regions where people have been practising agriculture for a long time. Profile development: Influence of humans is normally restricted to the surface horizons; the horizon differentiation of a buried soil may still be intact at some depth.

Regional distribution of Anthrosols Anthrosols are found wherever people have practised agriculture for a long time. Anthrosols with plaggic horizons are most common in northwest Europe. Together with Anthrosols with a terric horizon, they cover more than 500 000 ha. Anthrosols with irragric horizons are found in irrigation areas in dry regions, e.g. in Mesopotamia, near oases in desert regions and in parts of India. Anthrosols with an anthraquic horizon overlying a hydragric horizon (paddy soils) occupy vast areas in China and in parts of South and Southeast Asia (e.g. Sri Lanka, Viet Nam, Thailand and Indonesia). Anthrosols with hortic horizons are found all over the world where humans have fertilized the soil with household wastes and manure. The Terra Preta do Indio in the Amazon Region of Brazil belongs to this group.

Management and use of Anthrosols Plaggic horizons have favourable physical properties (porosity, rootability and moisture availability), but many have less satisfactory chemical characteristics (acidity, and nutrient deficiencies). Rye, oats, barley, potato, and also the more demanding sugar beet and summer wheat are common crops on European Anthrosols with a plaggic horizon. Prior to the advent of chemical fertilizers, rye yields were 700–1 100 kg/ha, or 4–5 times the quantity of seed used. Today, these soils receive generous doses of fertilizers and average per-hectare yield levels for rye, barley and summer wheat are 5 000, 4 500 and 5 500 kg, respectively. Sugar beet and potato produce 40–50 tonnes/ ha. Nowadays, they are increasingly used for production of silage maize and grass; per-hectare production levels of 12–13 tonnes of dry maize silage and 10–13 tonnes of dry grass are considered normal. In places, Anthrosols with plaggic horizons are used for tree nurseries and horticulture. The good drainage and the dark colour of the surface soil (early warming in spring) make it possible to till and sow or plant early in 72 World reference base for soil resources 2006

the season. Soils with deep plaggic horizons in the Netherlands were in demand for the cultivation of tobacco until the 1950s. Anthrosols with a hortic horizon are kitchen soils. Well-known examples are situated on river terraces in south Maryland, United States of America, and along the Amazon River in Brazil. They have deep, black topsoils formed in layers of kitchen refuse (mainly oyster shells, fish bones, etc.) from early Indian habitations. Many countries possess small areas of soils that were modified by early inhabitants. Long-continued wet cultivation of rice leads to an anthraquic horizon with an underlying hydragric horizon. Puddling of wetland rice fields (involving destruction of the natural soil structure by intensive tillage when the soil is saturated with water) is done intentionally, inter alia to reduce percolation losses. Anthrosols with irragric horizons are formed as a result of prolonged sedimentation (predominantly silt) from irrigation water. A special case is found in depression areas where dryland crops are commonly planted on constructed ridges that alternate with drainage furrows. The original soil profile of the ridge areas is buried under a thick layer of added soil material. The ridge-and-furrow system is known from such different environments as the wet forests of western Europe and the coastal swamps of Southeast Asia where the ridges are planted to dryland crops and rice is grown in the shallow ditch areas. In parts of western Europe, notably in Ireland and the United Kingdom, calcareous materials (e.g. beach sands) were carted to areas with acid Arenosols, Podzols, Albeluvisols and Histosols. Eventually these modified surface layers of mineral material turned into terric horizons that give the soil much improved properties for arable cropping than the original surface soil. In Central Mexico, deep soils were constructed of organic-matter-rich lacustrine sediments, thus forming a system of artificial islands and channels (chinampas). These soils have a terric horizon and were the most productive lands of the Aztec empire; now most of these soils are affected by salinization.

ARENOSOLS

Arenosols comprise sandy soils, including both soils developed in residual sands after in situ weathering of usually quartz-rich sediments or rock, and soils developed in recently deposited sands such as dunes in deserts and beach lands. Corresponding soils in other classification systems include of the US Soil Taxonomy and the sols minéraux bruts and sols peu évolués in the French classification system of the CPCS (1967). Many Arenosols belong to Arenic Rudosols (Australia), Psammozems (Russian Federation) and Neossolos (Brazil).

Summary description of Arenosols Connotation: Sandy soils; from Latin arena, sand. Parent material: Unconsolidated, in places calcareous, translocated materials of sandy texture; relatively small areas of Arenosols occur in extremely weathered siliceous rock. Environment: From arid to humid and perhumid, and from extremely cold to extremely hot; landforms vary from recent dunes, beach ridges and sandy plains to very old plateaus; the vegetation ranges from desert over scattered vegetation (mostly grassy) to light forest. Profile development: In the dry zone, there is little or no soil development. Arenosols in the perhumid tropics tend to develop thick albic eluviation horizons (with a spodic horizon below 200 m from the soil surface) whereas most Arenosols of the humid temperate zone show signs of alteration or transport of humus, Fe or clay, but too weak to be diagnostic. Chapter 4 – Description, distribution, use and management of reference soil groups 73

Regional distribution of Arenosols Arenosols are one of the most extensive RSGs in the world; including shifting sands and active dunes, they cover about 1 300 million ha, or 10 percent of the land surface. Vast expanses of deep aeolian sands are found on the Central African between the equator and 30 °S. These Kalahari Sands form the largest body of sands on earth. Other areas of Arenosols occur in the Sahelian region of Africa, various parts of the Sahara, central and , the Near East, and China. Sandy coastal plains and coastal dune areas are of smaller geographic extent. Although most Arenosols occur in arid and semi-arid regions, they are typical azonal soils; they are found in the widest possible range of climates, from very arid to very humid and from cold to hot. Arenosols are widespread in aeolian landscapes but occur also in marine, littoral, and lacustrine sands and in coarse-grained weathering mantles of siliceous rocks, mainly sandstone, quartzite and granite. There is no limitation as to age or period in which soil formation took place. Arenosols occur on very old surfaces as well as in very recent landforms, and may be associated with almost any type of vegetation.

Management and use of Arenosols Arenosols occur in widely different environments, and possibilities to use them for agriculture vary accordingly. The characteristic that all Arenosols have in common is their coarse texture, accounting for their generally high permeability and low water and nutrient storage capacity. On the other hand, Arenosols offer ease of cultivation, rooting and harvesting of root and tuber crops. Arenosols in arid lands, where the annual rainfall is less than 300 mm, are predominantly used for extensive (nomadic) grazing. Dry farming is possible where the annual rainfall exceeds 300 mm. Low coherence, low nutrient storage capacity and high sensitivity to erosion are serious limitations of Arenosols in the dry zone. Good yields of small grains, melons, pulses and fodder crops have been realized on irrigated Arenosols, but high percolation losses may make surface irrigation impracticable. Drip or trickle irrigation, possibly combined with careful dosage of fertilizers, may remedy the situation. Many areas with Arenosols in the Sahelian zone (annual rainfall of 300–600 mm) are transitional to the Sahara, and their soils are covered with sparse vegetation. Uncontrolled grazing and clearing for cultivation without appropriate measures can easily make these soils unstable and revert them to shifting dunes. Arenosols in the humid and subhumid temperate zone have similar limitations as those of the dry zone, albeit that drought is a less serious constraint. In some instances, e.g. in horticulture, the low water storage of Arenosols is considered advantageous because the soils warm up early in the season. In mixed farming systems (which are much more common) with cereals, fodder crops and grassland, supplemental sprinkler irrigation is applied during dry spells. A large part of the Arenosols of the temperate zone is under forest, either production forest or natural stands in carefully managed nature reserves. Arenosols in the humid tropics are best left under their natural vegetation, particularly so the deeply weathered Arenosols with an albic horizon. As nutrient elements are all concentrated in the biomass and in the , clearing of the land will inevitably produce infertile badlands without ecological or economic value. Under forest, the land can still produce some timber (e.g. Agathis spp.) and wood for the pulp and paper industry. Permanent cultivation of annual crops would require management inputs that are usually not economically justifiable. In places, Arenosols have been planted to perennial crops such as rubber and pepper; coastal sands are widely planted to estate crops such as coconut, cashew, casuarinas and pine, especially where good quality groundwater is within reach of the root system. Root 74 World reference base for soil resources 2006

and tuber crops benefit from the ease of harvesting, notably cassava, with its tolerance of low nutrient levels. Groundnut and bambara groundnut can be found on the better soils. Arenosols and related soils with a sandy surface texture in some regions (e.g. west Australia and parts of South Africa) may be prone to develop water-repellency, typically caused by hydrophobic exudates of soil fungi that coat sand grains. Water-repellency is most intense after lengthy spells of hot, dry weather and leads to differential water infiltration. This is thought to have ecological significance in promoting plant species diversity (e.g. in Namaqualand). Wetting agents (surfactants such as calcium lignosulphonate) are sometimes used to achieve more uniform water penetration under irrigation. Dryland wheat farmers in Australia mine clay and apply it to their sandy soils with specialized machinery. The results (more uniform germination and better herbicide efficiency) can be economically attractive where a local source of clay is available.

CALCISOLS

Calcisols accommodate soils in which there is substantial secondary accumulation of lime. Calcisols are common in highly calcareous parent materials and widespread in arid and semi-arid environments. Formerly used soil names for many Calcisols include Desert soils and Takyrs. In the US Soil Taxonomy, most of them belong to the Calcids.

Summary description of Calcisols Connotation: Soils with substantial accumulation of secondary lime; from Latin calx, lime. Parent material: Mostly alluvial, colluvial and aeolian deposits of base-rich weathering material. Environment: Level to hilly land in arid and semi-arid regions. The natural vegetation is sparse and dominated by xerophytic shrubs and trees and/or ephemeral grasses. Profile development: Typical Calcisols have a pale brown surface horizon; substantial secondary accumulation of lime occurs within 100 cm of the surface.

Regional distribution of Calcisols It is difficult to quantify the worldwide extent of Calcisols with any measure of accuracy. Many Calcisols occur together with Solonchaks that are actually salt- affected Calcisols and/or with other soils having secondary accumulation of lime that do not key out as Calcisols. The total Calcisol area may well amount to some 1 000 million ha, nearly all of it in the arid and semi-arid tropics and subtropics of both hemispheres.

Land use and management of Calcisols Vast areas of so-called natural Calcisols are under shrubs, grasses and herbs and are used for extensive grazing. Drought-tolerant crops such as sunflower might be grown rainfed, preferably after one or a few fallow years, but Calcisols reach their full productive capacity only where carefully irrigated. Extensive areas of Calcisols are used for production of irrigated winter wheat, melons, and cotton in the Mediterranean zone. Sorghum bicolor (el sabeem) and fodder crops, such as Rhodes grass and alfalfa, are tolerant of high Ca levels. Some 20 vegetable crops have been grown successfully on irrigated Calcisols fertilized with nitrogen (N), phosphorus (P) and trace elements (Fe and zinc [Zn]). Furrow irrigation is superior to basin irrigation on slaking Calcisols because it reduces surface crusting/caking and seedling mortality; pulse crops in particular are Chapter 4 – Description, distribution, use and management of reference soil groups 75

very vul nerable in the seedling stage. In places, arable farming is hindered by stoniness of the surface soil and/or a petrocalcic horizon at shallow depth.

CAMBISOLS

Cambisols combine soils with at least an incipient subsurface soil formation. Transformation of parent material is evident from structure formation and mostly brownish discoloration, increasing clay percentage, and/or carbonate removal. Other soil classification systems refer to many Cambisols as: Braunerden (Germany), Sols bruns (France), Brown soils/Brown Forest soils (older US systems), or Burozems (Russian Federation). FAO coined the name Cambisols, adopted by Brazil (Cambissolos); US Soil Taxonomy classifies most of these soils as .

Summary description of Cambisols Connotation: Soils with at least the beginnings of horizon differentiation in the subsoil evident from changes in structure, colour, clay content or carbonate content; from Italian cambiare, to change. Parent material: Medium and fine-textured materials derived from a wide range of rocks. Profile development: Cambisols are charac terized by slight or moderate weathering of parent material and by absence of appreciable quantities of illuviated clay, organic matter, Al and/or Fe compounds. Environment: Level to mountainous terrain in all climates; wide range of vegetation types.

Regional distribution of Cambisols Cambisols cover an estimated 1 500 million ha worldwide. This RSG is particularly well represented in temperate and boreal regions that were under the influence of glaciations during the Pleistocene, partly because the parent material of the soil is still young, but also because soil formation is slow in cool regions. Erosion and deposition cycles explain the occurrence of Cambisols in mountain regions. Cambisols also occur in dry regions but are less common in the humid tropics and subtropics where weathering and soil formation proceed at much faster rates than in temperate, boreal and dry regions. The young alluvial plains and terraces of the Ganges–Brahmaputra system are probably the largest continuous surface of Cambisols in the tropics. Cambisols are also common in areas with active geologic erosion, where they may occur in association with mature tropical soils.

Management and use of Cambisols Cambisols generally make good agricultural land and are used intensively. Cambisols with high base saturation in the temperate zone are among the most productive soils on earth. More acid Cambisols, although less fertile, are used for mixed arable farming and as grazing and forest land. Cambisols on steep slopes are best kept under forest; this is particularly true for Cambisols in highlands. Cambisols on irrigated alluvial plains in the dry zone are used intensively for production of food and oil crops. Cambisols in undulating or hilly terrain (mainly colluvial) are planted to a variety of annual and perennial crops or are used as grazing land. Cambisols in the humid tropics are typically poor in nutrients but are still richer than associated Acrisols or Ferralsols and they have a greater CEC. Cambisols with groundwater influence in alluvial plains are highly productive paddy soils. 76 World reference base for soil resources 2006

CHERNOZEMS

Chernozems accommodate soils with a thick black surface layer that is rich in organic matter. The Russian soil scientist Dokuchaev coined the name Chernozem in 1883 to denote the typical zonal soil of the tall grass steppes in continental . Many Chernozems correspond to: Calcareous Black Soils and Kalktschernoseme (Germany); Chernosols (France); Eluviated Black Soils (Canada); several suborders (especially Udolls) of the (United States of America); and Chernossolos (Brazil).

Summary description of Chernozems Connotation: Black soils rich in organic matter; from Russian chernij, black, and zemlja, earth or land. Parent material: Mostly aeolian and re-washed aeolian sediments (loess). Environment: Regions with a continental climate with cold winters and hot summers, which are dry at least in the late summer; in flat to undulating plains with tall-grass vegetation (forest in the northern transitional zone). Profile development: Dark brown to black mollic surface horizon, in many cases over a cambic or argic subsurface horizon; with secondary carbonates or a calcic horizon in the subsoil.

Regional distribution of Chernozems Chernozems cover an estimated 230 million ha worldwide, mainly in the middle latitude steppes of Eurasia and North America, north of the zone with Kastanozems.

Management and use of Chernozems Russian soil scientists rank the deep, central Chernozems among the best soils in the world. With less than half of all Chernozems in Eurasia being used for arable cropping, these soils constitute a formidable resource for the future. Preservation of the favourable soil structure through timely cultivation and careful irrigation at low watering rates prevents ablation and erosion. Application of P fertilizers is required for high yields. Wheat, barley and maize are the principal crops grown, alongside other food crops and vegetables. Part of the Chernozem area is used for livestock rearing. In the northern temperate belt, the possible growing period is short and the principal crops grown are wheat and barley, in places in rotation with vegetables. Maize is widely grown in the warm temperate belt. Maize production tends to stagnate in drier years unless the crop is irrigated adequately.

CRYOSOLS

Cryosols comprise mineral soils formed in a permafrost environment. Where water is present, it occurs primarily in the form of ice. Cryogenic processes are the dominant soil-forming processes. Cryosols are widely known as permafrost soils. Other common names for many Cryosols are: (United States of America), Cryozems (Russian Federation), Cryomorphic soils and Polar desert soils.

Summary description of Cryosols Connotation: Frost-affected soils; from Greek kryos, cold. Parent material: A wide variety of materials, including glacial till and aeolian, alluvial, colluvial and residual materials. Environment: Flat to mountainous areas in Antarctic, Arctic, subarctic and boreal regions affected by permafrost, notably in depressions. Cryosols are associated with sparsely to continuously vegetated tundra, open-canopy lichen coniferous forest and closed-canopy coniferous or mixed coniferous and deciduous forest. Chapter 4 – Description, distribution, use and management of reference soil groups 77

Profile development: In the presence of water, cryogenic processes produce cryoturbated horizons, frost heave, thermal cracking, ice segregation and patterned ground microrelief.

Regional distribution of Cryosols Geographically, Cryosols are circumpolar in both the Northern and Southern Hemispheres. They cover an estimated 180 million km2, or about 13 percent of the global land surface. Cryosols occur in the permafrost regions of the Arctic, and are widespread in the subarctic zone, discontinuous in the boreal zone, and sporadic in more temperate mountainous regions. Major areas with Cryosols are found in the Russian Federation (100 million ha), Canada (25 million ha), China (19 million ha), Alaska (11 million ha), and in parts of Mongolia. Smaller occurrences have been reported from northern Europe, Greenland and the ice-free areas of Antarctica.

Management and use of Cryosols Natural and human-induced biological activity in Cryosols is confined to the active surface layer that thaws every summer and also protects the underlying permafrost. Removal of the peat layer on top of the soil or of the vegetation and/or disturbance of the surface soil often lead to alterations of the permafrost depth and to rapid and drastic environmental changes, with possible damage to structures created by humans. Most areas of Cryosols in North America and Eurasia are in the natural state and support sufficient vegetation for grazing animals, such as caribou, reindeer and musk oxen. Large herds of caribou still migrate seasonally in the northern part of North America; reindeer herding is an important industry in the vast northern areas, especially in northern Europe. Overgrazing leads rapidly to erosion and other environmental damage. Human activities, mainly relating to agriculture, oil and gas production, and mining, have had a major impact on these soils. Severe thermokarsting has occurred on land cleared for agriculture. Improper management of pipelines and mining can cause oil spills and chemical pollution that affect large areas.

DURISOLS

Durisols are associated mainly with old surfaces in arid and semi-arid environments and accommodate very shallow to moderately deep, moderately well- to well-drained soils that contain cemented secondary silica (SiO2) within 100 cm of the soil surface. Many Durisols are known as: hardpan soils (Australia), dorbank (South Africa), Durids (United States of America), or as duripan phase of other soils, e.g. of Calcisols (FAO).

Summary description of Durisols Connotation: Soils with hardened secondary silica; from Latin durus, hard. Parent material: Silicate-rich materials, mainly alluvial and colluvial deposits of all texture classes. Environment: Level and slightly sloping alluvial plains, terraces and gently sloping piedmont plains in arid, semi-arid and Mediterranean regions. Profile development: Strongly weathered soils with a hard layer of secondary silica (petroduric horizon); eroded Durisols with exposed petroduric horizons are common in gently sloping terrain.

