Kheyrodin and Kheyrodin, World J. Biol. Med. Science Volume 2 (4), 89-99, 2015

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World Journal of Biology and Medical Sciences Published by Society for Advancement of Science® ISSN 2349-0063 (Online/Electronic) Volume 2, Issue- 4, 89-99, October to December, 2015

WJBMS 2/04/45/2015 All rights reserved www.sasjournals.com A Double Blind Peer Reviewed Journal / Refereed Journal [email protected]/[email protected] REVIEW ARTICLE Received: 03/07/2015 Revised: 03/09/2015 Accepted: 05/09/2015 Study of Desert Soil Horizons: A Review Sadaf Kheyrodin and *Hamid Kheyrodin Islamic Azad University, Mashhad Branch, Iran *Faculty of Desert Science, Semnan University, Iran

ABSTRACT Horizons are defined in most cases by obvious physical features, chiefly color. Desert soils form in areas where the demand for water by the atmosphere (evaporation) and plants (transpiration) is much greater than precipitation. Deserts cover 20 to 33% of the Earth’s land surface, and can be found in the tropics, at the poles, and in between. Most desert soils are called Aridisols (dry soil). However, in really dry regions of the Sahara and Australian outback, the soil orders are called Entisols. Entisols are new soils, like sand dunes, which are too dry for any major soil horizon development. They also occur in floodplains after a spring flood, which is why they can occur in the desert. The deserts are very fertile, which means that, if provided with water, they can grow a lot of food. This can be done by providing water with irrigation technology. The Central Valley in California produces 250 types of fruits and vegetables, and does so with river water, aquifers, and streams. Six master soil horizons are commonly recognized and are designated using the capital letters O, A, E, B, C, and R. O horizon definition, the layer of loose leaves and organic debris at the surface of soil. A-horizons may also be the result of a combination of soil bioturbation and surface processes that winnow fine particles from biologically mounded topsoil. B horizons: are commonly referred to as the subsoil. They are a zone of accumulation where rain water percolating through the soil has leached material. The C

89 Kheyrodin and Kheyrodin, World J. Biol. Med. Science Volume 2 (4), 89-99, 2015 horizon (parent material) is below the B Horizon. This layer is little affected by soil- forming. The R horizon is basically the deepest soil horizon in the soil profile. Unlike the horizon above, this horizon does not comprise rocks or boulders, but instead is made of continuous mass of bedrock. It is very difficult to dig through this layer. Key words: Desert Soil, Horizons, Aridisols, Entisols and Soil Farming.

INTRODUCTION represented as "ideal" soil in diagrams. A soil horizon is a layer generally parallel Each main horizon is denoted by a capital to the soil surface, whose physical letter, which may then be followed by characteristics differ from the layers several alphanumerical modifiers above and beneath. Each soil type usually highlighting particular outstanding has three or four horizons. Horizons are features of the horizon. While the general defined in most cases by obvious physical O-A-B-C-R sequence seems fairly features, chiefly color and texture. These universal, some variation exists between may be described both in absolute terms the classification systems in different (particle size distribution for texture, for parts of the world. In addition, the exact instance) and in terms relative to the definition of each main horizon may differ surrounding material (i.e., "coarser" or slightly – for instance, the US system uses "sandier" than the horizons above and the thickness of a horizon as a below). The differentiation of the soil into distinguishing feature, while the distinct horizons is largely the result of Australian system does not. It should be influences, such as air, water, solar emphasized that no one system is more radiation and plant material, originating at correct – as artificial constructs, their the soil-atmosphere interface. Since the utility lies in their ability to accurately weathering of the soil occurs first at the describe local conditions in a consistent surface and works its way down, the manner. Also, many subtropical and uppermost layers have been changed the tropical areas have soils such as oxisols or most, while the deepest layers are most aridisols that have very different horizons similar to the original parent material. from "ideal" soil or no horizons at all. Identification and description of the horizons present at a given site is the first Main Profile step in soil classification at higher levels, Six master soil horizons are commonly through the use of systems such as the recognized and are designated using the USDA soil taxonomy or the Australian Soil capital letters O, A, E, B, C, and R. The Classification. The World Reference Base following horizons are listed by their for Soil Resources lists 40 diagnostic position from top to bottom within the horizons. Isbell Raymond, (2002). Soil soil profile. Not all of these layers are scientists often dig a large hole, called a present in every location. For instance, P soil pit (usually several meters deep and horizons only form in areas which have about a meter wide) to expose soil been waterlogged for long periods of horizons for study. The vertical section time. Soils with a history of human exposing a set of horizons, from the interference, for instance through major ground surface to the parent rock, is earthworks or regular deep ploughing, termed a soil profile. Most soils, especially may lack distinct horizons almost in temperate climates, conform to a completely. When examining soils in the similar general pattern of horizons, often field, attention must be paid to the local