Regional distribution of Durisols Extensive areas of Durisols occur in Australia, in South Africa and Namibia, and in the United States of America (notably, Nevada, California and Arizona); minor occurrences have been reported from Central and South America and from Kuwait. Durisols are a 78 World reference base for soil resources 2006

relatively new introduction in international soil classification and have not often been mapped as such. A precise indication of their extent is not yet available.

Land use and management of Durisols The agricultural use of Durisols is limited to extensive grazing (rangeland). Durisols in natural environments generally support enough vegetation to contain erosion, but elsewhere erosion of the surface soil is widespread. Stable landscapes occur in dry regions where Durisols were eroded down to their resistant duripan. Durisols may be cultivated with some success where sufficient irrigation water is available. A petroduric horizon may need to be broken up or removed altogether if it forms a barrier to root and water penetration. Excess levels of soluble salts may affect Durisols in low-lying areas. Hard duripan material is widely used in road construction.

FERRALSOLS

Ferralsols represent the classical, deeply weathered, red or yellow soils of the humid tropics. These soils have diffuse horizon boundaries, a clay assemblage dominated by low-activity clays (mainly kaolinite) and a high content of sesquioxides. Local names usually refer to the colour of the soil. Many Ferralsols are known as: (United States of America); Latossolos (Brazil); Alítico, Ferrítico and Ferralítico (Cuba); Sols ferralitiques (France); and Ferralitic soils (Russian Federation).

Summary description of Ferralsols Connotation: Red and yellow tropical soils with a high content of sesquioxides; from Latin ferrum, iron, and alumen, alum. Parent material: Strongly weathered material on old, stable geomorphic surfaces; more commonly in material weathered from basic rock than from siliceous material. Environment: Typically in level to undulating land of Pleistocene age or older; less common on younger, easily weathering rocks. Perhumid or humid tropics; minor occurrences elsewhere are considered to be relics from past eras with a warmer and wetter climate than today. Profile development: Deep and intensive weathering has resulted in a residual concentration of resistant primary minerals (e.g. quartz) alongside sesquioxides and kaolinite. This mineralogy and the relatively low pH explain the stable microstructure (pseudo-sand) and yellowish (goethite) or reddish () soil colours.

Regional distribution of Ferralsols The worldwide extent of Ferralsols is estimated at some 750 million ha, almost exclusively in the humid tropics on the continental shields of South America (especially Brazil) and Africa (especially Congo, Democratic Republic of the Congo, southern Central African Republic, Angola, Guinea and eastern Madagascar). Outside the continental shields, Ferralsols are restricted to regions with easily weathering basic rock and a hot and humid climate, e.g. in Southeast Asia.

Management and use of Ferralsols Most Ferralsols have good physical properties. Great soil depth, good permeability and stable microstructure make Ferralsols less susceptible to erosion than most other intensely weathered tropical soils. Moist Ferralsols are friable and easy to work. They are well drained but may in times be droughty because of their low available water storage capacity. The chemical fertility of Ferralsols is poor; weatherable minerals are scarce or absent, and cation retention by the mineral soil fraction is weak. Under natural Chapter 4 – Description, distribution, use and management of reference soil groups 79

vegetation, nutrient elements that are taken up by the roots are eventually returned to the surface soil with falling leaves and other plant debris. The bulk of all cycling plant nutrients is contained in the biomass; available plant nutrients in the soil are concentrated in the soil organic matter. If the process of nutrient cycling is interrupted, e.g. upon introduction of low-input sedentary subsistence farming, the rootzone will rapidly become depleted of plant nutrients. Maintaining soil fertility by manuring, mulching and/or adequate (i.e. long enough) fallow periods or agroforestry practices, and prevention of surface soil erosion are important management requirements. Strong retention (fixing) of P is a characteristic problem of Ferralsols (and several other soils, e.g. Andosols). Ferralsols are normally also low in N, K, secondary nutrients (Ca, Mg and S), and some 20 micronutrients. Silica deficiency is possible where silica- demanding crops (e.g. grasses) are grown. In Mauritius, soils are tested for available silica and fertilized with silica amendments. Manganese and Zn, which are very soluble at low pH, may at some time reach toxic levels in the soil or become deficient after intense leaching of the soil. Boron and copper deficiencies may also be encountered. Liming is a means of raising the pH value of the rooted surface soil. Liming combats Al toxicity and raises the ECEC. On the other hand, it lowers the anion exchange capacity, which might lead to collapse of structure elements and slaking at the soil surface. Therefore, frequent small doses of lime or basic slag are preferable to one massive application; 0.5–2 tonnes/ha of lime or dolomite are normally enough to supply Ca as a nutrient and to buffer the low soil pH of many Ferralsols. Surface application of gypsum, as a suitably mobile form of Ca, can increase the depth of crop root development (in addition, the sulphate in the gypsum reacts with sesquioxides to produce a “self-liming” effect). This relatively recent innovation is now practised widely, especially in Brazil. Fertilizer selection and the mode and timing of fertilizer application determine to a great extent the success of agriculture on Ferralsols. Slow-release phosphate (phosphate rock) applied at a rate of several tonnes per hectare eliminates P deficiency for a number of years. For a quick fix, much more soluble double or triple superphosphate is used, needed in much smaller quantities, especially if placed in the direct vicinity of the roots. The phosphate rock option is probably only viable economically where it is locally available and when other P fertilizers are not easily purchased. Sedentary subsistence farmers and shifting cultivators on Ferralsols grow a variety of annual and perennial crops. Extensive grazing is also common and considerable areas of Ferralsols are not used for agriculture at all. The good physical properties of Ferralsols and the often level topography would encourage more intensive forms of land use if problems caused by poor chemical properties could be overcome.

FLUVISOLS

Fluvisols accommodate genetically young, azonal soils in alluvial deposits. The name Fluvisols may be misleading in the sense that these soils are not confined only to river sediments (Latin fluvius, river); they also occur in lacustrine and marine deposits. Many Fluvisols correlate with: Alluvial soils (Russian Federation); Hydrosols (Australia); Fluvents and Fluvaquents (United States of America); Auenböden, Marschen, Strandböden, Watten and Unterwasserböden (Germany); Neossolos (Brazil); and Sols minéraux bruts d’apport alluvial ou colluvial or Sols peu évolués non climatiques d’apport alluvial ou colluvial (France).

Summary description of Fluvisols Connotation: Soils developed in alluvial deposits; from Latin fluvius, river. Parent material: Predominantly recent, fluvial, lacustrine and marine deposits. 80 World reference base for soil resources 2006

Environment: Alluvial plains, river fans, valleys and tidal marshes on all continents and in all climate zones; many Fluvisols under natural conditions are flooded periodically. Profile development: Profiles with evidence of stratification; weak horizon differentiation but a distinct topsoil horizon may be present. Redoximorphic features are common, in particular in the lower part of the profile.

Regional distribution of Fluvisols Fluvisols occur on all continents and in all climates. They occupy some 350 million ha worldwide, of which more than half are in the tropics. Major concentrations of Fluvisols are found: ÿ along rivers and lakes, e.g. in the Amazon basin, the Ganges Plain of India, the plains near Chad in Central Africa, and the marshlands of Brazil, and northern Argentina; ÿ in deltaic areas, e.g. the deltas of the Ganges–Brahmaputra, Indus, Mekong, Mississippi, Nile, , Orinoco, Plate, Po, Rhine and Zambezi; ÿ in areas of recent marine deposits, e.g. the coastal lowlands of Sumatra, Kalimantan and Irian (Indonesia and Papua New Guinea). Major areas of Fluvisols with a thionic horizon or sulphidic material (Acid Sulphate Soils) are found in the coastal lowlands of Southeast Asia (Indonesia, Viet Nam and Thailand), West Africa (Senegal, Gambia, Guinea Bissau, Sierra Leone and Liberia) and along the northeast coast of South America (, , and Venezuela).

Management and use of Fluvisols The good natural fertility of most Fluvisols and attractive dwelling sites on river levees and on higher parts in marine landscapes were recognized in prehistoric times. Later, great civilizations developed in river landscapes and on marine plains. Paddy rice cultivation is widespread on tropical Fluvisols with satisfactory irrigation and drainage. Paddy land should be dry for at least a few weeks every year in order to prevent the redox potential of the soil from becoming so low that nutritional problems

(Fe or H2S) arise. A dry period also stimulates microbial activity and promotes mineralization of organic matter. Many dryland crops are grown on Fluvisols as well, normally with some form of water control. Tidal lands that are strongly saline are best kept under mangroves or some other salt-tolerant vegetation. Such areas are ecologically valuable and can, with caution, be used for fishing, hunting, salt pans or woodcutting for charcoal or fuelwood. Fluvisols with a thionic horizon or sulphidic material suffer from severe acidity and high levels of Al toxicity.

GLEYSOLS

Gleysols are wetland soils that, unless drained, are saturated with groundwater for long enough periods to develop a characteristic gleyic colour pattern. This pattern is essentially made up of reddish, brownish or yellowish colours at ped surfaces and/or in the upper soil layer or layers, in combination with greyish/bluish colours inside the peds and/or deeper in the soil. Common names for many Gleysols are: gley and meadow soils (former Soviet Union); Gleyzems (Russian Federation); Gleye (Germany); Gleissolos (Brazil); and groundwater soils. Many of the WRB Gleysols correlate with the aquic suborders of the US Soil Taxonomy (Aqualfs, Aquents, Aquepts, Aquolls, etc). Chapter 4 – Description, distribution, use and management of reference soil groups 81

Summary description of Gleysols Connotation: Soils with clear signs of groundwater influence; from Russian gley, mucky mass. Parent material: A wide range of unconsolidated materials, mainly fluvial, marine and lacustrine sediments of Pleistocene or Holocene age, with basic to acidic mineralogy. Environment: Depression areas and low landscape positions with shallow groundwater. Profile development: Evidence of reduction processes with segregation of Fe compounds within 50 cm of the soil surface.

Regional distribution of Gleysols Gleysols occupy an estimated 720 million ha worldwide. They are azonal soils and occur in nearly all climates, from perhumid to arid. The largest extent of Gleysols is in subarctic areas in the north of the Russian Federation (especially Siberia), Canada and Alaska, and in humid temperate and subtropical lowlands, e.g. in China and Bangladesh. An estimated 200 million ha of Gleysols are found in the tropics, mainly in the Amazon region, equatorial Africa, and the coastal swamps of Southeast Asia.

Management and use of Gleysols The main obstacle to utilization of Gleysols is the necessity to install a drainage system to lower the groundwater table. Adequately drained Gleysols can be used for arable cropping, and horticulture. Soil structure will be destroyed for a long time if soils are cultivated when too wet. Therefore, Gleysols in depression areas with unsatisfactory possibilities to lower the groundwater table are best kept under a permanent grass cover or swamp forest. Liming of drained Gleysols that are high in organic matter and/or of low pH value creates a better habitat for micro- and meso- organisms and enhances the rate of decomposition of soil organic matter (and the supply of plant nutrients). Gleysols can be put under tree crops only after the water table has been lowered with deep drainage ditches. Alternatively, the trees are planted on ridges that alternate with shallow depressions in which rice is grown. This sorjan system is applied widely in tidal swamp areas with pyritic sediments in Southeast Asia. Gleysols can be well used for wetland rice cultivation where the climate is appropriate. Gleysols with a thionic horizon or sulphidic material suffer from severe acidity and high levels of Al toxicity.

GYPSISOLS

Gypsisols are soils with substantial secondary accumulation of gypsum (CaSO4.2H2O). These soils are found in the driest parts of the arid climate zone, which explains why leading soil classification systems labelled many of them Desert soils (former Soviet Union), and Yermosols or Xerosols (FAO–UNESCO, 1971–1981). The US Soil Taxonomy terms most of them Gypsids.

Summary description of Gypsisols Connotation: Soils with substantial accumulation of secondary calcium sulphate; from Greek gypsos, gypsum. Parent material: Mostly unconsolidated alluvial, colluvial or aeolian deposits of base-rich weathering material. Environment: Predominantly level to hilly land and depression areas (e.g. former inland lakes) in regions with an arid climate. The natural vegetation is sparse and dominated by xerophytic shrubs and trees and/or ephemeral grasses. 82 World reference base for soil resources 2006

Profile development: Light-coloured surface horizon; accumulation of calcium sulphate, with or without carbonates, is concentrated in the subsoil.

Regional distribution of Gypsisols Gypsisols are exclusive to arid regions; their worldwide extent is probably of the order of 100 million ha. Major occurrences are in and around Mesopotamia, in desert areas in the Near East and adjacent Central Asian republics, in the Libyan and Namib deserts, in southeast and central Australia and in the southwest of the United States of America.

Management and use of Gypsisols Gypsisols that contain only a low percentage of gypsum in the upper 30 cm can be used for the production of small grains, cotton, alfalfa, etc. Dry farming on deep Gypsisols makes use of fallow years and other water harvesting techniques but is rarely very rewarding because of the adverse climate conditions. Gypsisols in young alluvial and colluvial deposits have a relatively low gypsum content. Where such soils are in the vicinity of water resources, they can be very productive; many irrigation projects are established on such soils. However, even soils containing 25 percent powdery gypsum or more could still produce excellent yields of alfalfa hay (10 tonnes/ha), wheat, apricots, dates, maize and grapes if irrigated at high rates in combination with forced drainage. Irrigated agriculture on Gypsisols is plagued by rapid dissolution of soil gypsum, resulting in irregular subsidence of the land surface, caving in canal walls, and corrosion of concrete structures. Large areas with Gypsisols are in use for extensive grazing.

HISTOSOLS

Histosols comprise soils formed in organic material. These vary from soils developed in predominantly moss peat in boreal, arctic and subarctic regions, via moss peat, reeds/ sedge peat (fen) and forest peat in temperate regions to mangrove peat and swamp forest peat in the humid tropics. Histosols are found at all altitudes, but the vast majority occurs in lowlands. Common names are peat soils, muck soils, bog soils and organic soils. Many Histosols belong to: Moore, Felshumusböden and Skeletthumusböden (Germany); Organosols (Australia); Organossolos (Brazil); Organic order (Canada); and Histosols and Histels (United States of America).

Summary description of Histosols Connotation: Peat and muck soils; from Greek histos, tissue. Parent material: Incompletely decomposed plant remains, with or without admixtures of sand, silt or clay. Environment: Histosols occur extensively in boreal, arctic and subarctic regions. Elsewhere, they are confined to poorly drained basins and depressions, swamp and marshlands with shallow groundwater, and highland areas with a high precipitation– evapotranspiration ratio. Profile development: Mineralization is slow and transformation of plant remains through biochemical disintegration, and formation of humic substances creates a surface layer of mould with or without prolonged water saturation. Translocated organic material may accumulate in deeper tiers but is more often leached from the soil.

Regional distribution of Histosols The total extent of Histosols in the world is estimated at some 325–375 million ha, the majority located in the boreal, subarctic and low arctic regions of the Northern Hemisphere. Most of the remaining Histosols occur in temperate lowlands and cool montane areas; only one-tenth of all Histosols are found in the tropics. Extensive areas Chapter 4 – Description, distribution, use and management of reference soil groups 83

of Histosols occur in the United States of America and Canada, western Europe and northern Scandinavia, and in northern regions east of the Ural mountain range. Some 20 million ha of tropical forest peat border the Sunda shelf in Southeast Asia. Smaller areas of tropical Histosols are found in river deltas, e.g. in the Orinoco Delta and the delta of the River Mekong, and in depression areas at some altitude.

Management and use of Histosols The properties of the organic material (botanical composition, stratification, degree of decomposition, packing density, wood content, mineral admixtures, etc.) and the type of peat bog (basin peat [fen], raised bog, etc.) determine the management requirements and use possibilities of Histosols. Histosols without prolonged water saturation are often formed in cold environments unattractive for agricultural use. Natural need to be drained and, normally, also limed and fertilized in order to permit cultivation of normal crops. Centrally guided reclamation projects are almost exclusive to the temperate zone, where millions of hectares have been opened. In many instances, this has initiated the gradual degradation, and ultimately the loss, of the precious peat. In the tropics, increasing numbers of landless farmers venture onto the peat lands, where they clear the forest and cause raging peat fires in the process. Many of them abandon their land again after only a few years; the few that succeed are on shallow, topogenous peat. In recent decades, increasing areas of tropical peat land have been planted to oil-palm and pulp wood tree species such as Acacia mangium, Acacia crassicarpa and Eucalyptus sp. This practice may be less than ideal but it is far less destructive than arable subsistence farming. Another common problem encountered when Histosols are drained is the oxidation of sulphidic minerals, which accumulate under anaerobic conditions, especially in coastal regions. The sulphuric acid produced effectively destroys productivity unless lime is applied copiously, making the cost of reclamation prohibitive. In summary, it is desirable to protect and conserve fragile peat lands because of their intrinsic value (especially their common function as sponges in regulating stream flow and in supporting wetlands containing unique species of animals) and because prospects for their sustained agricultural use are meagre. Where their use is imperative, sensible forms of forestry or plantation cropping are to be preferred over annual cropping, horticulture or, the worst option, harvesting of the peat material for power generation or production of horticultural growth substrate, active carbon, flower pots, etc. Peat that is used for arable crop production will mineralize at sharply increased rates because it must be drained, limed and fertilized in order to ensure satisfactory crop growth. Under these circumstances, the drain depth should be kept as shallow as possible and prudence exercised when applying lime and fertilizers.

KASTANOZEMS

Kastanozems accommodate dry grassland soils, among them the zonal soils of the short-grass steppe belt, south of the Eurasian tall-grass steppe belt with Chernozems. Kastanozems have a similar profile to that of Chernozems but the humus-rich surface horizon is thinner and not as dark as that of the Chernozems and they show more prominent accumulation of secondary carbonates. The chestnut-brown colour of the surface soil is reflected in the name ; common names for many Kastanozems are: (Dark) Chestnut Soils (Russian Federation), Kalktschernoseme (Germany), (Dark) Brown Soils (Canada), and Ustolls and Xerolls (United States of America).

Summary description of Kastanozems Connotation: Dark brown soils rich in organic matter; from Latin castanea and Russian kashtan, chestnut, and zemlja, earth or land. 84 World reference base for soil resources 2006

Parent material: A wide range of unconsolidated materials; a large part of all Kastanozems has developed in loess. Environment: Dry and continental with relatively cold winters and hot summers; flat to undulating dominated by ephemeral short grasses. Profile development: A brown mollic horizon of medium depth, in many cases over a brown to cinnamon cambic or argic horizon; with secondary carbonates or a calcic horizon in the subsoil, in some cases also with secondary gypsum.

Regional distribution of Kastanozems The total extent of Kastanozems is estimated to be about 465 million ha. Major areas are in the Eurasian short-grass steppe belt (southern Ukraine, the south of the Russian Federation, Kazakhstan and Mongolia), in the Great Plains of the United States of America, Canada and Mexico, and in the and chaco regions of northern Argentina, Paraguay and southern .

Management and use of Kastanozems Kastanozems are potentially rich soils; periodic lack of soil moisture is the main obstacle to high yields. Irrigation is nearly always necessary for high yields; care must be taken to avoid secondary salinization of the surface soil. Phosphate fertilizers might be necessary for good yields. Small grains and irrigated food and vegetable crops are the principal crops grown. Wind and water erosion is a problem on Kastanozems, especially on fallow lands. Extensive grazing is another important land use on Kastanozems. However, the sparsely vegetated grazing lands are inferior to the tall-grass steppes on Chernozems, and overgrazing is a serious problem.

LEPTOSOLS

Leptosols are very shallow soils over continuous rock and soils that are extremely gravelly and/or stony. Leptosols are azonal soils and particularly common in mountainous regions. Leptosols include the: Lithosols of the Soil Map of the World (FAO–UNESCO, 1971–1981); Lithic subgroups of the order (United States of America); Leptic Rudosols and Tenosols (Australia); and Petrozems and Litozems (Russian Federation). In many national systems, Leptosols on calcareous rocks belong to , and those on other rocks to Rankers. Continuous rock at the surface is considered non-soil in many soil classification systems.

Summary description of Leptosols Connotation: Shallow soils; from Greek leptos, thin. Parent material: Various kinds of continuous rock or of unconsolidated materials with less than 20 percent (by volume) fine earth. Environment: Mostly land at high or medium altitude and with strongly dissected topography. Leptosols are found in all climate zones (many of them in hot or cold dry regions), in particular in strongly eroding areas. Profile development: Leptosols have continuous rock at or very close to the surface or are extremely gravelly. Leptosols in calcareous weathering material may have a mollic horizon.

Regional distribution of Leptosols Leptosols are the most extensive RSG on earth, extending over about 1 655 million ha. Leptosols are found from the tropics to the cold polar tundra and from sea level to the highest mountains. Leptosols are particularly widespread in montane areas, notably in Asia and South America, in the Sahara and the Arabian deserts, the Ungava Peninsula Chapter 4 – Description, distribution, use and management of reference soil groups 85

of northern Canada and in the Alaskan mountains. Elsewhere, Leptosols can be found on rocks that are resistant to weathering or where erosion has kept pace with soil formation, or has removed the top of the soil profile. Leptosols with continuous rock at less than 10 cm depth in montane regions are the most extensive Leptosols.

Management and use of Leptosols Leptosols have a resource potential for wet-season grazing and as forest land. Leptosols to which the rendzic qualifier applies are planted to teak and mahogany in Southeast Asia; those in the temperate zone are under mainly deciduous mixed forest whereas acid Leptosols are commonly under coniferous forest. Erosion is the greatest threat to Leptosol areas, particularly in montane regions in the temperate zones where high population pressure (tourism), overexploitation and increasing environmental pollution lead to deterioration of forests and threaten large areas of vulnerable Leptosols. Leptosols on hill slopes are generally more fertile than their counterparts on more level land. One or a few good crops could perhaps be grown on such slopes but at the price of severe erosion. Steep slopes with shallow and stony soils can be transformed into cultivable land through terracing, the removal of stones by hand and their use as terrace fronts. Agroforestry (a combination of rotation of arable crops and forest under strict control) holds promise but is still largely in an experimental stage. The excessive internal drainage and the shallowness of many Leptosols can cause drought even in a humid environment.

LIXISOLS

Lixisols comprise soils that have a higher clay content in the subsoil than in the topsoil as a result of pedogenetic processes (especially clay migration) leading to an argic subsoil horizon. Lixisols have a high base saturation and low-activity clays at certain depths. Many Lixisols are included in: Red Yellow Podzolic soils (e.g. Indonesia); Argissolos (Brazil); sols ferralitiques faiblement desaturés appauvris (France); and Red and Yellow Earths, or with low-activity clays (United States of America).

Summary description of Lixisols Connotation: Soils with a pedogenetic clay differentiation (especially clay migration) between a topsoil with a lower and a subsoil with a higher clay content, low-activity clays and a high base saturation at some depths; from Latin lixivia, washed-out substances. Parent material: In a wide variety of parent materials, notably in unconsolidated, strongly weathered and strongly leached, finely textured materials. Environment: Regions with a tropical, subtropical or warm temperate climate with a pronounced dry season, notably on old erosion or deposition surfaces. Many Lixisols are surmised to be polygenetic soils with characteristics formed under a more humid climate in the past.

Regional distribution of Lixisols Lixisols are found in seasonally dry tropical, subtropical and warm temperate regions on Pleistocene and older surfaces. These soils cover a total area of about 435 million ha, of which more than half occur in sub-Sahelian and East Africa, about one-quarter in South and Central America, and the remainder on the Indian subcontinent and in Southeast Asia and Australia.

Management and use of Lixisols Areas with Lixisols that are still under natural savannah or open woodland vegetation are widely used for low volume grazing. Preservation of the surface soil with its all- 86 World reference base for soil resources 2006

important organic matter is of utmost importance. Degraded surface soils have low aggregate stability and are prone to slaking and/or erosion where exposed to the direct impact of raindrops. Tillage of wet soil or use of excessively heavy machinery compacts the soil and causes serious structure deterioration. Tillage and measures such as terracing, contour ploughing, mulching and use of cover crops help to conserve the soil. The low absolute level of plant nutrients and the low cation retention by Lixisols makes recurrent inputs of fertilizers and/or lime a precondition for continuous cultivation. Chemically and/or physically deteriorated Lixisols regenerate very slowly where not reclaimed actively. Perennial crops are to be preferred to annual crops, particularly on sloping land. Cultivation of tuber crops (cassava and sweet potato) or groundnut increases the danger of soil deterioration and erosion. Rotation of annual crops with improved pasture has been recommended in order to maintain or improve the content of soil organic matter.

LUVISOLS

Luvisols are soils that have a higher clay content in the subsoil than in the topsoil as a result of pedogenetic processes (especially clay migration) leading to an argic subsoil horizon. Luvisols have high-activity clays throughout the argic horizon and a high base saturation at certain depths. Many Luvisols are or were known as: Textural- metamorphic soils (Russian Federation), sols lessivés (France), Parabraunerden (Germany), Chromosols (Australia), Luvissolos (Brazil), Grey- soils (earlier terminology of the United States of America), and Alfisols with high-activity clays (US Soil Taxonomy).

Summary description of Luvisols Connotation: Soils with a pedogenetic clay differentiation (especially clay migration) between a topsoil with a lower and a subsoil with a higher clay content, high-activity clays and a high base saturation at some depth; from Latin luere, to wash. Parent material: A wide variety of unconsolidated materials including glacial till, and aeolian, alluvial and colluvial deposits. Environment: Most common in flat or gently sloping land in cool temperate regions and in warm regions (e.g. Mediterranean) with distinct dry and wet seasons. Profile development: Pedogenetic differentiation of clay content with a lower content in the topsoil and a higher content in the subsoil without marked leaching of base cations or advanced weathering of high-activity clays; highly leached Luvisols might have an albic eluviation horizon between the surface horizon and an argic subsurface horizon, but lack the albeluvic tonguing of Albeluvisols.

Regional distribution of Luvisols Luvisols extend over 500–600 million ha worldwide, mainly in temperate regions such as in the west and centre of the Russian Federation, the United States of America, and Central Europe, but also in the Mediterranean region and southern Australia. In subtropical and tropical regions, Luvisols occur mainly on young land surfaces.

Management and use of Luvisols Most Luvisols are fertile soils and suitable for a wide range of agricultural uses. Luvisols with a high silt content are susceptible to structure deterioration where tilled when wet or with heavy machinery. Luvisols on steep slopes require erosion control measures. The eluvial horizons of some Luvisols are depleted to the extent that an unfavourable platy structure is formed. In places, the dense subsoil causes temporarily reducing Chapter 4 – Description, distribution, use and management of reference soil groups 87

conditions with a stagnic colour pattern. These are the reasons why truncated Luvisols are in many instances better soils for farming than the original, non-eroded soils. Luvisols in the temperate zone are widely grown to small grains, sugar beet and fodder; in sloping areas, they are used for orchards, forests and/or grazing. In the Mediterranean region, where Luvisols (many with the chromic, calcic or vertic qualifier) are common in colluvial deposits of limestone weathering, the lower slopes are widely sown to wheat and/or sugar beet while the often eroded upper slopes are used for extensive grazing or planted to tree crops.

NITISOLS

Nitisols are deep, well-drained, red, tropical soils with diffuse horizon boundaries and a subsurface horizon with more than 30 percent clay and moderate to strong angular blocky structure elements that easily fall apart into characteristic shiny, polyhedric (nutty) elements. Weathering is relatively advanced but Nitisols are far more productive than most other red, tropical soils. Many Nitisols correlate with: Nitossolos (Brazil); kandic Great Groups of Alfisols and Ultisols, and different Great Groups of Inceptisols and Oxisols (United States of America); Sols Fersialitiques or Ferrisols (France); and Red Earths.

Summary description of Nitisols Connotation: Deep, well-drained, red, tropical soils with a clayey nitic subsurface horizon that has typical nutty, polyhedric, blocky structure elements with shiny ped faces; from Latin nitidus, shiny. Parent material: Finely textured weathering products of intermediate to basic parent rock, in some regions rejuvenated by recent admixtures of volcanic ash. Environment: Nitisols are predominantly found in level to hilly land under tropical rain forest or savannah vegetation. Profile development: Red or reddish-brown clayey soils with a nitic subsurface horizon of high aggregate stability. The clay assemblage of Nitisols is dominated by kaolinite/(meta)halloysite. Nitisols are rich in Fe and have little water-dispersible clay.

Regional distribution of Nitisols There are about 200 million ha of Nitisols worldwide. More than half of all Nitisols are found in tropical Africa, notably in the highlands (> 1 000 m) of Ethiopia, Kenya, Congo and Cameroon. Elsewhere, Nitisols are well represented at lower altitudes, e.g. in tropical Asia, South America, Central America, Southeast Africa and Australia.

Management and use of Nitisols Nitisols are among the most productive soils of the humid tropics. The deep and porous solum and the stable soil structure of Nitisols permit deep rooting and make these soils quite resistant to erosion. The good workability of Nitisols, their good internal drainage and fair water holding properties are complemented by chemical (fertility) properties that compare favourably with those of most other tropical soils. Nitisols have relatively high contents of weathering minerals, and surface soils may contain several percent of organic matter, in particular under forest or tree crops. Nitisols are planted to plantation crops such as cocoa, coffee, rubber and pineapple, and are also widely used for food crop production on smallholdings. High P sorption calls for application of P fertilizers, usually provided as slow-release, low-grade phosphate rock (several tonnes per hectare, with maintenance doses every few years) in combination with smaller applications of better soluble superphosphate for short-term response by the crop. 88 World reference base for soil resources 2006

PHAEOZEMS

Phaeozems accommodate soils of relatively wet grassland and forest regions in moderately continental climates. Phaeozems are much like Chernozems and Kastanozems but are leached more intensively. Consequently, they have dark, humus- rich surface horizons that, in comparison with Chernozems and Kastanozems, are less rich in bases. Phaeozems may or may not have secondary carbonates but have a high base saturation in the upper metre of the soil. Commonly used names for many Phaeozems are: Brunizems (Argentina and France); Dark grey forest soils and Leached and podzolized chernozems (former Soviet Union); Tschernoseme (Germany); Dusky- red prairie soils (older classification of the United States of America); Udolls and Albolls (US Soil Taxonomy); and Phaeozems (including most of the former Greyzems) (FAO).

Summary description of Phaeozems Connotation: Dark soils rich in organic matter; from Greek phaios, dusky, and Russian zemlja, earth or land. Parent material: Aeolian (loess), glacial till and other unconsolidated, predominantly basic materials. Environment: Warm to cool (e.g. tropical highlands) moderately continental regions, humid enough that there is, in most years, some percolation through the soil, but also with periods in which the soil dries out; flat to undulating land; the natural vegetation is grassland such as tall-grass steppe and/or forest. Profile development: A mollic horizon (thinner and in many soils less dark than in Chernozems), mostly over a cambic or argic subsurface horizon.

Regional distribution of Phaeozems Phaeozems cover an estimated 190 million ha worldwide. Some 70 million ha of Phaeozems are found in the humid and subhumid Central Lowlands and easternmost parts of the Great Plains of the United States of America. Another 50 million ha of Phaeozems are in the subtropical pampas of Argentina and . The third largest area of Phaeozems (18 million ha) is in northeast China, followed by extensive areas in the centre of the Russian Federation. Smaller, mostly discontinuous areas are found in Central Europe, notably the Danube area of Hungary and adjacent countries and in montane areas in the tropics.

Management and use of Phaeozems Phaeozems are porous, fertile soils and make excellent farmland. In the United States of America and Argentina, Phaeozems are in use for the production of soybean and wheat (and other small grains). Phaeozems on the high plains of Texas produce good yields of irrigated cotton. Phaeozems in the temperate belt are planted to wheat, barley and vegetables alongside other crops. Wind and water erosion are serious hazards. Vast areas of Phaeozems are used for cattle rearing and fattening on improved pastures.

PLANOSOLS

Planosols are soils with a light-coloured, surface horizon that shows signs of periodic water stagnation and abruptly overlies a dense, slowly permeable subsoil with significantly more clay than the surface horizon. The US Soil Classification coined the name Planosols in 1938; its successor, the US Soil Taxonomy, includes most of the original Planosols in the Great Groups of the Albaqualfs, Albaquults and Argialbolls. The name has been adopted in Brazil (Planossolos). Chapter 4 – Description, distribution, use and management of reference soil groups 89

Summary description of Planosols Connotation: Soils with a coarse-textured surface horizon abruptly over a dense and finer textured subsoil, typically in seasonally waterlogged flat lands; from Latin planus, flat. Parent material: Mostly clayey alluvial and colluvial deposits. Environment: Seasonally or periodically wet, level (plateau) areas, mainly in subtropical and temperate, semi-arid and subhumid regions with light forest or grass vegetation. Profile development: Geological stratification or pedogenesis (destruction and/or removal of clay), or both, has produced relatively coarse-textured, light-coloured surface soil abruptly overlying finer textured subsoil; impeded downward percolation of water causes temporarily reducing conditions with a stagnic colour pattern, at least close to the abrupt textural change.

Regional distribution of Planosols The world’s major Planosol areas occur in subtropical and temperate regions with clear alternation of wet and dry seasons, e.g. in Latin America (southern Brazil, Paraguay and Argentina), Africa (Sahelian zone, East and Southern Africa), the east of the United States of America, Southeast Asia (Bangladesh and Thailand), and Australia. Their total extent is estimated at some 130 million ha.

Management and use of Planosols Natural Planosol areas support a sparse grass vegetation, often with scattered shrubs and trees that have shallow root systems and can cope with temporary waterlogging. Land use on Planosols is normally less intensive than that on most other soils under the same climate conditions. Vast areas of Planosols are used for extensive grazing. Wood production on Planosols is much lower than on other soils under the same conditions. Planosols in the temperate zone are mainly in grass or they are planted to arable crops such as wheat and sugar beet. Yields are modest even on drained and deeply loosened soils. Root development on natural unmodified Planosols is hindered severely by oxygen deficiency in wet periods, dense subsoil and, in places, by toxic levels of Al in the rootzone. The low hydraulic conductivity of the dense subsurface soil makes narrow drain spacing necessary. Surface modification such as ridge and furrow can lessen crop yield losses from waterlogging. Planosols in Southeast Asia are widely planted to a single crop of paddy rice, produced on bunded fields that are inundated in the rainy season. Efforts to produce dryland crops on the same land during the dry season have met with little success; the soils seem better suited to a second crop of rice with supplemental irrigation. Fertilizers are needed for good yields. Paddy fields should be allowed to dry out at least once a year in order to prevent or minimize microelement deficiencies or toxicity associated with prolonged soil reduction. Some Planosols require application of more than just NPK fertilizers, and their low fertility level may prove difficult to correct. Where temperature permits paddy rice cultivation, this is probably superior to any other kind of land use. Grasslands with supplemental irrigation in the dry season are a good land use in climates with long dry periods and short infrequent wet spells. Strongly developed Planosols with a very silty or sandy surface soil are perhaps best left untouched.

PLINTHOSOLS

Plinthosols are soils with plinthite, petroplinthite or pisoliths. Plinthite is an Fe-rich (in some cases also Mn-rich), humus-poor mixture of kaolinitic clay (and other products 90 World reference base for soil resources 2006

of strong weathering such as gibbsite) with quartz and other constituents that changes irreversibly to a layer with hard nodules, a hardpan or irregular aggregates on exposure to repeated wetting and drying. Petroplinthite is a continuous, fractured or broken sheet of connected, strongly cemented to indurated nodules or mottles. Pisoliths are discrete strongly cemented to indurated nodules. Both petroplinthite and pisoliths develop from plinthite by hardening. Many of these soils are known as: Groundwater Laterite Soils, Perched Water Laterite Soils and Plintossolos (Brazil); Sols gris latéritiques (France); and Plinthaquox, Plinthaqualfs, Plinthoxeralfs, Plinthustalfs, Plinthaquults, Plinthohumults, Plinthudults and Plinthustults (United States of America).

Summary description of Plinthosols Connotation: Soils with plinthite, petroplinthite or pisoliths; from Greek plinthos, brick. Parent material: Plinthite is more common in weathering material from basic rock than in acidic rock weathering. In any case, it is crucial that sufficient Fe be present, originating either from the parent material itself or brought in by seepage water or ascending groundwater from elsewhere. Environment: Formation of plinthite is associated with level to gently sloping areas with fluctuating groundwater or stagnating surface water. A widely held view is that plinthite is associated with rain forest areas whereas petroplinthic and pisolithic soils are more common in the savannah zone. Profile development: Strong weathering with subsequent segregation of plinthite at the depth of groundwater fluctuation or impeded surface water drainage. Hardening of plinthite to pisoliths or petroplinthite takes place upon repeated drying and wetting. This may occur during the intervals of recession of a seasonally fluctuating water table or after geological uplift of the terrain, topsoil erosion, lowering of the groundwater level, increasing drainage capacity, and/or climate change towards drier conditions. Petroplinthite may break up into irregular aggregates or gravels, which may be transported to form colluvial or alluvial deposits. Hardening or induration requires a certain minimum concentration of iron oxides.

Regional distribution of Plinthosols The global extent of Plinthosols is estimated at some 60 million ha. Soft plinthite is most common in the wet tropics, notably in the eastern Amazon basin, the central Congo basin and parts of Southeast Asia. Extensive areas with pisoliths and petroplinthite occur in the Sudano-Sahelian zone, where petroplinthite forms hard caps on top of uplifted/exposed landscape elements. Similar soils occur in the Southern African savannah, on the Indian subcontinent, and in drier parts of Southeast Asia and northern Australia.