90 Kheyrodin and Kheyrodin, World J. Biol. Med. Science Volume 2 (4), 89-99, 2015 geomorphology and the historical uses to Layers: Soil generally consists of visually which the land has been put in order to and texturally distinct layers, also called ensure that the appropriate names are profiles, which can be summarized as applied to the observed horizons. The follows from top to bottom: horizon not listed is the O horizon which is grass and animal/plant life.

Figure 1. Soil Profile.

O) Organic matter: Surficial organic mounded topsoil. In this case, the A- deposit with litter layer of plant residues horizon is regarded as a "biomantle". in relatively non-decomposed form. B) Subsoil: Subsurface layer reflecting A) Surface soil: Organics mixed with chemical or physical alteration of parent mineral matter. The Layer of mineral soil material. This layer accumulates iron, clay, with the most organic matter aluminum and organic compounds, a accumulation and soil life. This layer process referred to as illuviation. eluviates (is depleted of) iron, clay, C) Parent rock: The parent material in aluminum, organic compounds, and other sedimentary deposits. Layer of large soluble constituents. When eluviation is unbroken rocks. This layer may pronounced, a lighter colored "E" accumulate the more soluble compounds subsurface soil horizon is apparent at the . base of the "A" horizon. A-horizons may R) Bedrock: The parent material in also be the result of a combination of soil bedrock landscapes. This layer denotes bioturbation and surface processes that the layer of partially weathered bedrock winnow fine particles from biologically at the base of the soil profile. Unlike the above layers, R horizons largely comprise

91 Kheyrodin and Kheyrodin, World J. Biol. Med. Science Volume 2 (4), 89-99, 2015 continuous masses of hard rock that horizons may be divided into O1 and O2 cannot be excavated by hand. Soils categories, whereby O1 horizons contain formed in situ will exhibit strong decomposed matter whose origin can be similarities to this bedrock layer. These spotted on sight (for instance, fragments areas of bedrock are under 50 feet of the of rotting leaves), and O2 horizons contain other profiles. only well-decomposed organic matter, the origin of which is not readily visible. O O Horizon horizons may also be divided into three This layer generally forms above the subordinate O horizons denoted as: Oi, mineral soil or occurs in an organic soil Oe, and Oa. profile. The "O" stands for organic matter. P Horizon It is a surface layer dominated by the These horizons also heavily organic, but presence of large amounts of organic are distinct from O horizons in that they material derived from dead plant and/or form under waterlogged conditions. The animal residues which is in varying stages "P" designation comes from their of decomposition. The O horizon is common name, peats. They may be generally absent in grassland regions. The divided into P1 and P2 in the same way as O horizon usually occurs in forested areas O Horizons. This layer accumulates iron, and is commonly referred to as the forest clay, aluminium and organic compounds, floor. The O horizon should be considered a process referred to as illuviation distinct from the layer of leaf litter (Johnson, D.L., J.E.J. Domier, and D.N. covering many heavily vegetated areas, Johnson 2005). which contains no weathered mineral particles and is not part of the soil itself. O A Horizon

Figure 2. Show some realistic soil profiles. The A horizon is the topmost mineral that of the lower horizons. The A horizons horizon, often referred to as the 'topsoil'. are often coarser in texture, having lost This layer generally contains enough some of the finer materials by partially decomposed (humified) organic translocation to lower horizons and by matter to give the soil a color darker than erosion. This layer is known as the zone in

92 Kheyrodin and Kheyrodin, World J. Biol. Med. Science Volume 2 (4), 89-99, 2015 which the most biological activity occurs. to as the biomantle (McDonald, R. C. Soil organisms such as earthworms, 1990 and Wilkinson, M.T. and G.S. potworms (enchytraeids), arthropods, Humphreys 2005). However, since nematodes, fungi, and many species of biological activity extends far deeper into bacteria and archaea are concentrated the soil, it cannot be used as a chief here, often in close association with plant distinguishing feature of an A horizon. roots. Thus the A horizon may be referred E Horizon

Figure 3. Show soil horizons in forest.

Albeluvisol – dark surface horizon on a characteristics, and then appended with bleached subsurface horizon (an albic the designation "e" (see the section below horizon) that tongues into a clay on horizon suffixes). In soils that contain illuviation (Bt) horizon gravels, due to animal bioturbation, a Main article: Eluvium stonelayer commonly forms near or at the "E", being short for eluviated, is most base of the E horizon. commonly used to label a horizon that has The above layers may be referred to been significantly leached of clay, iron, collectively as the "solum". The layers and aluminum oxides, which leaves a below have no collective name but are concentration of resistant minerals, such distinct in that they are noticeably less as quartz, in the sand and silt sizes. These affected by surface soil-forming are present only in older, well-developed processes. soils, and generally occur between the A B Horizon and B horizons. The E horizon often has a B horizons form below an O, A, or E pale color that is generally lighter in color horizon and they have undergone than either the horizon above or below it. sufficient changes during soil genesis, E horizons are commonly found in soils such that the properties of their original developed under forests, but are rare in parent material are no longer discernible. soils developed under grasslands. In The B horizon is commonly referred to as regions where this designation is not the "subsoil". In humid regions, B horizons employed, leached layers are classified are the layers of maximum accumulation firstly as an A or B according to other of materials such as silicate clays, iron (Fe)

93 Kheyrodin and Kheyrodin, World J. Biol. Med. Science Volume 2 (4), 89-99, 2015 and aluminum (Al) oxides, and organic brownish or reddish due to residual clay material. These materials typically and iron oxides. accumulate through a process termed C Horizon illuviation, wherein the materials The C horizon (parent material) is below gradually wash in from the overlying the B Horizon. This layer is little affected horizons. Accordingly, this layer is also by soil-forming processes and they thus referred to as the "illuviated" horizon or have a lack of pedological development. the "zone of accumulation". In addition, it In other words, the C horizon is the is defined as having a distinctly different unconsolidated material underlying the structure or consistency than the solum (A and B horizons). It may or may horizon(s) above and the horizon(s) not be the same as the parent material below. The B horizon may also have from which the solum formed. The C stronger colors (higher chroma) than the horizon forms as the R horizon weathers A horizon. In arid and semiarid regions, and rocks break up into smaller particles. calcium carbonate or calcium sulfate may The C horizon is below the zones of accumulate in the B horizon. greatest biological activity and it has not As with the A horizon, the B horizon may been sufficiently altered by soil genesis to be divided into B1, B2, and B3 types under qualify as a B horizon. In dry regions, the Australian system. B1 is a transitional carbonates and gypsum may be horizon of the opposite nature to an A3 – concentrated in the C horizon. While dominated by the properties of the B loose enough to be dug with a shovel, C horizons below it, but containing some A- horizon material often retains some of the horizon characteristics. B2 horizons have a structural features of the parent rock or concentration of clay, minerals, or geologic deposits from which it formed. organics and feature the highest soil The A and B layers usually originated from development within the profile. B3 the C horizon. The upper layers of the C horizons are transitional between the horizon may in time become a part of the overlying B layers and the material solum as weathering and erosion beneath it, whether C or D horizon. continue. The C Horizon may contain The A3, B1, and B3 horizons are not tightly lumps or more likely large shelves of defined, and their use is generally at the unweathered rock, rather than being discretion of the individual worker. made up solely of small fragments as in Plant roots penetrate through this layer, the solum. It contains rocks with cracks but it has little humus. It is usually and crevices.

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Figure 4. Show soil profiles in 120 cm.