Management and use of Plinthosols Plinthosols present considerable management problems. Poor natural soil fertility caused by strong weathering, waterlogging in bottomlands and drought on Plinthosols with petroplinthite, pisoliths or gravels are serious limitations. Many Plinthosols outside of the wet tropics have shallow, continuous petroplinthite, which limits the rooting volume to the extent that arable farming is not possible; such land can at best be used for low-volume grazing. Soils with high contents of pisoliths (up to 80 percent) are still planted to food crops and tree crops (e.g. cocoa in West Africa, and cashew in India) but the crops suffer from drought in the dry season. Many soil and water conservation techniques are used to improve these soils for urban and peri-urban agriculture in West Africa. Civil engineers have a different appreciation of petroplinthite and plinthite than do agronomists. To them, plinthite is a valuable material for making bricks, and massive Chapter 4 – Description, distribution, use and management of reference soil groups 91

petroplinthite is a stable surface for building or it can be cut to building blocks. Gravels of broken petroplinthite can be used in foundations and as surfacing material on roads and airfields. In some instances, petroplinthite is a valuable of Fe, Al, Mn and/or Ti.

PODZOLS

Podzols are soils with a typically ash-grey upper subsurface horizon, bleached by loss of organic matter and iron oxides, on top of a dark accumulation horizon with brown, reddish or black illuviated humus and/or reddish Fe compounds. Podzols occur in humid areas in the boreal and temperate zones and locally also in the tropics. The name Podzol is used in most national soil classification systems; other names for many of these soils are: Spodosols (China and United States of America), Espodossolos (Brazil), and Podosols (Australia).

Summary description of Podzols Connotation: Soils with a spodic illuviation horizon under a subsurface horizon that has the appearance of ash and is covered by an organic layer; from Russian pod, underneath, and zola, ash. Parent material: Weathering materials of siliceous rock, including glacial till and alluvial and aeolian deposits of quartzite sands. Podzols in the boreal zone occur on almost any rock. Environment: Mainly in humid temperate and boreal regions of the Northern Hemisphere, in level to hilly land under heather and/or coniferous forest; in the humid tropics under light forest. Profile development: Complexes of Al, Fe and organic compounds migrate from the surface soil downwards with percolating rainwater. The metal–humus complexes precipitate in an illuvial spodic horizon; the overlying eluvial horizon remains bleached and is in many Podzols an albic horizon. This is covered by an organic layer whereas dark mineral topsoil horizons are absent in most boreal Podzols.

Regional distribution of Podzols Podzols cover an estimated 485 million ha worldwide, mainly in the temperate and boreal regions of the Northern Hemisphere. They are extensive in Scandinavia, the northwest of the Russian Federation, and Canada. Besides these zonal Podzols, there are smaller occurrences of intrazonal Podzols in both the temperate zone and the tropics. Tropical Podzols occur on less than 10 million ha, mainly in residual sandstone weathering in perhumid regions and in alluvial quartz sands, e.g. in uplifted coastal areas. The exact distribution of tropical Podzols is not known; important occurrences are found along the Rio Negro and in French Guiana, Guyana and Suriname in South America, in the Malaysian region (Kalimantan, Sumatra and Irian), and in northern and southern Australia. They seem to be less common in Africa.

Management and use of Podzols Zonal Podzols occur in regions with unattractive climate conditions for most arable land uses. Intrazonal Podzols are more frequently reclaimed for arable use than are zonal Podzols, particularly those in temperate climates. The low nutrient status, low level of available moisture and low pH make Podzols unattractive soils for arable farming. Aluminium toxicity and P deficiency are common problems. Deep ploughing (to improve the moisture storage capacity of the soil and/or to eliminate a dense illuviation horizon or hardpan), liming and fertilization are the main ameliorative measures taken. Trace elements may migrate with the metal–humus complexes. In the 92 World reference base for soil resources 2006

Western Cape region of South Africa, deeper rooted orchards and vineyards suffer fewer trace element deficiencies than do shallow-rooted vegetable crops. Most zonal Podzols are under forest; intrazonal Podzols in temperate regions are mostly under forest or shrubs (heath). Tropical Podzols normally sustain a light forest that recovers only slowly after cutting or burning. Mature Podzols are generally best used for extensive grazing or left idle under their natural (climax) vegetation.

REGOSOLS

Regosols form a taxonomic remnant group containing all soils that could not be accommodated in any of the other RSGs. In practice, Regosols are very weakly developed mineral soils in unconsolidated materials that do not have a mollic or umbric horizon, are not very shallow or very rich in gravels (Leptosols), sandy (Arenosols) or with fluvic materials (Fluvisols). Regosols are extensive in eroding lands, particularly in arid and semi-arid areas and in mountainous terrain. Many Regosols correlate with soil taxa that are marked by incipient soil formation such as: (United States of America); Rudosols (Australia); Regosole (Germany); Sols peu évolués régosoliques d’érosion or even Sols minéraux bruts d’apport éolien ou volcanique (France); and Neossolos (Brazil).

Summary description of Regosols Connotation: Weakly developed soils in unconsolidated material; from Greek rhegos, blanket. Parent material: unconsolidated, finely grained material. Environment: All climate zones without permafrost and at all elevations. Regosols are particularly common in arid areas (including the dry tropics) and in mountain regions. Profile development: No diagnostic horizons. Profile development is minimal as a consequence of young age and/or slow soil formation, e.g. because of aridity. Regional distribution of Regosols Regosols cover an estimated 260 million ha worldwide, mainly in arid areas in the mid-west of the United States of America, northern Africa, the Near East, and Australia. Some 50 million ha of Regosols occur in the dry tropics and another 36 million ha in mountain areas. The extent of most Regosol areas is only limited; consequently, Regosols are common inclusions in other map units on small-scale maps.

Management and use of Regosols Regosols in desert areas have minimal agricultural significance. Regosols with rainfall of 500–1 000 mm/year need irrigation for satisfactory crop production. The low moisture holding capacity of these soils calls for frequent applications of irrigation water; sprinkler or trickle irrigation solves the problem but is rarely economic. Where rainfall exceeds 750 mm/year, the entire profile is raised to its water holding capacity early in the ; improvement of dry farming practices may then be a better investment than installation of costly irrigation facilities. Many Regosols are used for extensive grazing. Regosols on colluvial deposits in the loess belt of northern Europe and North America are mostly cultivated; they are planted to small grains, sugar beet and fruit trees. Regosols in mountainous regions are delicate and best left under forest.

SOLONCHAKS

Solonchaks are soils that have a high concentration of soluble salts at some time in the year. Solonchaks are largely confined to the arid and semi-arid climate zones and Chapter 4 – Description, distribution, use and management of reference soil groups 93

to coastal regions in all climates. Common international names are saline soils and salt-affected soils. In national soil classification systems, many Solonchaks belong to: halomorphic soils (Russian Federation), Halosols (China), and Salids (United States of America).

Summary description of Solonchaks Connotation: Saline soils; from Russian sol, salt. Parent material: Virtually any unconsolidated material. Environment: Arid and semi-arid regions, notably in areas where ascending groundwater reaches the solum, with vegetation of grasses and/or halophytic herbs, and in inadequately managed irrigation areas. Solonchaks in coastal areas occur in all climates. Profile development: From weakly to strongly weathered, many Solonchaks have a gleyic colour pattern at some depth. In low-lying areas with a shallow water table, salt accumulation is strongest at the soil surface of the soil (external Solonchaks). Solonchaks where ascending groundwater does not reach the topsoil (or even the solum) have the greatest accumulation of salts at some depth below the soil surface (internal Solonchaks).

Regional distribution of Solonchaks The total extent of Solonchaks in the world is estimated at about 260 million ha. Solonchaks are most extensive in the Northern Hemisphere, notably in the arid and semi-arid parts of northern Africa, the Near East, the former Soviet Union and Central Asia; they are also widespread in Australia and the Americas.

Management and use of Solonchaks Excessive accumulation of salts in soil affects plant growth in two ways: ÿ The salts aggravate drought stress because dissolved electrolytes create an osmotic potential that affects water uptake by plants. Before any water can be taken up from the soil, plants must compensate the combined forces of the matrix potential of the soil, i.e. the force with which the soil matrix retains water, and the osmotic potential. As a rule of thumb, the osmotic potential of a soil solution (in hectoPascals) amounts to some 650 × EC (dS/m). The total potential that can be compensated by plants (known as the critical leaf water head) varies strongly between plant species. Plant species that stem from the humid tropics have a comparatively low critical leaf water head. For example, green peppers can compensate a total soil moisture potential (matric plus osmotic forces) of only some 3 500 hPa whereas cotton, a crop that evolved in arid and semi-arid climates, survives some 25 000 hPa. ÿ The salts upset the balance of ions in the soil solution because nutrients are proportionately less available. Antagonistic effects are known to exist, e.g. between Na and K, between Na and Ca, and between Mg and K. In higher concentrations, the salts may be directly toxic to plants. Very harmful in this respect are Na ions and chloride ions (they disturb N metabolism). Farmers on Solonchaks adapt their cultivation methods. For example, plants on furrow-irrigated fields are not planted on the top of the ridges but at half height. This ensures that the roots benefit from the irrigation water while salt accumulation is strongest near the top of the ridge, away from the root systems. Strongly salt-affected soils have little agricultural value. They are used for extensive grazing of , goats, camels and cattle, or lie idle. Only after the salts have been flushed from the soil (which then ceases to be a ) may good yields be hoped for. Application of irrigation water must not only satisfy the needs of the crop, but excess water must be applied above the irrigation requirement in order to maintain a downward water flow in the 94 World reference base for soil resources 2006

soil and to flush excess salts from the rootzone. Irrigation of crops in arid and semi- arid regions must be accompanied by drainage whereby drainage facilities should be designed to keep the groundwater table below the critical depth. Use of gypsum assists in maintaining hydraulic conductivity while salts are being flushed out with irrigation water.

SOLONETZ

Solonetz are soils with a dense, strongly structured, clayey subsurface horizon that has a high proportion of adsorbed Na and/or Mg ions. Solonetz that contain free

soda (Na2CO3) are strongly alkaline (field pH > 8.5). Common international names are alkali soils and sodic soils. In national soil classification systems many Solonetz correlate with: Sodosols (Australia), the Solonetzic order (Canada), various Solonetz types (Russian Federation), and to the natric Great Groups of several Orders (United States of America).

Summary description of Solonetz Connotation: Soils with a high content of exchangeable Na and/or Mg ions; from Russian sol, salt. Parent material: Unconsolidated materials, mostly fine-textured sediments. Environment: Solonetz are normally associated with flat lands in a climate with hot, dry summers, or with (former) coastal deposits that contain a high proportion of Na ions. Major concentrations of Solonetz are in flat or gently sloping grasslands with loess, loam or clay in semi-arid, temperate and subtropical regions. Profile development: A black or brown surface soil over a natric horizon with strong round-topped columnar structure elements. Well-developed Solonetz can have an albic eluviation horizon (beginning) directly over the natric horizon. A calcic or gypsic horizon may be present below the natric horizon. Many Solonetz have a field pH of about 8.5, indicative of the presence of free sodium carbonate.

Regional distribution of Solonetz Solonetz occur predominantly in areas with a steppe climate (dry summers and an annual precipitation sum of not more than 400–500 mm), in particular in flat lands with impeded vertical and lateral drainage. Smaller occurrences are found on inherently saline parent materials (e.g. marine clays or saline alluvial deposits). Worldwide, Solonetz cover some 135 million ha. Major Solonetz areas are found in Ukraine, Russian Federation, Kazakhstan, Hungary, Bulgaria, Romania, China, United States of America, Canada, South Africa, Argentina and Australia.

Management and use of Solonetz The suitability of virgin Solonetz for agricultural uses is dictated almost entirely by the depth and properties of the surface soil. A deep (> 25 cm) humus-rich surface soil is needed for successful arable crop production. However, most Solonetz have only a much shallower surface horizon, or have lost the surface horizon altogether. Solonetz amelioration has two basic elements: ÿ improvement of the porosity of the surface or subsurface soil; ÿ lowering of the ESP. Most reclamation attempts start with incorporation of gypsum or, exceptionally, calcium chloride in the soil. Where lime or gypsum occur at shallow depth in the soil body, deep ploughing (mixing the carbonate or gypsum containing subsoil with the surface soil) may make expensive amendments superfluous. Traditional reclamation strategies start with the planting of an Na-resistant crop, e.g. Rhodes grass, to gradually improve the permeability of the soil. Once a functioning pore system is in place, Na Chapter 4 – Description, distribution, use and management of reference soil groups 95

ions are carefully leached from the soil with good-quality (Ca-rich) water (relatively pure water should be avoided because it exacerbates the problem). An extreme reclamation method (developed in Armenia and applied successfully to Solonetz with a calcic or petrocalcic horizon in the Arax Valley) uses diluted sulphuric acid (a waste product of the metallurgical industry) to dissolve CaCO3 contained in the soil. This brings Ca ions in the soil solution, which displace exchangeable Na. The practice improves soil aggregation and soil permeability. The resulting sodium sulphate (in the soil solution) is subsequently flushed out of the soil. In India, pyrite was applied to Solonetz to produce sulphuric acid, thus lowering extreme alkalinity and overcoming Fe deficiency. Ameliorated Solonetz can produce a fair crop foodgrain or forage. The majority of the world’s Solonetz have never been reclaimed and are used for extensive grazing or lie idle.

STAGNOSOLS

Stagnosols are soils with a perched water table showing redoximorphic features caused by surface water. Stagnosols are periodically wet and mottled in the topsoil and subsoil, with or without concretions and/or bleaching. A common name in many national classification systems for most Stagnosols is pseudogley. In the US Soil Taxonomy, many of them belong to the Aqualfs, Aquults, Aquents, Aquepts and Aquolls.

Summary description of Stagnosols Connotation: From Latin stagnare, to flood. Parent material: A wide variety of unconsolidated materials including glacial till, and loamy aeolian, alluvial and colluvial deposits, but also physically weathered silt stone. Environment: Most common in flat or gently sloping land in cool temperate to subtropical regions with humid to perhumid climate conditions. Profile development: Similar to strongly mottled Luvisols, Cambisols or Umbrisols; the topsoil can also be completely bleached (albic horizon). Regional distribution of Stagnosols Stagnosols cover 150–200 million ha worldwide; for the greater part in humid to perhumid temperate regions of West and Central Europe, North America, southeast Australia, and Argentina, associated with Luvisols as well as silty to clayey Cambisols and Umbrisols. They also occur in humid to perhumid subtropical regions, associated with Acrisols and Planosols.

Management and use of Stagnosols The agricultural suitability of Stagnosols is limited because of their oxygen deficiency resulting from stagnating water above a dense subsoil. Therefore, they have to be drained. However, in contrast to Gleysols, drainage with channels or pipes is in many cases insufficient. It is necessary to have a higher porosity in the subsoil in order to improve the hydraulic conductivity. This may be achieved by deep loosening or deep ploughing. Drained Stagnosols can be fertile soils owing to their moderate degree of leaching.

TECHNOSOLS

Technosols comprise a new RSG and combine soils whose properties and pedogenesis are dominated by their technical origin. They contain a significant amount of artefacts (something in the soil recognizably made or extracted from the earth by humans), or are sealed by technic hard rock (material created by humans, having properties unlike natural rock). They include soils from wastes (landfills, sludge, cinders, mine spoils 96 World reference base for soil resources 2006

and ashes), pavements with their underlying unconsolidated materials, soils with geomembranes and constructed soils in human-made materials. Technosols are often referred to as urban or mine soils. They are recognized in the new Russian soil classification system as Technogenic Superficial Formations.

Summary description of Technosols Connotation: Soils dominated or strongly influenced by human-made material; from Greek technikos, skilfully made Parent material: All kinds of materials made or exposed by human activity that otherwise would not occur at the Earth’s surface; pedogenesis in these soils is affected strongly by materials and their organization. Environment: Mostly in urban and industrial areas, in small areas, although in a complex pattern associated with other groups. Profile development: Generally none, although in old dumps (e.g. Roman rubble) evidence of natural pedogenesis can be observed, such as clay translocation. Lignite and fly ash deposits may exhibit over time vitric or andic properties (Zikeli, Kastler and Jahn, 2004; Zevenbergen et al., 1999). Original profile development may still be present in contaminated natural soils.

Regional distribution of Technosols Technosols are found throughout the world where human activity has led to the construction of artificial soil, sealing of natural soil, or extraction of material normally not affected by surface processes. Thus, cities, roads, mines, refuse dumps, oil spills, coal fly ash deposits and the like are included in Technosols.

Management and use of Technosols Technosols are affected strongly by the nature of the material or the human activity that placed it. They are more likely to be contaminated than soils from other RSGs. Many Technosols have to be treated with care as they may contain toxic substances resulting from industrial processes. Many Technosols, in particular the ones in refuse dumps, are currently covered with a layer of natural soil material in order to permit revegetation. Such a layer forms part of the Technosol, provided that the requirement of 20 percent or more (by volume, by weighted average) artefacts in the upper 100 cm of the soil surface or to continuous rock or a cemented or indurated layer, whichever is shallower, of the Technosol definition is met.

UMBRISOLS

Umbrisols accommodate soils in which organic matter has accumulated within the mineral surface soil (in most cases with low base saturation) to the extent that it significantly affects the behaviour and utilization of the soil. Umbrisols are the logical counterpart of soils with a mollic horizon and a high base saturation throughout (Chernozems, Kastanozems and Phaeozems). Not previously recognized at such a high taxonomic level, many of these soils are classified in other systems as: several Great Groups of Entisols and Inceptisols (United States of America); Humic Cambisols and Umbric Regosols (FAO); Sombric Brunisols and Humic Regosols (France); Much dark-humus soils (Russian Federation); Brown Podzolic soils (e.g. Indonesia); and Umbrisols (Romania).

Summary description of Umbrisols Connotation: Soils with dark topsoil; from Latin umbra, shade. Parent material: Weathering material of siliceous rock. Chapter 4 – Description, distribution, use and management of reference soil groups 97

Environment: Humid climates; common in mountainous regions with little or no moisture deficit, in mostly cool areas but including tropical and subtropical mountains. Profile development: Dark brown umbric (seldom: mollic) surface horizon, in many cases over a cambic subsurface horizon with low base saturation.