D Horizon the master horizon symbols for horizons developed in the second layer of parent D horizons are not universally material. For example, a soil would have a distinguished, but in the Australian system sequence of horizons designated O-A-B- are "any soil material below the solum 2C if C horizon developed in a different that is unlike the solum in general parent material. Where a layer of mineral character, is not C horizon, and cannot be soil material was transported by humans given reliable designation… [it] may be from a source outside the pedon, the recognized by the contrast in pedologic caret symbol (^) is inserted before the organization between it and the overlying master horizon designation. horizons" (MacDonald et al., 1990, p. 106). Transition Horizons Master horizon letter combinations with a R Horizon slash in between, such as E/B, are used to R horizons are the layers of partially designate transition horizons where weathered bedrock at the base of the soil distinct parts of the horizon have profile. Unlike the above layers, R properties of E while other parts have horizons are composed largely of properties of B. Transitional layers consolidated masses of hard rock that between the master horizons may be cannot be excavated by hand. Soils dominated by properties of one horizon formed in situ will exhibit strong but also have characteristics of another. similarities to this bedrock layer. The two master horizon letters are used to designate these transition horizons L Horizon (e.g., AE, EB, BE, and BC), wherein the L (limnic) horizons or layers indicate dominant horizon is placed before the mineral or organic material that has been non-dominant horizon. For example, BE is deposited in water by precipitation or a transition to B that is more like B than E, through the actions of aquatic organisms. while EB is a transition to B that is more Included are coprogenous earth like E than B. (sedimentary peat), diatomaceous earth, and marl; and is usually found as a Subordinate Distinctions remnant of past bodies of standing water. Subordinate horizons may occur within a master horizon and these are designated Subdivisions within master horizons by lowercase letters following the capital Often, distinctive layers exist within a master horizon letter (e.g., Ap, Bt, or Oi). given master horizon, and these are Since the nature of a master horizon is indicated by a numeral following the only generally described by the capital letter designation. For example, if three master horizon letter, the lowercase letter different combinations of structure and symbols following the master horizon colors can be seen in the B horizon, then designation is often used to indicate more the profile may include a sequence such specific horizon characteristics. as B1-B2-B3. If two different geologic Subordinate distinctions include, but are parent materials (e.g., loess overlying not limited to, special physical glacial till) are present within the soil characteristics and the accumulation of profile, the numeral 2 is placed in front of particular materials.

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Table 1. Lowercase Letter Symbols to Designate Subordinate Distinctions within Master. Horizons

Letter Distinction Letter Distinction

Organic matter, highly a b Buried soil horizon decomposed c Concretions or nodules d Dense unconsolidated materials

Organic matter, intermediate e f Frozen soil decomposition

Illuvial accumulation of organic g Strong gleying (mottling) h matter

Organic matter, slightly i j Jarosite (yellow sulfate mineral) decomposed jj Cryoturbation (frost churning) k Accumulation of carbonates m Cementation or Induration n Accumulation of sodium o Accumulation of Fe and Al oxides p Plowing or other disturbance q Accumulation of silica r Weathered or soft bedrock

Illuvial organic matter and Fe and s ss Slickensides (shiny clay wedges) Al oxides

Presence of human-manufactured t Accumulation of silicate clays u materials (artifacts)

Distinctive color or structure v Plinthite (high iron, red material) w without clay accumulation x Fragipan (high bulk density, brittle) y Accumulation of gypsum z Accumulation of soluble salts

Buried Soils profile. This sort of occurrence is most While soil formation is generally described common in coastal areas, and descriptions as occurring in situ, as rock breaks down are modified by numerical prefixes. Thus, and is mixed with other materials, the a profile containing a buried sequence process is often far more complicated. For could be structured O, A1, A2, B2, 2A2, instance, a fully formed profile may have 2B21, 2B22, 2C with the buried profile developed in an area only to be buried by commencing at 2A2. wind- or water-deposited sediments Diagnostic surface horizons of mineral which later formed into another soil soils

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The diagnostic horizons at the soil surface areas, and has a layer of peat or muck are called epipedons (from the Greek 'epi, with a black to dark brown color and a "over", and pedon, "soil"). The presence very low density. or absence of these particular diagnostic Surface horizons are usually characterized surface horizons help soil scientists by roundish granular structure that determine the place of a soil within the exhibits a hierarchy in which relatively Soil classification system. The epipedon large macroaggregates (0.25 to 5 mm in includes the upper part of the soil diameter) are composed of smaller darkened by organic matter and/or the microaggregates (2 to 250 µm). The latter, upper eluvial horizons. It may also include in turn, are composed of tiny packets of the B horizon if the eluvial horizons are clay and organic matter only a few µm in sufficiently darkened by organic matter. size. You may easily demonstrate the All soils must have one epipedon. Eight existence of this hierarchy of aggregation epipedons are recognized, but only five by selecting a few of the largest occur naturally over wide areas. The five aggregates in a soil and gently crumbling naturally-occurring epipedons are called them into many smaller-sized pieces. You the mollic epipedon, umbric epipedon, will find that even the smallest specks of ochric epipedon, melanic epipedon, and soil usually are not individual particles but histic epipedon. can be rubbed into a smear of still smaller The mollic epipedon is a mineral surface particles of silt, clay, and humus. At each horizon that is distinguished by its dark level in the hierarchy of aggregates, color, an associated accumulation of different factors are responsible for organic matter (>0.6% throughout), for its binding together the subunits. thickness (>25 cm), and for its softness Diagnostic subsurface horizons even when dry. The mollic epipedon also Many subsurface diagnostic horizons are has a high base saturation (>50%). The used to characterize different soils in soil umbric epipedon has the same general taxonomy. Each diagnostic horizon characteristics as the mollic epipedon provides a characteristic that helps place a except it has a low base saturation soil in its proper class in the system. A soil (<50%). The umbric epipedon commonly may or may not have a subsurface develops in areas with slightly higher diagnostic horizon. There are rainfall than mollic epipedon, and forms approximately 20 subsurface diagnostic where the parent material has lower horizons, of which include the argillic content of calcium and magnesium, The horizon, natric horizon, kandic horizon, ochric epipedon is a mineral horizon that oxic horizon, spodic horizon, and albic is either too thin, too light in color, or too horizon. The argillic horizon (designated low in organic matter to be considered a as Bt) is characterized by a thick mollic or umbric horizon. The melanic accumulation of silicate clays that have epipedon is a mineral horizon that is very moved downward from the upper dark in color due to its high organic horizons or have formed in place. The matter content (>0.6%). It is characteristic argillic horizon often contains clays with of soils high in such minerals as allophane, shiny coatings termed "argillans" or "clay developed from volcanic ash. Melanic skins", and accompanied by less than 15% epipedon is extremely fluffly for a mineral exchangeable sodium on the colloidal soil. Histic epipedon is a 20 to 60 cm thick complex. The natric horizon (designated layer of organic soil materials (>20%) as Btn) similarly has silicate clay overlying a mineral soil. It is formed in wet accumulation with clay skins, but the clays

97 Kheyrodin and Kheyrodin, World J. Biol. Med. Science Volume 2 (4), 89-99, 2015 are accompanied by more than 15% to get well-versed with different horizons. exchangeable sodium on the colloidal The oxic horizon (designated as Bo) is a complex and by columnar or prismatic soil very weathered horizon with a high structural units. The natric horizon is most content of Fe/Al oxides and low-activity commonly found in arid and semiarid silicate clays. It is generally physically regions. The kandic horizon (designated as stable, crumbly, and not very sticky, Bt, Bts), sometimes referred to as "old" despite its high clay content. It is found clays, has an accumulation of Fe and Al most commonly in humid tropical and oxides as well as low-activity silicate clays subtropical regions. The spodic horizon (e.g., kaolinite), but clay skins need not be (designated as Bh, Bhs) is an illuvial and evident. The clays are low in activity as dark-colored horizon that is characterized shown by their low cation-holding by the accumulation of colloidal organic capacities. matter and aluminum oxide with or CONCLUSION without iron oxide. It is commonly found Soil is made of a number of distinct, in highly leached forest soils of cool humid horizontal layers placed one above the climates, typically on sandy-textured other, which are known as soil horizons. parent materials. The albic horizon Most soils conform to a general pattern (designated as E) is an eluvial and light- consisting of six horizons. A vertical cross colored horizon that is characterized by a section of the soil, which is known as the low content of clay and Fe/Al oxides. soil profile, is your best bet if you intend

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Figure 5. Show variation of soils horizon.

ACKNOWLEDGEMENTS Wilkinson, M.T. and G.S. Humphreys Research conducted and supported by (2005). Exploring pedogenesis via nuclide- Semnan University for help with the based soil production rates and OSL-based manuscript. bioturbation rates. Australian Journal of Soil Research, v. 43, pp. 767-779. REFERENCES McDonald, R. C. (1990). Australian Soil and Land Survey Field Handbook, 2nd Ed. Isbell Raymond, (2002). The Australian Melbourne: Inkata Press. Soil-Net section Soil Classification, Revised Edition. on soil horizons. Australian Soil and Land Survey Food and Agricultural Organization of the Handbooks Series 4, CSIRO Publishing, United Nations (1998). Online version ISBN92-5-104141-5 "Appendix 1: Soil Johnson, D.L., J.E.J. Domier, and D.N. horizon designations". Retrieved 2008-02- Johnson (2005). Reflections on the nature 02. of soil and its biomantle. Annals, "Soil Horizon letter designations" (2008). Association of American Geographers, v. Encyclopedia Britannica Online. 95 (1), pp. 11-31. Encyclopedia Britannica. Retrieved 2008- 02-02.

Corresponding author: Dr. Hamid Kheyrodin, Faculty of Desert Science, Semnan University, Iran Email: [email protected]

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