Regional distribution of Umbrisols Umbrisols occur in cool, humid regions, mostly mountainous and with little or no soil moisture deficit. They occupy about 100 million ha throughout the world. In South America, Umbrisols are common in the Andean ranges of Colombia, Ecuador and, to a lesser extent, in Venezuela, Bolivia and Peru. They also occur in Brazil, e.g. in the Serra do Mar, and in and South Africa, e.g. in the Drakensberg range. Umbrisols in North America are confined largely to the northwest Pacific seaboard. In Europe, Umbrisols occur along the northwest Atlantic seaboard, e.g. in Iceland, on the British Isles and in northwest and . In Asia, they are found in the mountain ranges east and west of Lake Baikal, and on fringes of the , notably in India, , China and . Umbrisols occur at lower altitudes in Manipur (eastern India), in the Chin Hills (western Myanmar) and in Sumatra (Barisan range). In , Umbrisols are found in the mountain ranges of Papua New Guinea and southeast Australia and in the eastern parts of , New Zealand.

Management and use of Umbrisols Many Umbrisols are under a natural or near-natural vegetation cover. Umbrisols above the tree line in the Andean, Himalayan and Central Asian mountain ranges, or at lower altitudes in northern and western Europe where the former forest vegetation has been largely cleared, carry a vegetation of short grasses of low nutritional value. Coniferous forest predominates in Brazil (e.g. Araucaria spp.) and in the United States of America (mainly Thuja, Tsuga and Pseudotsuga spp.). Umbrisols in tropical mountain areas in and Oceania are under montane evergreen forest. In the mountains of southern Mexico, the vegetation varies from tropical semi-deciduous forest to much cooler montane . The predominance of sloping land and wet and cool climate conditions restricts utilization of many Umbrisols to extensive grazing. Management focuses on the introduction of improved grasses and correction of the soil pH by liming. Many Umbrisols are susceptible to erosion. The planting of perennial crops and bench or contour terracing offer possibilities for permanent agriculture on gentler slopes. Where conditions are suitable, cash crops may be grown, e.g. cereals and root crops in the United States of America, Europe and South America, or tea and in South Asia (Indonesia). Highland coffee on Umbrisols demands high management inputs to meet its stringent nutrient requirements. In New Zealand, Umbrisols have been transformed into highly productive soils, used for intensive sheep and dairy farming, and production of cash crops.

VERTISOLS

Vertisols are churning, heavy clay soils with a high proportion of swelling clays. These soils form deep wide cracks from the surface downward when they dry out, which happens in most years. The name Vertisols (from Latin vertere, to turn) refers to the constant internal turnover of soil material. Common local names for many Vertisols are: black cotton soils, regur (India), black turf soils (South Africa), margalites (Indonesia), Vertosols (Australia), Vertissolos (Brazil), and Vertisols (United States of America). 98 World reference base for soil resources 2006

Summary description of Vertisols Connotation: Churning, heavy clay soils; from Latin vertere, to turn. Parent material: Sediments that contain a high proportion of swelling clays, or products of rock weathering that have the characteristics of swelling clays. Environment: Depressions and level to undulating areas, mainly in tropical, subtropical, semi-arid to subhumid and humid climates with an alternation of distinct wet and dry seasons. The climax vegetation is savannah, natural grassland and/or woodland. Profile development: Alternate swelling and shrinking of expanding clays results in deep cracks in the dry season, and formation of slickensides and wedge-shaped structural elements in the subsurface soil. Gilgai microrelief is peculiar to Vertisols although not commonly encountered.

Regional distribution of Vertisols Vertisols cover 335 million ha worldwide. An estimated 150 million ha is potential cropland. Vertisols in the tropics cover some 200 million ha; one-quarter of this is considered to be useful land. Most Vertisols occur in the semi-arid tropics, with an average annual rainfall of 500–1 000 mm, but Vertisols are also found in the wet tropics, e.g. Trinidad (where the annual rainfall sum amounts to 3 000 mm). The largest Vertisol areas are on sediments that have a high content of smectitic clays or that produce such clays upon post-depositional weathering (e.g. in the Sudan), and on extensive basalt plateaus (e.g. in India and Ethiopia). Vertisols are also prominent in South Africa, Australia, the southwest of the United States of America (Texas), Uruguay, Paraguay and Argentina. Vertisols are typically found in lower landscape positions such as dry lake bottoms, river basins, lower river terraces, and other lowlands that are periodically wet in their natural state.

Management and use of Vertisols Large areas of Vertisols in the semi-arid tropics are still unused or are used only for extensive grazing, wood chopping, charcoal burning and the like. These soils have considerable agricultural potential, but adapted management is a precondition for sustained production. The comparatively good chemical fertility and their occurrence on extensive level plains where reclamation and mechanical cultivation can be envisaged are assets of Vertisols. Their physical soil characteristics and, notably, their difficult water management cause problems. Buildings and other structures on Vertisols are at risk, and engineers have to take special precautions to avoid damage. The agricultural uses of Vertisols range from very extensive (grazing, collection of fuelwood, and charcoal burning) through smallholder post-rainy season crop production (millet, sorghum, cotton and chickpeas) to small-scale (rice) and large-scale irrigated agriculture (cotton, wheat, barley, sorghum, chickpeas, flax, noug [Guzotia abessynica] and sugar cane). Cotton is known to perform well on Vertisols, allegedly because cotton has a vertical root system that is not damaged severely by cracking of the soil. Tree crops are generally less successful because tree roots find it difficult to establish themselves in the subsoil and are damaged as the soil shrinks and swells. Management practices for crop production should be directed primarily at water control in combination with conservation or improvement of soil fertility. The physical properties and the soil moisture regime of Vertisols represent serious management constraints. The heavy soil texture and domination of expanding clay minerals result in a narrow soil moisture range between moisture stress and water excess. Tillage is hindered by stickiness when the soil is wet and hardness when it is dry. The susceptibility of Vertisols to waterlogging may be the single most important factor that reduces the actual growing period. Excess water in the rainy season must be stored for post-rainy season use (water harvesting) on Vertisols with very slow infiltration rates. Chapter 4 – Description, distribution, use and management of reference soil groups 99

One compensation for the shrink–swell characteristics is the phenomenon of self- mulching that is common on many Vertisols. Large clods produced by primary tillage break down with gradual drying into fine peds, which provide a passable seed bed with minimal effort. For the same reason, gully erosion on overgrazed Vertisols is seldom severe because gully walls soon assume a shallow angle of repose, which allows grass to become re-established more readily.

101

Chapter 5 Definitions of formative elements for second-level units of the WRB

The definitions of the formative elements for the second-level units relate to RSGs, diagnostic horizons, properties and materials, attributes such as colour, chemical conditions, texture, etc. Reference to the RSGs defined in Chapters 3 and 4 and the diagnostic features listed in Chapter 2 is given in italics. Usually, only a limited number of combinations will be possible; most of the definitions are mutually exclusive.

Abruptic (ap) Having an abrupt textural change within 100 cm of the soil surface.

Aceric (ae) Having a pH (1:1 in water) between 3.5 and 5 and jarosite mottles in some layer within 100 cm of the soil surface (in Solonchaks only).

Acric (ac) -1 Having an argic horizon that has a CEC (by 1 M NH4OAc) of less than 24 cmolc kg clay in some part to a maximum depth of 50 cm below its upper limit, either starting within 100 cm of the soil surface or within 200 cm of the soil surface if the argic horizon is overlain by loamy sand or coarser textures throughout, and a base saturation (by 1 M

NH4OAc) of less than 50 percent in the major part between 50 and 100 cm from the soil surface.

Acroxic (ao) -1 Having less than 2 cmolc kg fine earth exchangeable bases plus 1 M KCl exchangeable Al3+ in one or more layers with a combined thickness of 30 cm or more within 100 cm of the soil surface (in Andosols only).

Albic (ab) Having an albic horizon starting within 100 cm of the soil surface.

Hyperalbic (ha) Having an albic horizon starting within 50 cm of the soil surface and its lower boundary at a depth of 100 cm or more from the soil surface.

Glossalbic (gb) Showing tonguing of an albic into an argic or natric horizon.

Alcalic (ax) Having a pH (1:1 in water) of 8.5 or more throughout within 50 cm of the soil surface or to continuous rock or a cemented or indurated layer, whichever is shallower.

Alic (al) -1 Having an argic horizon that has a CEC (by 1 M NH4OAc) of 24 cmolc kg clay or more throughout or to a depth of 50 cm below its upper limit, whichever is shallower, 102 World reference base for soil resources 2006

either starting within 100 cm of the soil surface or within 200 cm of the soil surface if the argic horizon is overlain by loamy sand or coarser textures throughout, and a base

saturation (by 1 M NH4OAc) of less than 50 percent in the major part between 50 and 100 cm from the soil surface.

Aluandic (aa) Having one or more layers, cumulatively 30 cm or more thick, with andic properties and an acid oxalate (pH 3) extractable silica content of less than 0.6 percent, and an 1 2 Alpy /Alox of 0.5 or more, within 100 cm of the soil surface (in Andosols only).

Thaptaluandic (aab) Having one or more buried layers, cumulatively 30 cm or more thick, with andic properties and an acid oxalate (pH 3) extractable silica content of less than 3 4 0.6 percent, or an Alpy /Alox of 0.5 or more, within 100 cm of the soil surface.

Alumic (au) Having an Al saturation (effective) of 50 percent or more in some layer between 50 and 100 cm from the surface.

Andic (an) Having one or more layers, cumulatively 30 cm or more thick, with andic properties, within 100 cm of the soil surface.

Thaptandic (ba) Having one or more buried layers, cumulatively 30 cm or more thick, with andic properties, within 100 cm of the soil surface.

Anthraquic (aq) Having an anthraquic horizon.

Anthric (am) Having an anthric horizon.

Arenic (ar) Having a texture of loamy fine sand or coarser in a layer, 30 cm or more thick, within 100 cm of the soil surface.

Epiarenic (arp) Having a texture of loamy fine sand or coarser in a layer, 30 cm or more thick, within 50 cm of the soil surface.

Endoarenic (arn) Having a texture of loamy fine sand or coarser in a layer, 30 cm or more thick, between 50 and 100 cm from the soil surface.

1 Alpy: pyrophosphate-extractable aluminium, expressed as percent of the fine earth (0–2 mm) fraction on an oven-dried (105 °C) basis. 2 Alox: acid oxalate-extractable aluminium (Blakemore, Searle and Daly, 1981), expressed as percent of the fine earth (0–2 mm) fraction on an oven-dried (105 °C) basis. 3 Alpy: pyrophosphate-extractable aluminium, expressed as percent of the fine earth (0–2 mm) fraction on an oven-dried (105o C) basis. 4 Alox: acid oxalate-extractable aluminium (Blakemore, Searle and Daly, 1981), expressed as percent of the fine earth (0–2 mm) fraction on an oven-dried (105 °C) basis. Chapter 5 – Definitions of formative elements for second-level units of the WRB 103

Aric (ai) Having only remnants of diagnostic horizons – disturbed by deep ploughing.

Aridic (ad) Having aridic properties without a takyric or yermic horizon.

Arzic (az) Having sulphate-rich groundwater in some layer within 50 cm of the soil surface during some time in most years and containing 15 percent or more gypsum averaged over a depth of 100 cm from the soil surface or to continuous rock or a cemented or indurated layer, whichever is shallower (in Gypsisols only).

Brunic (br) Having a layer, 15 cm or more thick, which meets criteria 2–4 of the cambic horizon but fails criterion 1, starting within 50 cm of the soil surface.

Calcaric (ca) Having calcaric material between 20 and 50 cm from the soil surface or between 20 cm and continuous rock or a cemented or indurated layer, whichever is shallower.

Calcic (cc) Having a calcic horizon or concentrations of secondary carbonates starting within 100 cm of the soil surface.

Cambic (cm) Having a cambic horizon starting within 50 cm of the soil surface.

Carbic (cb) Having a spodic horizon that does not turn redder on ignition (in Podzols only).

Carbonatic (cn) Having a salic horizon with a soil solution (1:1 in water) with a pH of 8.5 or more and - 2- - [HCO3 ] > [SO4 ] >> [Cl ] (in Solonchaks only).

Chloridic (cl) - 2- - Having a salic horizon with a soil solution (1:1 in water) with [Cl ] >> [SO4 ] > [HCO3 ] (in Solonchaks only).

Chromic (cr) Having within 150 cm of the soil surface a subsurface layer, 30 cm or more thick, that has a Munsell hue redder than 7.5 YR or that has both, a hue of 7.5 YR and a chroma, moist, of more than 4.

Clayic (ce) Having a texture of clay in a layer, 30 cm or more thick, within 100 cm of the soil surface.

Epiclayic (cep) Having a texture of clay in a layer, 30 cm or more thick, within 50 cm of the soil surface. 104 World reference base for soil resources 2006

Endoclayic (cen) Having a texture of clay in a layer, 30 cm or more thick, within 50 and 100 cm of the soil surface.

Colluvic (co) Having colluvic material, 20 cm or more thick, created by human-induced lateral movement.

Cryic (cy) Having a cryic horizon starting within 100 cm of the soil surface or a cryic horizon starting within 200 cm of the soil surface with evidence of cryoturbation in some layer within 100 cm of the soil surface.

Cutanic (ct) Having clay coatings in some parts of an argic horizon either starting within 100 cm of the soil surface or within 200 cm of the soil surface if the argic horizon is overlain by loamy sand or coarser textures throughout.

Densic (dn) Having natural or artificial compaction within 50 cm of the soil surface to the extent that roots cannot penetrate.

Drainic (dr) Having a histic horizon that is drained artificially starting within 40 cm of the soil surface.

Duric (du) Having a duric horizon starting within 100 cm of the soil surface.

Endoduric (nd) Having a duric horizon starting between 50 and 100 cm from the soil surface.

Hyperduric (duh) Having a duric horizon with 50 percent or more (by volume) durinodes starting within 100 cm of the soil surface.

Dystric (dy)

Having a base saturation (by 1 M NH4OAc) of less than 50 percent in the major part between 20 and 100 cm from the soil surface or between 20 cm and continuous rock or a cemented or indurated layer, or, in Leptosols, in a layer, 5 cm or more thick, directly above continuous rock.

Endodystric (ny)

Having a base saturation (by 1 M NH4OAc) of less than 50 percent throughout between 50 and 100 cm from the soil surface.

Epidystric (ed)

Having a base saturation (by 1 M NH4OAc) of less than 50 percent throughout between 20 and 50 cm from the soil surface. Chapter 5 – Definitions of formative elements for second-level units of the WRB 105

Hyperdystric (hd)

Having a base saturation (by 1 M NH4OAc) of less than 50 percent throughout between 20 and 100 cm from the soil surface, and less than 20 percent in some layer within 100 cm of the soil surface.

Orthodystric (dyo)

Having a base saturation (by 1 M NH4OAc) of less than 50 percent throughout between 20 and 100 cm from the soil surface.

Ekranic (ek) Having technic hard rock starting within 5 cm of the soil surface and covering 95 percent or more of the horizontal extent of a pedon (in Technosols only).

Endoduric (nd) See Duric.

Endodystric (ny) See Dystric.

Endoeutric (ne) See Eutric.

Endofluvic (nf) See Fluvic.

Endogleyic (ng) See Gleyic.

Endoleptic (nl) See Leptic.

Endosalic (ns) See Salic.

Entic (et) Not an albic horizon and a loose spodic horizon (in Podzols only).

Epidystric (ed) See Dystric.

Epieutric (ee) See Eutric.

Epileptic (el) See Leptic.

Episalic (ea) See Salic.

Escalic (ec) Occurring in human-made terraces. 106 World reference base for soil resources 2006

Eutric (eu)

Having a base saturation (by 1 M NH4OAc) of 50 percent or more in the major part between 20 and 100 cm from the soil surface or between 20 cm and continuous rock or a cemented or indurated layer, or, in Leptosols, in a layer, 5 cm or more thick, directly above continuous rock.

Endoeutric (ne)

Having a base saturation (by 1 M NH4OAc) of 50 percent or more throughout between 50 and 100 cm from the soil surface.

Epieutric (ee)

Having a base saturation (by 1 M NH4OAc) of 50 percent or more throughout between 20 and 50 cm from the soil surface.

Hypereutric (he)

Having a base saturation (by 1 M NH4OAc) of 50 percent or more throughout between 20 and 100 cm from the soil surface and 80 percent or more in some layer within 100 cm of the soil surface.

Orthoeutric (euo)

Having a base saturation (by 1 M NH4OAc) of 50 percent or more throughout between 20 and 100 cm from the soil surface.

Eutrosilic (es) Having one or more layers, cumulatively 30 cm or more thick, with andic properties -1 and a sum of exchangeable bases of 15 cmolc kg fine earth or more within 100 cm of the surface (in Andosols only).

Ferralic (fl) Having a ferralic horizon starting within 200 cm of the soil surface (in Anthrosols only), or ferralic properties in at least some layer starting within 100 cm of the soil surface (in other soils).

Hyperferralic (flh) 1 -1 Having a Ferralic properties and a CEC (by 1 M NH4OAc) of less than 16 cmolc kg clay in at least some layer starting within 100 cm of the soil surface.

Hypoferralic (flw) Having in a layer, 30 cm or more thick, within 100 cm of the soil surface a CEC (by -1 1 M NH4OAc) of less than 4 cmolc kg fine earth and a Munsell chroma, moist, of 5 or more or a hue redder than 10 YR (in Arenosols only).

Ferric (fr) Having a ferric horizon starting within 100 cm of the soil surface.

Hyperferric (frh) Having a ferric horizon with 40 percent or more of the volume discrete reddish to blackish nodules starting within 100 cm of the soil surface.

1 See Annex 1. Chapter 5 – Definitions of formative elements for second-level units of the WRB 107

Fibric (fi) Having, after rubbing, two-thirds or more (by volume) of the organic material consisting of recognizable plant tissue within 100 cm of the soil surface (in Histosols only).

Floatic (ft) Having organic material floating on water (in Histosols only).

Fluvic (fv) Having fluvic material in a layer, 25 cm or more thick, within 100 cm of the soil surface.

Endofluvic (nf) Having fluvic material in a layer, 25 cm or more thick, between 50 and 100 cm from the soil surface.

Folic (fo) Having a folic horizon starting within 40 cm of the soil surface.

Thaptofolic (fob) Having a buried folic horizon starting between 40 and 100 cm from the soil surface.

Fractipetric (fp) Having a strongly cemented or indurated horizon consisting of fractured or broken clods with an average horizontal length of less than 10 cm, starting within 100 cm of the soil surface.

Fractiplinthic (fa) Having a petroplinthic horizon consisting of fractured or broken clods with an average horizontal length of less than 10 cm, starting within 100 cm of the soil surface.

Fragic (fg) Having a fragic horizon starting within 100 cm of the soil surface.

Fulvic (fu) Having a fulvic horizon starting within 30 cm of the soil surface.

Garbic (ga) Having a layer, 20 cm or more thick within 100 cm of the soil surface, with 20 percent or more (by volume, by weighted average) artefacts containing 35 percent or more (by volume) organic waste materials (in Technosols only).

Gelic (ge) Having a layer with a soil temperature of 0 °C or less for two or more consecutive years starting within 200 cm of the soil surface.

Gelistagnic (gt) Having temporary water saturation at the soil surface caused by a frozen subsoil.

Geric (gr) Having geric properties in some layer within 100 cm of the soil surface. 108 World reference base for soil resources 2006

Gibbsic (gi) Having a layer, 30 cm or more thick, containing 25 percent or more gibbsite in the fine earth fraction within 100 cm of the soil surface.

Glacic (gc) Having a layer, 30 cm or more thick, containing 75 percent (by volume) or more ice within 100 cm of the soil surface.

Gleyic (gl) Having within 100 cm of the mineral soil surface in some parts reducing conditions and in 25 percent or more of the soil volume a gleyic colour pattern.

Endogleyic (ng) Having between 50 and 100 cm from the mineral soil surface in some parts reducing conditions and in 25 percent or more of the soil volume a gleyic colour pattern.

Epigleyic (glp) Having within 50 cm of the mineral soil surface in some parts reducing conditions and in 25 percent or more of the soil volume a gleyic colour pattern.

Glossalbic (gb) See Albic.

Glossic (gs) Showing tonguing of a mollic or umbric horizon into an underlying layer.

Molliglossic (mi) Showing tonguing of a mollic horizon into an underlying layer.

Umbriglossic (ug) Showing tonguing of an umbric horizon into an underlying layer.

Greyic (gz) Having Munsell colours with a chroma of 3 or less when moist, a value of 3 or less when moist and 5 or less when dry and uncoated silt and sand grains on structural faces within 5 cm of the mineral soil surface.

Grumic (gm) Having a soil surface layer with a thickness of 3 cm or more with a strong structure finer than very coarse granular (in Vertisols only).

Gypsic (gy) Having a gypsic horizon starting within 100 cm of the soil surface.

Gypsiric (gp) Having a gypsiric material between 20 and 50 cm from the soil surface.

Haplic (ha) Having a typical expression of certain features (typical in the sense that there is no further or meaningful characterization) and only used if none of the preceding qualifiers applies. Chapter 5 – Definitions of formative elements for second-level units of the WRB 109

Hemic (hm) Having, after rubbing, between two-thirds and one-sixth (by volume) of the organic material consisting of recognizable plant tissue within 100 cm from the soil surface (in Histosols only).

Histic (hi) Having a histic horizon starting within 40 cm of the soil surface.

Thaptohistic (hib) Having a buried histic horizon starting between 40 and 100 cm from the soil surface.

Hortic (ht) Having a hortic horizon.

Humic (hu) Having the following organic carbon contents in the fine earth fraction as a weighted average in Ferralsols and Nitisols, 1.4 percent or more to a depth of 100 cm from the mineral soil surface; in Leptosols, 2 percent or more to a depth of 25 cm from the mineral soil surface; in other soils, 1 percent or more to a depth of 50 cm from the mineral soil surface.

Hyperhumic (huh) Having an organic carbon content of 5 percent or more as a weighted average in the fine earth fraction to a depth of 50 cm from the mineral soil surface.

Hydragric (hg) Having an anthraquic horizon and an underlying hydragric horizon, the latter starting within 100 cm of the soil surface.

Hydric (hy) Having within 100 cm of the soil surface one or more layers with a combined thickness of 35 cm or more, which have a water retention at 1 500 kPa (in undried samples) of 100 percent or more (in Andosols only).

Hydrophobic (hf) Water-repellent, i.e. water stands on a dry soil for the duration of 60 seconds or more (in Arenosols only).

Hyperalbic (ha) See Albic.

Hyperalic (hl) Having an argic horizon that has a silt to clay ratio of less than 0.6 and an Al saturation (effective) of 50 percent or more, throughout or to a depth of 50 cm below its upper limit, whichever is shallower (in Alisols only).

Hypercalcic (hc) Having a calcic horizon with 50 percent or more (by mass) calcium carbonate equivalent (in Calcisols only).

Hyperdystric (hd) See Dystric. 110 World reference base for soil resources 2006

Hypereutric (he) See Eutric.

Hypergypsic (hp) Having a gypsic horizon with 50 percent or more (by mass) gypsum (in Gypsisols only).

Hyperochric (ho) Having a mineral topsoil layer, 5 cm or more thick, with a Munsell value, dry, of 5.5 or more that turns darker on moistening, an organic carbon content of less than 0.4 percent, a platy structure in 50 percent or more of the volume, and a surface crust.

Hypersalic (hs) See Salic.

Hyperskeletic (hk) Containing less than 20 percent (by volume) fine earth averaged over a depth of 75 cm from the soil surface or to continuous rock, whichever is shallower.

Hypocalcic (wc) Having a calcic horizon with a calcium carbonate equivalent content in the fine earth fraction of less than 25 percent and starting within 100 cm of the soil surface (in Calcisols only).

Hypogypsic (wg) Having a gypsic horizon with a gypsum content in the fine earth fraction of less than 25 percent and starting within 100 cm of the soil surface (in Gypsisols only).

Hypoluvic (wl) Having an absolute clay increase of 3 percent or more within 100 cm of the soil surface (in Arenosols only).

Hyposalic (ws) See Salic.

Hyposodic (wn) See Sodic.

Irragric (ir) Having an irragric horizon.

Lamellic (ll) Having clay lamellae with a combined thickness of 15 cm or more within 200 cm of the soil surface.

Laxic (la) Having a bulk density of less than 0.8 kg dm-3, in a mineral soil layer, 20 cm or more thick, starting within 75 cm of the soil surface.

Leptic (le) Having continuous rock starting within 100 cm of the soil surface. Chapter 5 – Definitions of formative elements for second-level units of the WRB 111

Endoleptic (nl) Having continuous rock starting between 50 and 100 cm from the soil surface.

Epileptic (el) Having continuous rock starting within 50 cm of the soil surface.

Lignic (lg) Having inclusions of intact wood fragments, which make up one-quarter or more of the soil volume, within 50 cm of the soil surface (in Histosols only).

Limnic (lm) Having limnic material, cumulatively 10 cm or more thick, within 50 cm of the soil surface.

Linic (lc) Having a continuous, very slowly permeable to impermeable constructed geomembrane of any thickness starting within 100 cm of the soil surface.

Lithic (li) Having continuous rock starting within 10 cm of the soil surface (in Leptosols only).

Nudilithic (nt) Having continuous rock at the soil surface (in Leptosols only).

Lixic (lx) -1 Having an argic horizon that has a CEC (by 1 M NH4OAc) of 24 cmolc kg clay or more in some part to a maximum depth of 50 cm below its upper limit, either starting within 100 cm of the soil surface or within 200 cm of the soil surface if the argic horizon is overlain by loamy sand or coarser textures throughout, and a base saturation (by 1 M

NH4OAc) of 50 percent or more in the major part between 50 and 100 cm from the soil surface.

Luvic (lv) -1 Having an argic horizon that has a CEC of 24 cmolc kg clay or more throughout or to a depth of 50 cm below its upper limit, whichever is shallower, either starting within 100 cm of the soil surface or within 200 cm of the soil surface if the argic horizon is overlain by loamy sand or coarser textures throughout, and a base saturation (by 1 M

NH4OAc) of 50 percent or more in the major part between 50 and 100 cm from the soil surface.

Magnesic (mg) Having an exchangeable Ca to Mg ratio of less than 1 in the major part within 100 cm of the soil surface or to continuous rock or a cemented or indurated layer, whichever is shallower.

Manganiferric (mf) Having a ferric horizon starting within 100 cm of the soil surface in which half or more of the nodules or mottles are black.

Mazic (mz) Massive and hard to very hard in the upper 20 cm of the soil (in Vertisols only). 112 World reference base for soil resources 2006

Melanic (ml) Having a melanic horizon starting within 30 cm of the soil surface (in Andosols only).

Mesotrophic (ms)

Having a base saturation (by 1 M NH4OAc) of less than 75 percent at a depth of 20 cm from the soil surface (in Vertisols only).

Mollic (mo) Having a mollic horizon.

Molliglossic (mi) See Glossic.

Natric (na) Having a natric horizon starting within 100 cm of the soil surface.

Nitic (ni) Having a nitic horizon starting within 100 cm of the soil surface.

Novic (nv) Having above the soil that is classified at the RSG level, a layer with recent sediments (new material), 5 cm or more and less than 50 cm thick.

Areninovic (anv) Having above the soil that is classified at the RSG level, a layer with recent sediments (new material), 5 cm or more and less than 50 cm thick, which has a texture of loamy fine sand or coarser in its major part.

Clayinovic (cnv) Having above the soil that is classified at the RSG level, a layer with recent sediments (new material), 5 cm or more and less than 50 cm thick, which has a texture of clay in its major part.

Siltinovic (snv) Having above the soil that is classified at the RSG level, a layer with recent sediments (new material), 5 cm or more and less than 50 cm thick, which has a texture of silt, silt loam, silty clay loam or silty clay in its major part.

Nudilithic (nt) See Lithic.

Ombric (om) Having a histic horizon saturated predominantly with rainwater starting within 40 cm of the soil surface (in Histosols only).

Ornithic (oc) Having a layer 15 cm or more thick with ornithogenic material starting within 50 cm of the soil surface.

Ortsteinic (os) Having a cemented spodic horizon (ortstein) (in Podzols only). Chapter 5 – Definitions of formative elements for second-level units of the WRB 113

Oxyaquic (oa) Saturated with oxygen-rich water during a period of 20 or more consecutive days and not a gleyic or stagnic colour pattern in some layer within 100 cm of the soil surface.

Pachic () Having a mollic or umbric horizon 50 cm or more thick.

Pellic (pe) Having in the upper 30 cm of the soil a Munsell value, moist, of 3.5 or less and a chroma, moist, of 1.5 or less (in Vertisols only).

Petric (pt) Having a strongly cemented or indurated layer starting within 100 cm of the soil surface.

Endopetric (ptn) Having a strongly cemented or indurated layer starting between 50 and 100 cm from the soil surface.

Epipetric (ptp) Having a strongly cemented or indurated layer starting within 50 cm of the soil surface.

Petrocalcic (pc) Having a petrocalcic horizon starting within 100 cm of the soil surface.

Petroduric (pd) Having a petroduric horizon starting within 100 cm of the soil surface.

Petrogleyic (py) Having a layer, 10 cm or more thick, with an oximorphic colour pattern1, 15 percent or more (by volume) of which is cemented (bog iron), within 100 cm of the surface.

Petrogypsic (pg) Having a petrogypsic horizon starting within 100 cm of the soil surface.

Petroplinthic (pp) Having a petroplinthic horizon starting within 100 cm of the soil surface.

Petrosalic (ps) Having within 100 cm of the soil surface, a layer, 10 cm or more thick, which is cemented by salts more soluble than gypsum.

Pisoplinthic (px) Having a pisoplinthic horizon starting within 100 cm of the soil surface.

Placic (pi) Having within 100 cm of the soil surface, an iron pan, between 1 and 25 mm thick, that is continuously cemented by a combination of organic matter, Fe and/or Al.

1 As defined in the gleyic colour pattern. 114 World reference base for soil resources 2006

Plaggic (pa) Having a plaggic horizon.

Plinthic (pl) Having a plinthic horizon starting within 100 cm of the soil surface.

Posic (po)

Having a zero or positive charge (pHKCl - pHwater, both 1:1) in a layer, 30 cm or more thick, starting within 100 cm of the soil surface (in Plinthosols and Ferralsols only).

Profondic (pf) Having an argic horizon in which the clay content does not decrease by 20 percent or more (relative) from its maximum within 150 cm of the soil surface.

Protic (pr) Showing no soil horizon development (in Arenosols only).

Puffic (pu) Having a crust pushed up by salt crystals (in Solonchaks only).

Reductaquic (ra) Saturated with water during the thawing period and at some time of the year reducing conditions above a cryic horizon and within 100 cm of the soil surface (in Cryosols only).

Reductic (rd) Having reducing conditions in 25 percent or more of the soil volume within 100 cm of the soil surface caused by gaseous emissions, e.g. methane or carbon dioxide (in Technosols only).

Regic (rg) Not having buried horizons (in Anthrosols only).

Rendzic (rz) Having a mollic horizon that contains or immediately overlies calcaric materials containing 40 percent or more calcium carbonate equivalent.

Rheic (rh) Having a histic horizon saturated predominantly with groundwater or flowing surface water starting within 40 cm of the soil surface (in Histosols only).

Rhodic (ro) Having within 150 cm of the soil surface a subsurface layer, 30 cm or more thick, with a Munsell hue redder than 5 YR (3.5 YR or redder), a value, moist, of less than 3.5 and a value, dry, no more than one unit higher than the moist value.

Rubic (ru) Within 100 cm of the soil surface a subsurface layer, 30 cm or more thick, with a Munsell hue redder than 10 YR or a chroma, moist, of 5 or more (in Arenosols only).

Ruptic (rp) Having a lithological discontinuity within 100 cm of the soil surface. Chapter 5 – Definitions of formative elements for second-level units of the WRB 115

Rustic (rs) Having a spodic horizon that turns redder on ignition (in Podzols only).

Salic (sz) Having a salic horizon starting within 100 cm of the soil surface.

Endosalic (ns) Having a salic horizon starting between 50 and 100 cm from the soil surface.

Episalic (ea) Having a salic horizon starting within 50 cm of the soil surface.

Hypersalic (hs) -1 Having an ECe of 30 dS m or more at 25 °C in some layer within 100 cm of the soil surface.

Hyposalic (ws) -1 Having an ECe of 4 dS m or more at 25 °C in some layer within 100 cm of the soil surface.

Sapric (sa) Having, after rubbing, less than one-sixth (by volume) of the organic material consisting of recognizable plant tissue within 100 cm of the soil surface (in Histosols only).

Silandic (sn) Having one or more layers, cumulatively 30 cm or more thick, with andic properties and an acid oxalate (pH 3) extractable silica (Siox) content of 0.6 percent or more, or an

Alpy to Alox ratio of less than 0.5 within 100 cm of the soil surface (in Andosols only).

Thaptosilandic (snb) Having one or more buried layers, cumulatively 30 cm or more thick, with andic

properties and an acid oxalate (pH 3) extractable silica (Siox) content of 0.6 percent

or more, or an Alpy to Alox ratio of less than 0.5 within 100 cm of the soil surface.

Siltic (sl) Having a texture of silt, silt loam, silty clay loam or silty clay in a layer, 30 cm or more thick, within 100 cm of the soil surface.

Endosiltic (sln) Having a texture of silt, silt loam, silty clay loam or silty clay in a layer, 30 cm or more thick, within 50 and 100 cm of the soil surface.

Episiltic (slp) Having a texture of silt, silt loam, silty clay loam or silty clay in a layer, 30 cm or more thick, within 50 cm of the soil surface.

Skeletic (sk) 40 percent or more (by volume) gravel or other coarse fragments averaged over a depth of 100 cm from the soil surface or to continuous rock or a cemented or indurated layer, whichever is shallower. 116 World reference base for soil resources 2006

Endoskeletic (skn) Having 40 percent or more (by volume) gravel or other coarse fragments averaged over a depth between 50 and 100 cm from the soil surface.

Episkeletic (skp) Having 40 percent or more (by volume) gravel or other coarse fragments averaged over a depth of 50 cm from the soil surface.

Sodic (so) Having 15 percent or more exchangeable Na plus Mg on the exchange complex within 50 cm of the soil surface throughout.

Endosodic (son) Having 15 percent or more exchangeable Na plus Mg on the exchange complex between 50 and 100 cm from the soil surface throughout.

Hyposodic (sow) Having 6 percent or more exchangeable Na on the exchange complex in a layer, 20 cm or more thick, within 100 cm of the soil surface.

Solodic (sc) Having a layer, 15 cm or more thick within 100 cm of the soil surface, with the columnar or prismatic structure of the natric horizon, but lacking its sodium saturation requirements.

Sombric (sm) Having a sombric horizon starting within 150 cm of the soil surface.

Spodic (sd) Having a spodic horizon starting within 200 cm of the mineral soil surface.

Spolic (sp) Having a layer, 20 cm or more thick within 100 cm of the soil surface, with 20 percent or more (by volume, by weighted average) artefacts containing 35 percent or more (by volume) of industrial waste (mine spoil, dredgings, rubble, etc.) (in Technosols only).

Stagnic (st) Having within 100 cm of the mineral soil surface in some parts reducing conditions for some time during the year and in 25 percent or more of the soil volume, single or in combination, a stagnic colour pattern or an albic horizon.

Endostagnic (stn) Having between 50 and 100 cm from the mineral soil surface in some parts reducing conditions for some time during the year and in 25 percent or more of the soil volume, single or in combination, a stagnic colour pattern or an albic horizon.

Epistagnic (stn) Having within 50 cm of the mineral soil surface in some parts reducing conditions for some time during the year and in 25 percent or more of the soil volume, single or in combination, a stagnic colour pattern or an albic horizon.

Subaquatic (sq) Being permanently submerged under water not deeper than 200 cm. Chapter 5 – Definitions of formative elements for second-level units of the WRB 117

Sulphatic (su) 2- - Having a salic horizon with a soil solution (1:1 in water) with [SO4 ] >> [HCO3 ] > [Cl-] (in Solonchaks only).

Takyric (ty) Having a takyric horizon.

Technic (te) Having 10 percent or more (by volume, by weighted average) artefacts in the upper 100 cm from the soil surface or to continuous rock or a cemented or indurated layer, whichever is shallower.

Tephric (tf) Having tephric material to a depth of 30 cm or more from the soil surface or to continuous rock, whichever is shallower.

Terric (tr) Having a terric horizon.

Thaptandic (ba) See Andic.

Thaptovitric (bv) See Vitric.

Thionic (ti) Having a thionic horizon or a layer with sulphidic material, 15 cm or more thick, starting within 100 cm of the soil surface.

Hyperthionic (tih) Having a thionic horizon starting within 100 cm of the soil surface and a pH (1:1 in water) less than 3.5.

Orthothionic (tio) Having a thionic horizon starting within 100 cm of the soil surface and a pH (1:1 in water) between 3.5 and 4.0.

Protothionic (tip) Having a layer with sulphidic material, 15 cm or more thick, starting within 100 cm of the soil surface.

Thixotropic (tp) Having in some layer within 50 cm of the soil surface material that changes, under pressure or by rubbing, from a plastic solid into a liquefied stage and back into the solid condition.

Tidalic (td) Being flooded by tidewater but not covered by water at mean low tide.

Toxic (tx) Having in some layer within 50 cm of the soil surface toxic concentrations of organic or inorganic substances other than ions of Al, Fe, Na, Ca and Mg. 118 World reference base for soil resources 2006

Anthrotoxic (atx) Having in some layer within 50 cm of the soil surface sufficiently high and persistent concentrations of organic or inorganic substances to markedly affect the health of humans who come in regular contact with the soil.

Ecotoxic (etx) Having in some layer within 50 cm of the soil surface sufficiently high and persistent concentrations of organic or inorganic substances to markedly affect , in particular the populations of the mesofauna.

Phytotoxic (ptx) Having in some layer within 50 cm of the soil surface sufficiently high or low concentrations of ions other than Al, Fe, Na, Ca and Mg, to markedly affect plant growth.

Zootoxic (ztx) Having in some layer within 50 cm of the soil surface sufficiently high and persistent concentrations of organic or inorganic substances to markedly affect the health of animals, including humans, that ingest plants grown on these soils.

Transportic (tn) Having a layer, 30 cm or more thick, with solid or liquid material that has been moved from a source area outside the immediate vicinity of the soil by intentional human activity, usually with the aid of machinery, and without substantial reworking or displacement by natural forces

Turbic (tu) Having cryoturbation features (mixed material, disrupted soil horizons, involutions, organic intrusions, frost heave, separation of coarse from fine materials, cracks or patterned ground) at the soil surface or above a cryic horizon and within 100 cm of the soil surface.

Umbric (um) Having an umbric horizon.

Umbriglossic (ug) See Glossic.

Urbic (ub) Having a layer, 20 cm or more thick within 100 cm of the soil surface, with 20 percent or more (by volume, by weighted average) artefacts containing 35 percent or more (by volume) of rubble and refuse of human settlements (in Technosols only).

Vermic (vm) Having 50 percent or more (by volume, by weighted average) of worm holes, casts, or filled animal burrows in the upper 100 cm of the soil or to continuous rock or a cemented or indurated layer, whichever is shallower.

Vertic (vr) Having a vertic horizon or vertic properties starting within 100 cm of the soil surface. Chapter 5 – Definitions of formative elements for second-level units of the WRB 119

Vetic (vt) Having an ECEC (sum of exchangeable bases plus exchangeable acidity in 1 M KCl) of -1 less than 6 cmolc kg clay in some subsurface layer within 100 cm of the soil surface.

Vitric (vi) Having one or more layers, cumulatively 30 cm or more thick, with vitric properties, within 100 cm of the soil surface.

Thaptovitric (bv) Having one or more buried layers, cumulatively 30 cm or more thick, with vitric properties, within 100 cm of the soil surface.

Voronic (vo) Having a voronic horizon (in Chernozems only).

Xanthic (xa) Having a ferralic horizon that has in a subhorizon, 30 cm or more thick within 150 cm of the soil surface, a Munsell hue of 7.5 YR or yellower and a value, moist, of 4 or more and a chroma, moist, of 5 or more.

Yermic (ye) Having a yermic horizon, including a desert pavement.

Nudiyermic (yes) Having a yermic horizon without a desert pavement.

SPECIFIERS

The following specifiers may be used to indicate depth of occurrence, or to express the intensity of soil characteristics. Their code is always added after the qualifier code. The specifiers are combined with other elements into one word, e.g. Endoskeletic. A triple combination, e.g. Epihyperdystric, is allowed.

Bathy (..d) Horizon, property or material starting between 100 and 200 cm from the soil surface.

Cumuli (..c) Having a repetitive accumulation of material of 50 cm or more at the soil surface (e.g. cumulinovic and cumulimollic).

Endo (..n) Horizon, property or material starting between 50 and 100 cm from the soil surface.

Epi (..p) Horizon, property or material starting within 50 cm of the soil surface.

Hyper (..h) Having a strong expression of certain features.

Hypo (..w) Having a weak expression of certain features. 120 World reference base for soil resources 2006

Ortho (..o) Having a typical expression of certain features (typical in the sense that no further or meaningful characterization is made).

Para (..r) Having a resemblance to certain features (e.g. Paralithic).

Proto (..t) Indicating a precondition or an early stage of development of certain features (e.g. Protothionic).

Thapto (..b) Having a buried horizon within 100 cm of the surface (given in combination with the buried diagnostic horizon, e.g. Thaptomollic). 121

References

Asiamah, R.D. 2000. Plinthite and conditions for its hardening in agricultural soils in Ghana. Kwame Nkrumah University of Science and Technology, Kumasi, Ghana. (Thesis) Blakemore, L.C., Searle, P.L. & Daly, B.K. 1981. Soil Bureau analytical methods. A method for chemical analysis of soils. NZ Soil Bureau Sci. Report 10A. DSIRO. Bridges, E.M. 1997. World soils. 3rd edition. Cambridge, UK, Cambridge University Press. Buivydaité, V.V., Vaičys, M., Juodis, J. & Motuzas, A. 2001. Lietuvos dirvožemių klasifikacija. Vilnius, Lievos mokslas. Burt, R., ed. 2004. Soil survey laboratory methods manual. Soil Survey Investigations Report No. 42, Version 4.0. Lincoln, USA, Natural Resources Conservation Service. Cooperative Research Group on Chinese Soil Taxonomy (CRGCST). 2001. Chinese soil taxonomy. Beijing and New York, USA, Science Press. CPCS. 1967. Classification des sols. Grignon, France, Ecole nationale supérieure agronomique. 87 pp. European Soil Bureau Network/European Commission. 2005. Soil atlas of Europe. Luxembourg, Office for Official Publications of the European Communities. FAO. 1966. Classification of Brazilian soils, by J. Bennema. Report to the Government of Brazil. FAO EPTA Report No. 2197. Rome. FAO. 1988. Soil map of the world. Revised legend, by FAO–UNESCO–ISRIC. World Soil Resources Report No. 60. Rome. FAO. 1994. World Reference Base for Soil Resources, by ISSS–ISRIC–FAO. Draft. Rome/ Wageningen, Netherlands. FAO. 1998. World Reference Base for Soil Resources, by ISSS–ISRIC–FAO. World Soil Resources Report No. 84. Rome. FAO. 2001a. Lecture notes on the major soils of the world (with CD-ROM), by P. Driessen, J. Deckers, O. Spaargaren & F, Nachtergaele, eds. World Soil Resources Report No. 94. Rome. FAO. 2001b. Major soils of the world. Land and Water Digital Media Series No. 19. Rome. FAO. 2003. Properties and management of soils of the tropics. Land and Water Digital Media Series No. 24. Rome. FAO. 2005. Properties and management of drylands. Land and Water Digital Media Series No. 31. Rome. FAO. 2006. Guidelines for soil description. 4th edition. Rome. FAO–UNESCO. 1971–1981. Soil map of the world 1:5 000 000. 10 Volumes. Paris, UNESCO. Fieldes, M. & Perrott, K.W. 1966. The nature of allophane soils: 3. Rapid field and laboratory test for allophane. N. Z. J. Sci., 9: 623–629. Gong, Z., Zhang, X., Luo, G., Shen, H. & Spaargaren, O.C. 1997. Extractable phosphorus in soils with a fimic epipedon. Geoderma, 75: 289–296. Hewitt, A.E. 1992. New Zealand soil classification. DSIR Land Resources Scientific Report 19. Lower Hutt. Ito, T., Shoji, S., Shirato, Y. & Ono, E. 1991. Differentiation of a spodic horizon from a buried A horizon. Soil Sci. Soc. Am. J., 55: 438–442. Krogh, L. & Greve, M.H. 1999. Evaluation of World Reference Base for Soil Resources and FAO Soil Map of the World using nationwide grid soil data from Denmark. Soil Use & Man., 15(3):157–166. Nachtergaele, F. 2005. The “soils” to be classified in the World Reference Base for Soil Resources. Euras. Soil Sci., 38(Suppl. 1): 13–19. 122 World reference base for soil resources 2006

Němecěk, J. Macků, J., Vokoun, J., Vavříě, D. & Novák, P. 2001. Taxonomický klasifikační system půd České Republiky. Prague, ČZU. Olsen, S.R., Cole, C.V., Watanabe, F.S. & Dean, L.A. 1954. Estimation of available phosphorus by extraction with sodium bicarbonate. USDA Circ. 939. Washington, DC, United States Department of Agriculture. Poulenard, J. & Herbillon, A.J. 2000. Sur l’existence de trois catégories d’horizons de référence dans les Andosols. C. R. Acad. Sci. Paris, Sci. Terre & plan., 331: 651–657. Shishov, L.L., Tonkonogov, V.D., Lebedeva, I.I. & Gerasimova, M.I., eds. 2001. Russian soil classification system. Moscow, V.V. Dokuchaev Soil Science Institute. Shoji, S., Nanzyo, M., Dahlgren, R.A. & Quantin, P. 1996. Evaluation and proposed revisions of criteria for Andosols in the World Reference Base for Soil Resources. Soil Sci., 161(9): 604–615. Soil Survey Staff. 1999. Soil taxonomy. A basic system of soil classification for making and interpreting soil surveys. 2nd Edition. Agric. Handbook 436. Washington, DC, Natural Resources Conservation Service, United States Department of Agriculture. Soil Survey Staff. 2003. Keys to soil taxonomy. 9th Edition. Washington, DC, Natural Resources Conservation Service, United States Department of Agriculture. Sombroek, W.G. 1986. Identification and use of subtypes of the argillic horizon. In: Proceedings of the International Symposium on Red Soils, pp. 159–166, Nanjing, November 1983. Beijing, Institute of Soil Science, Academia Sinica, Science Press, and Amsterdam, Netherlands, Elsevier. Takahashi, T., Nanzyo, M. & Shoji, S. 2004. Proposed revisions to the diagnostic criteria for andic and vitric horizons and qualifiers of Andosols in the World Reference Base for Soil Resources. Soil Sci. Plant Nutr., 50 (3): 431–437. Van Reeuwijk, L.P. 2006. Procedures for soil analysis. 7th Edition. Technical Report 9. Wageningen, Netherlands, ISRIC – World Soil Information. Varghese, T. & Byju, G. 1993. Laterite soils. Their distribution, characteristics, classification and management. Technical Monograph 1. Thirivananthapuram, Sri Lanka, State Committee on Science, Technology and Environment. Zevenbergen, C., Bradley, J.P., van Reeuwijk, L.P., Shyam, A.K., Hjelmar, O. & Comans, R.N.J. 1999. Clay formation and metal fixation during weathering of coal fly ash. Env. Sci. & Tech., 33(19): 3405–3409. Zikeli, S., Kastler, M. & Jahn, R. 2005. Classification of Anthrosols with vitric/andic properties derived from lignite ash. Geoderma, 124: 253–265. 123

Annex 1 Summary of analytical procedures for soil characterization

This annex provides summaries of recommended analytical procedures to be used for soil characterization for the World Reference Base for Soil Resources. Full descriptions can be found in Procedures for soil analysis (Van Reeuwijk, 2006) and the USDA Soil Survey Laboratory Methods Manual (Burt, 2004).

1. SAMPLE PREPARATION Samples are air-dried or, alternatively, oven-dried at a maximum of 40 °C. The fine earth fraction is obtained by sieving the dry sample with a 2-mm sieve. Clods not passing through the sieve are crushed (not ground) and sieved again. Gravel, rock fragments, etc. not passing through the sieve are treated separately. In special cases where air-drying causes unacceptable irreversible changes in certain soil properties (e.g. in peat and soils with andic properties), samples are kept and treated in the field-moist state.

2. MOISTURE CONTENT Calculation of results of soil analysis is done on the basis of oven-dry (105 °C) soil mass.

3. PARTICLE-SIZE ANALYSIS The mineral part of the soil is separated into various size fractions and the proportion of these fractions is determined. The determination comprises all material, i.e. including gravel and coarser material, but the procedure itself is applied to the fine earth (< 2 mm) only. The pre-treatment of the sample is aimed at complete dispersion of the primary particles. Therefore, cementing materials (usually of secondary origin) such as organic matter and calcium carbonate may have to be removed. In some cases, de-ferration also needs to be applied. However, depending on the aim of study, it may be fundamentally wrong to remove cementing materials. Thus, all pre-treatments are to be considered optional. However, for soil characterization purposes, removal of organic matter by

H2O2 and of carbonates by HCl is routinely carried out. After this pre-treatment, the sample is shaken with a dispersing agent and sand is separated from clay and silt with a 63-μm sieve. The sand is fractionated by dry sieving, the clay and silt fractions are determined by the pipette method or, alternatively, by the hydrometer method.

4. WATER-DISPERSIBLE CLAY This is the clay content found when the sample is dispersed with water without any pre-treatment to remove cementing compounds and without use of a dispersing agent. The proportion of natural clay to total clay can be used as a structure stability indicator.

5. RETENTION The water content is determined of soil samples that have been equilibrated with water at various suction (tension) values. For low suction values, undisturbed core samples are equilibrated on a silt and kaolin bath; for high suction values, disturbed samples are 124 World reference base for soil resources 2006

equilibrated in pressure plate extractors. The bulk density is calculated from the core sample mass.

6. BULK DENSITY Soil bulk density is the mass per unit volume of soil. As bulk density changes with water content, the water status of the sample must be specified. Two different procedures can be used: ÿ Undisturbed core samples. A metal cylinder of known volume is pressed into the soil. The moist sample mass is recorded. This may be the field-moist state or the state after equilibrating the sample at a specified water tension. The sample is then oven-dried and weighed again. The bulk density is the ratio of dry mass to volume at the determined water content and/or the specified water tension. ÿ Coated clods. Field-occurring clods are coated with plastic lacquer (e.g. Saran dissolved in methyl ethyl ketone) to allow determination of underwater mass. This gives the volume of the clod. The moist sample mass is recorded. This may be the field-moist state or the state after equilibrating the clod at specified water suction. The sample is then oven-dried and weighed again. The bulk density is the ratio of dry mass to volume at the specified water suction. Note: The determination of bulk density is very sensitive to errors, particularly caused by non-representativeness of the samples (stones, cracks, roots, etc.). Therefore, determinations should always be made in triplicate.

7. COEFFICIENT OF LINEAR EXTENSIBILITY (COLE) The COLE gives an indication of the reversible shrink–swell capacity of a soil. It is calculated from the dry bulk density and the bulk density at 33 kPa water suction. The COLE value is expressed in centimetres per centimetre or as a percentage value.

8. PH The pH of the soil is potentiometrically measured in the supernatant suspension of a

1:2½ soil:liquid mixture. The liquid is either distilled water (pH-H2O) or a 1 M KCl solution (pH-KCl). In some cases definitions for classifcation specify a 1:1 soil:water ratio.

9. ORGANIC CARBON The Walkley–Black procedure is followed. This involves a wet combustion of the organic matter with a mixture of potassium dichromate and sulphuric acid at about 125 °C. The residual dichromate is titrated against ferrous sulphate. To compensate for incomplete destruction, an empirical correction factor of 1.3 is applied in the calculation of the result. Note: Other procedures, including carbon analysers (dry combustion) may also be used. In these cases a qualitative test for carbonates on effervescence with HCl is recommended and, if present, a correction for inorganic C (see Carbonate below) is required.

10. CARBONATE The rapid titration method by Piper (also called acid neutralization method) is used. The sample is treated with dilute HCl and the residual acid is titrated. The results are referred to as calcium carbonate equivalent as the dissolution is not selective for calcite and also other carbonates such as dolomite are dissolved to some extent. Note: Other procedures such as the Scheibler volumetric method may also be used. Annex 1 – Summary of analytical procedures for soil characterization 125

11. GYPSUM Gypsum is dissolved by shaking the sample with water. It is then selectively precipitated from the extract by adding acetone. This precipitate is re-dissolved in water and the Ca concentration is determined as a measure for gypsum.

12. CATION EXCHANGE CAPACITY (CEC) AND EXCHANGEABLE BASES The ammonium acetate pH 7 method is used. The sample is percolated with ammonium acetate (pH 7) and the bases are measured in the percolate. The sample is subsequently percolated with sodium acetate (pH 7), the excess salt is then removed and the adsorbed Na exchanged by percolation with ammonium acetate (pH 7). The Na in this percolate is a measure for the CEC. Alternatively, after percolation with ammonium acetate, the sample can be washed free of excess salt, the whole sample distilled and the evolved ammonia determined. Percolation in tubes may also be replaced by shaking in flasks. Each extraction must be repeated three times and the three extracts should be combined for analysis. Note 1: Other procedures for CEC may be used provided the determination is done at pH 7. Note 2: In special cases where CEC is not a diagnostic criterion, e.g. saline and alkaline soils, the CEC may be determined at pH 8.2. Note 3: The base saturation of saline, calcareous and gypsiferous soils can be considered to be 100 percent. Note 4: Where low-activity clays are involved, the CEC of the organic matter has to be deducted. This can be done by the graphical method (FAO, 1966), or by analysing the CEC of the organic matter or the mineral colloids separately.

13. EXCHANGEABLE ACIDITY This is the acidity (H + Al) released upon exchange by an unbuffered 1 M KCl solution. It may also be designated actual acidity (as opposed to potential or extractable acidity). It is used to determine the so-called effective cation exchange capacity (ECEC) defined as: sum of bases + (H + Al), with bases being determined by ammonium acetate extraction. When the exchangeable acidity is substantial, the Al may be determined separately in the extract as it may be toxic to plants. Note: Because the contribution of H+ is often negligible, some laboratories only determine exchangeable Al. In that case, the ECEC is calculated as: sum of bases + Al.

14. EXTRACTABLE IRON, ALUMINIUM, MANGANESE AND SILICON These analyses comprise: ÿ Free Fe, Al and Mn compounds in the soil extracted by a dithionite-citrate solution. (Both the Mehra and Jackson and Holmgren procedures may be used.) ÿ Active, short-range-order or amorphous Fe, Al and silica compounds extracted by an acid oxalate solution. ÿ Organically bound Fe and Al extracted by a pyrophosphate solution.

15. SALINITY Attributes associated with salinity in soils are determined in the saturation extract. The attributes include: pH, electrical conductivity (ECe), sodium adsorption ratio (SAR) and the cations and anions of the dissolved salts. These include Ca, Mg, Na, K, carbonate and bicarbonate, chloride, nitrate and sulphate. The SAR and the exchangeable sodium percentage (ESP) may be estimated from the concentrations of the dissolved cations.

16. PHOSPHATE RETENTION The Blakemore procedure is used. The sample is equilibrated with a phosphate solution at pH 4.6 and the proportion of phosphate withdrawn from solution is determined. 126 World reference base for soil resources 2006

17. OPTICAL DENSITY OF OXALATE EXTRACT (ODOE) The sample is percolated or shaken with an acid ammonium oxalate solution. The optical density of the extract is measured at 430-nm wavelength.

18. MELANIC INDEX The sample is shaken with a 0.5 M NaOH solution and the absorbance of the extract is measured at 450 and 520 nm, respectively. The melanic index is obtained by dividing the absorbance at 450 nm by the absorbance at 520 nm.

19. MINERALOGICAL ANALYSIS OF THE SAND FRACTION After removal of cementing and coating materials, the sand is separated from the clay and silt by wet sieving. From the sand, the fraction 63–420 μm is separated by dry sieving. This fraction is divided into a heavy fraction and a light fraction with the aid of a high-density liquid: a solution of sodium polytungstate1 with a specific density of 2.85 kg dm-3. Of the heavy fraction, a microscopic slide is made; the light fraction is stained selectively for microscopic identification of feldspars and quartz. Volcanic glass can usually be recognized as isotropic grains with vesicles.

20. X-RAY DIFFRACTOMETRY The clay fraction is separated from the fine earth and deposited in an oriented fashion on glass slides or porous ceramic plates to be analysed on an X-ray diffractometer. Unoriented powder specimens of clay and other fractions are analysed on the same apparatus or with a Guinier X-ray camera (photographs).

1 Bromoform can also be used as high density liquid but its use is discouraged because of its highly toxic vapour. 127

Annex 2 Recommended codes for the reference soil groups, qualifiers and specifiers

Reference Soil Groups AC Chernozem CH Kastanozem KS Podzol PZ Albeluvisol AB Cryosol CR Leptosol LP Regosol RG AL DU Lixisol LX Solonchak SC Andosol AN Ferralsol FR Luvisol LV Solonetz SN AT Fluvisol FL NT Stagnosol ST Arenosol AR Gleysol GL Phaeozem PH Technosol TC Calcisol CL GY Planosol PL Umbrisol UM Cambisol CM Histosol HS PT Vertisol VR

Qualifiers Abruptic ap Duric du Gelistagnic gt Hypoluvic wl Aceric ae Dystric dy Geric gr Hyposalic ws Acric ac Ekranic ek Gibbsic gi Hyposodic wn Acroxic ao Endoduric nd Glacic gc Irragric ir Albic ab Endodystric ny Gleyic gl Lamellic ll Alcalic ax Endoeutric ne Glossalbic gb Laxic la Alic al Endofluvic nf Glossic gs Leptic le Aluandic aa Endogleyic ng Greyic gz Lignic lg Alumic au Endoleptic nl Grumic gm Limnic lm Andic an Endosalic ns Gypsic gy Linic lc Anthraquic aq Entic et Gypsiric gp Lithic li Anthric am Epidystric ed Haplic ha Lixic lx Arenic ar Epieutric ee Hemic hm Luvic lv Aric ai Epileptic el Histic hi Magnesic mg Aridic ad Episalic ea Hortic ht Manganiferric mf Arzic az Escalic ec Humic hu Mazic mz Brunic br Eutric eu Hydragric hg Melanic ml Calcaric ca Eutrosilic es Hydric hy Mesotrophic ms Calcic cc Ferralic fl Hydrophobic hf Mollic mo Cambic cm Ferric fr Hyperalbic ha Molliglossic mi Carbic cb Fibric fi Hyperalic hl Natric na Carbonatic cn Floatic ft Hypercalcic hc Nitic ni Chloridic cl Fluvic fv Hyperdystric hd Novic nv Chromic cr Folic fo Hypereutric he Nudilithic nt Clayic ce Fractipetric fp Hypergypsic hp Ombric om Colluvic co Fractiplinthic fa Hyperochric ho Ornithic oc Cryic cy Fragic fg Hypersalic hs Ortsteinic os Cutanic ct Fulvic fu Hyperskeletic hk Oxyaquic oa Densic dn Garbic ga Hypocalcic wc Pachic ph Drainic dr Gelic ge Hypogypsic wg Pellic pe 128 World reference base for soil resources 2006

Qualifiers (Continued) Petric pt Reductaquic ra Solodic sc Tidalic td Petrocalcic pc Reductic rd Sombric sm Toxic tx Petroduric pd Regic rg Spodic sd Transportic tn Petrogleyic py Rendzic rz Spolic sp Turbic tu Petrogypsic pg Rheic rh Stagnic st Umbric um Petroplinthic pp Rhodic ro Subaquatic sq Umbriglossic ug Petrosalic ps Rubic ru Sulphatic su Urbic ub Pisoplinthic px Ruptic rp Takyric ty Vermic vm Placic pi Rustic rs Technic te Vertic vr Plaggic pa Salic sz Tephric tf Vetic vt Plinthic pl Sapric sa Terric tr Vitric vi Posic po Silandic sn Thaptandic ba Voronic vo Profondic pf Siltic sl Thaptovitric bv Xanthic xa Protic pr Skeletic sk Thionic ti Yermic ye Puffi c pu Sodic so Thixotropic tp

Specifiers Bathy ..d Epi ..p Ortho ..o Thapto ..b Cumuli ..c Hyper ..h Para ..r Endo ..n Hypo ..w Proto ..t WORLD SOIL RESOURCES REPORTS

1. Report of the First Meeting of the Advisory Panel on the Soil Map of the World, Rome, 19–23 June 1961 (E)** 2. Report of the First Meeting on Soil Survey, Correlation and Interpretation for Latin America, Rio de Janeiro, Brazil, 2831 May 1962 (E)** 3. Report of the First Soil Correlation Seminar for Europe, Moscow, USSR, 16–28 July 1962 (E)** 4. Report of the First Soil Correlation Seminar for South and Central Asia, Tashkent, Uzbekistan, USSR, 14 September–2 October 1962 (E)** 5. Report of the Fourth Session of the Working Party on Soil Classification and Survey (Subcommission on Land and Water Use of the European Commission on Agriculture), Lisbon, Portugal, 6–10 March 1963 (E)** 6. Report of the Second Meeting of the Advisory Panel on the Soil Map of the World, Rome, 9–11 July 1963 (E)** 7. Report of the Second Soil Correlation Seminar for Europe, Bucharest, Romania, 29 July–6 August 1963 (E)** 8. Report of the Third Meeting of the Advisory Panel on the Soil Map of the World, Paris, 3 January 1964 (E)** 9. Adequacy of Soil Studies in Paraguay, Bolivia and Peru, November–December 1963 (E)** 10. Report on the Soils of Bolivia, January 1964 (E)** 11. Report on the Soils of Paraguay, January 1964 (E)** 12. Preliminary Definition, Legend and Correlation Table for the Soil Map of the World, Rome, August 1964 (E)** 13. Report of the Fourth Meeting of the Advisory Panel on the Soil Map of the World, Rome, 16–21 May 1964 (E)** 14. Report of the Meeting on the Classification and Correlation of Soils from Volcanic Ash, Tokyo, Japan, 11–27 June 1964 (E)** 15. Report of the First Session of the Working Party on Soil Classification, Survey and Soil Resources of the European Commission on Agriculture, Florence, Italy, 1–3 October 1964 (E)** 16. Detailed Legend for the Third Draft on the Soil Map of South America, June 1965 (E)** 17. Report of the First Meeting on Soil Correlation for North America, Mexico, 1–8 February 1965 (E)** 18. The Soil Resources of Latin America, October 1965 (E)** 19. Report of the Third Correlation Seminar for Europe: Bulgaria, Greece, Romania, Turkey, Yugoslavia, 29 August–22 September 1965 (E)** 20. Report of the Meeting of Rapporteurs, Soil Map of Europe (Scale 1:1 000 000) (Working Party on Soil Classification and Survey of the European Commission on Agriculture), Bonn, Federal Republic of Germany, 29 November–3 December 1965 (E)** 21. Report of the Second Meeting on Soil Survey, Correlation and Interpretation for Latin America, Rio de Janeiro, Brazil, 13–16 July 1965 (E)** 22. Report of the Soil Resources Expedition in Western and Central Brazil, 24 June–9 July 1965 (E)** 23. Bibliography on Soils and Related Sciences for Latin America (1st edition), December 1965 (E)** 24. Report on the Soils of Paraguay (2nd edition), August 1964 (E)** 25. Report of the Soil Correlation Study Tour in Uruguay, Brazil and Argentina, June– August 1964 (E)** 26. Report of the Meeting on Soil Correlation and Soil Resources Appraisal in India, New Delhi, India, 5–15 April 1965 (E)** 27. Report of the Sixth Session of the Working Party on Soil Classification and Survey of the European Commission on Agriculture, Montpellier, France, 7–11 March 1967 (E)** 28. Report of the Second Meeting on Soil Correlation for North America, Winnipeg- Vancouver, Canada, 25 July–5 August 1966 (E)** 29. Report of the Fifth Meeting of the Advisory Panel on the Soil Map of the World, Moscow, USSR, 20–28 August 1966 (E)** 30. Report of the Meeting of the Soil Correlation Committee for South America, Buenos Aires, Argentina, 12–19 December 1966 (E)** 31. Trace Element Problems in Relation to Soil Units in Europe (Working Party on Soil Classification and Survey of the European Commission on Agriculture), Rome, 1967 (E)** 32. Approaches to Soil Classification, 1968 (E)** 33. Definitions of Soil Units for the Soil Map of the World, April 1968 (E)** 34. Soil Map of South America 1:5 000 000, Draft Explanatory Text, November 1968 (E)** 35. Report of a Soil Correlation Study Tour in Sweden and Poland, 27 September–14 October 1968 (E)** 36. Meeting of Rapporteurs, Soil Map of Europe (Scale 1:1 000 000) (Working Party on Soil Classification and Survey of the European Commission on Agriculture), Poitiers, France 21–23 June 1967 (E)** 37. Supplement to Definition of Soil Units for the Soil Map of the World, July 1969 (E)** 38. Seventh Session of the Working Party on Soil Classification and Survey of the European Commission on Agriculture, Varna, Bulgaria, 11–13 September 1969 (E)** 39. A Correlation Study of Red and Yellow Soils in Areas with a Mediterranean Climate (E)** 40. Report of the Regional Seminar of the Evaluation of Soil Resources in West Africa, Kumasi, Ghana, 14–19 December 1970 (E)** 41. Soil Survey and Soil Fertility Research in Asia and the Far East, New Delhi, 15–20 February 1971 (E)** 42. Report of the Eighth Session of the Working Party on Soil Classification and Survey of the European Commission on Agriculture, Helsinki, Finland, 5–7 July 1971 (E)** 43. Report of the Ninth Session of the Working Party on Soil Classification and Survey of the European Commission on Agriculture, Ghent, Belgium 28–31 August 1973 (E)** 44. First Meeting of the West African Sub-Committee on Soil Correlation for Soil Evaluation and Management, Accra, Ghana, 12–19 June 1972 (E)** 45. Report of the Ad Hoc Expert Consultation on Land Evaluation, Rome, Italy, 6–8 January 1975 (E)** 46. First Meeting of the Eastern African Sub-Committee for Soil Correlation and Land Evaluation, Nairobi, Kenya, 11–16 March 1974 (E)** 47. Second Meeting of the Eastern African Sub-Committee for Soil Correlation and Land Evaluation, Addis Ababa, Ethiopia, 25–30 October 1976 (E) 48. Report on the Agro-Ecological Zones Project, Vol. 1 – Methodology and Results for Africa, 1978. Vol. 2 – Results for Southwest Asia, 1978 (E) 49. Report of an Expert Consultation on Land Evaluation Standards for Rainfed Agriculture, Rome, Italy, 25–28 October 1977 (E) 50. Report of an Expert Consultation on Land Evaluation Criteria for Irrigation, Rome, Italy, 27 February–2 March 1979 (E) 51. Third Meeting of the Eastern African Sub-Committee for Soil Correlation and Land Evaluation, Lusaka, Zambia, 18–30 April 1978 (E) 52. Land Evaluation Guidelines for Rainfed Agriculture, Report of an Expert Consultation, 12–14 December 1979 (E) 53. Fourth Meeting of the West African Sub-Committee for Soil Correlation and Land Evaluation, Banjul, The Gambia, 20–27 October 1979 (E) 54. Fourth Meeting of the Eastern African Sub-Committee for Soil Correlation and Land Evaluation, Arusha, Tanzania, 27 October–4 November 1980 (E) 55. Cinquième réunion du Sous-Comité Ouest et Centre africain de corrélation des sols pour la mise en valeur des terres, Lomé, Togo, 7–12 décembre 1981 (F) 56. Fifth Meeting of the Eastern African Sub-Committee for Soil Correlation and Land Evaluation, Wad Medani, Sudan, 5–10 December 1983 (E) 57. Sixième réunion du Sous-Comité Ouest et Centre Africain de corrélation des sols pour la mise en valeur des terres, Niamey, Niger, 6–12 février 1984 (F) 58. Sixth Meeting of the Eastern African Sub-Committee for Soil Correlation and Land Evaluation, Maseru, Lesotho, 9–18 October 1985 (E) 59. Septième réunion du Sous-Comité Ouest et Centre africain de corrélation des sols pour la mise en valeur des terres, Ouagadougou, Burkina Faso, 10–17 novembre 1985 (F) 60. Revised Legend, Soil Map of the World, FAO-Unesco-ISRIC, 1988. Reprinted 1990 (E) 61. Huitième réunion du Sous-Comité Ouest et Centre africain de corrélation des sols pour la mise en valeur des terres, Yaoundé, Cameroun, 19–28 janvier 1987 (F) 62. Seventh Meeting of the East and Southern African Sub-Committee for Soil Correlation and Evaluation, Gaborone, Botswana, 30 March–8 April 1987 (E) 63. Neuvième réunion du Sous-Comité Ouest et Centre africain de corrélation des sols pour la mise en valeur des terres, Cotonou, Bénin, 14–23 novembre 1988 (F) 64. FAO-ISRIC Soil Database (SDB), 1989 (E) 65. Eighth Meeting of the East and Southern African Sub-Committee for Soil Correlation and Land Evaluation, Harare, Zimbabwe, 9–13 October 1989 (E) 66. World soil resources. An explanatory note on the FAO World Soil Resources Map at 1: 25 000 000 scale, 1991. Rev. 1, 1993 (E) 67. Digitized Soil Map of the World, Volume 1: Africa. Volume 2: North and Central America. Volume 3: Central and South America. Volume 4: Europe and West of the Urals. Volume 5: North East Asia. Volume 6: Near East and Far East. Volume 7: South East Asia and Oceania. Release 1.0, November 1991 (E) 68. Land Use Planning Applications. Proceedings of the FAO Expert Consultation 1990, Rome, 10–14 December 1990 (E) 69. Dixième réunion du Sous-Comité Ouest et Centre africain de corrélation des sols pour la mise en valeur des terres, Bouaké, Odienné, Côte d’Ivoire, 5–12 novembre 1990 (F) 70. Ninth Meeting of the East and Southern African Sub-Committee for Soil Correlation and Land Evaluation, Lilongwe, Malawi, 25 November – 2 December 1991 (E) 71. Agro-ecological land resources assessment for agricultural development planning. A case study of Kenya. Resources data base and land productivity. Main Report. Technical Annex 1: Land resources. Technical Annex 2: Soil erosion and productivity. Technical Annex 3: Agro-climatic and agro-edaphic suitabilities for barley, oat, cowpea, green gram and pigeonpea. Technical Annex 4: Crop productivity. Technical Annex 5: Livestock productivity. Technical Annex 6: Fuelwood productivity. Technical Annex 7: Systems documentation guide to computer programs for land productivity assessments. Technical Annex 8: Crop productivity assessment: results at district level. 1991. Main Report 71/9: Making land use choices for district planning, 1994 (E) 72. Computerized systems of land resources appraisal for agricultural development, 1993 (E) 73. FESLM: an international framework for evaluating sustainable , 1993 (E) 74. Global and national soils and terrain digital databases (SOTER), 1993. Rev. 1, 1995 (E) 75. AEZ in Asia. Proceedings of the Regional Workshop on Agro-ecological Zones Methodology and Applications, Bangkok, Thailand, 17–23 November 1991 (E) 76. Green manuring for soil productivity improvement, 1994 (E) 77. Onzième réunion du Sous-Comité Ouest et Centre africain de corrélation des sols pour la mise en valeur des terres, Ségou, Mali, 18–26 janvier 1993 (F) 78. Land degradation in South Asia: its severity, causes and effects upon the people, 1994 (E) 79. Status of sulphur in soils and plants of thirty countries, 1995 (E) 80. Soil survey: perspectives and strategies for the 21st century, 1995 (E) 81. Multilingual soil database, 1995 (Multil) 82. Potential for forage legumes of land in West Africa, 1995 (E) 83. Douzième réunion du Sous-Comité Ouest et Centre africain de corrélation des sols pour la mise en valeur des terres, Bangui, République Centrafricain, 5–10 décembre 1994 (F) 84. World reference base for soil resources, 1998 (E) 85. Soil Fertility Initiative for sub-Saharan Africa, 1999 (E) 86. Prevention of land degradation, enhancement of carbon sequestration and conservation of biodiversity through land use change and sustainable land management with a focus on Latin America and the Caribbean, 1999 (E) 87. AEZWIN: An interactive multiple-criteria analysis tool for land resources appraisal, 1999 (E)

88. Sistemas de uso de la tierra en los trópicos húmedios y la emisión y secuestro de CO2, 2000 (S) 89. Land resources information systems for food security in SADC countries, 2000 (E) 90. Land resource potential and constraints at regional and country levels, 2000 (E) 91. The European soil information system, 2000 (E) 92. Carbon sequestration projects under the clean development mechanism to address land degradation, 2000 (E) 93. Land resources information systems in Asia, 2000 (E) 94. Lecture notes on the major soils of the world, 2001 (E) 95. Land resources information systems in the Caribbean, 2001 (E) 96. sequestration for improved land management, 2001 (E F S) 97. Land degradation assessment in drylands – LADA project, 2002 (E) 98. Quatorzième réunion du Sous-Comité Ouest et Centre africain de corrélation des sols pour la mise en valeur des terres, Abomey, Bénin, 9–13 octobre 2000, 2002 (F) 99. Land resources information systems in the Near East, 2002 (E) 100. Data sets, indicators and methods to assess land degradation in drylands, 2003 (E) 101. Biological management of soil ecosystems for sustainable agriculture, 2003 (E) 102. Carbon sequestration in dryland soils, 2004 (E) 103 World reference base for soil resources 2006 – A framework for international classification, correlation and communication, 2006 (E)

Availability: June 2006

E – English Multil – Multilingual F – French ** Out of print S – Spanish A0510E-Cover-A4-Pant 4725 [ConvePage 1 30-05-2006 12:03:10 ISSN 0532-0488

WORLD SOIL 103 RESOURCES 103 REPORTS WORLD SOIL RESOURCES REPORTS 103

World reference base W orld reference base for soil resources 2006 for soil resources 2006 A framework for international classification, World reference base correlation and communication for soil resources 2006 A framework for international classification, correlation and communication

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MY an agreed common scientific language to enhance communication across disciplines using soil information. It CY contains definitions and diagnostic criteria to recognize soil CMY horizons, properties and materials and gives rules and K guidelines for classifying and subdividing soil reference groups.

ISBN 92-5-105511-4 ISSN 0532-0488 F 9 7 8 9 2 5 1 0 5 5 1 1 3 AO TC/M/A0510E/1/05.06/5200 International Union of Soil Sciences