Chapter 4

Soils of the Tropics

Fig. 4.1 Soil order map of the tropics. Adapted from NRCS (2005)

This chapter describes the main soils of the tropics, grouped technical. Natural systems classify soils as natural bodies in natural soil classification systems, those that describe and in their own right, while technical systems classify soils classify soils in their natural state. Classification is an according to their suitability for a specificuse,suchas important element of science, as knowledge is organized agriculture, engineering or waste disposal (Chapter 5). into categories and classes that are useful, or necessary for Naturalsoilclassification systems are divided into those communications among scientists (Cline 1949). Categories based on soil genesis (what the soil should be according to are hierarchical levels (for example states and municipalities the factors of soil formation) and those based on soil in a federal government system). Classes are the individual morphology, on actual soil properties (what the soils units that fall in a category (for example the individual are). The factors of soil formation are climate, organisms, states in a particular country and the individual municipal- parent material, topography and time (Jenny 1941). ities in a particular state). Initially, soil classification systems were all genetic; now- Many soil classification systems have been used in the adays, they are morphological but underpinned by tropics. They fall into two broad groups, natural and concepts of soil genesis. 4.1 SOIL TAXONOMY 83

The strong genetic orientation of early Russian and 4.1 Soil Taxonomy American pedologists1 led to the zonality concept. Zonal soils were those that have properties according to what Themountingevidenceagainsttheusefulnessofthedirect genesis theory indicates in terms of climate, vegetation, application of genetic theories in practical classification topography, parent material and age. The simplistic concept gradually eroded the confidence of American scientists in of a hot, humid climate, lush vegetation, old parent material their 1938 United States Department of Agriculture and old landscapes resulted in zonal tropical soils, called (USDA) system and its modifications. Soils considered lateritic soils, as discussed in Chapter 3. zonal for the United States, such as Podzols and The numerous soil classification systems are extensively Chernozems, were found in the tropics, where genetic described by Krasilnikov et al. (2009). There are five major theories dictated they should not exist. Likewise, lateritic systems that are important in the tropics, now all based on soils were found in the United States. This led to the quantitative morphological properties: the Soil Taxonomy development of a completely new soil classification system (Soil Survey Staff 1999) – see Figure 4.1; the World system, based on morphological and chemical properties Reference Base for Soil Resources (WRB) (Bridges et al. 1998, that can be quantified by specifictechniques.TheSoil Deckers et al. 1998, IUSS Working Group 2014); the Austra- Taxonomy system ended the grouping of soils according lian system (Isbell 2002); the Brazilian system (Embrapa to what they should be according to genesis theory and con- 1999); the Chinese system (Gong 1994, Li 2001 and others). centrated on what they actually are, and the system is, as the The Cuban system (Instituto de Suelos 1999, Hernández et al. subtitle states: “a basic system of soil classification 2006) is an example of a national system that was originally for making and interpreting soil surveys” (Soil Survey based on soil genesis and is now quantitative. Staff 1975). The difference between the genetic and the quantitative After about 10 years of development, the first version of morphological approaches was vividly illustrated to me Soil Taxonomy, called the Seventh Approximation (Soil during a study tour of African savanna soils in 1975. While Survey Staff 1960), was presented at the 7th International examining a soil pit in Ghana, a leading scientist of the Soil Science Congress in Madison, Wisconsin. Since then it genetic school, Professor Georges Aubert, looked up and said: has been updated by two major revisions (Soil Survey Staff “The climate is so, the vegetation is that, the topography is 1975, 1999). Keys to the Soil Taxonomy system, similar to such, the parent material is this and the age is that; therefore, those used in plant taxonomy, have been updated periodic- voilà, the soil is classified as that.” He barely looked at the soil ally (Soil Survey Staff 1998, 2014) and made available in the profile or at data sheets with physical and chemical analysis. USDA Natural Resource Conservation Service (NRCS) website A soil scientist from the morphological school, Professor (www.nrcs.usda.gov). Stanley Buol, carefully examined the profile, studied the Many of the revisions of Soil Taxonomy have resulted data, went through the keys to Soil Taxonomy and ended up from the active involvement of tropical soil scientists who with a classification according to quantitative properties. participated and often led international committees deal- The two most extensively used natural classification ing with ten soil orders (for example Beinroth and Osman systems, the Soil Taxonomy and the WRB systems, are 1981, Moormann 1985, Buol and Eswaran 1988). The imple- described in the following sections. The WRB, being a United mentation of these changes scaled-up Soil Taxonomy from Nations sponsored system, has superseded national systems a classification system for the United States into a of European countries that have been commonly used in the global one. tropics, such as the French and Belgian systems. The Cuban Soil Taxonomy is a hierarchical system, with six categor- system is presented as an example of a national classifica- ies: order, suborder, great group, subgroup and family (Soil tion system. Survey Staff 1999). It is unique in introducing quantitative This book deals with the properties and management of definitions of soil temperature and soil moisture regimes, soils as they presently exist. The genesis of soils in the based on normal climatic conditions where the soil is tropics (how soils are formed) is extremely interesting located, and on soil water availability. A normal year is a but will not be discussed in detail in this volume. This topic year that has plus or minus one standard deviation from the is worthy of an entire book of its own. Readers interested in long-term (30-year) mean (Soil Survey Staff 1999). Families tropical soil genesis and its development through time are are further subdivided into two additional categories, series referred to books by Mohr and van Baren (1954), Mohr et al. and phase, in local soil surveys. (1972), van Wambeke (1992) and Buol et al. (2011). Of specificsignificance to soils in tropical latitudes is the definition of “iso” soil-temperature-regime families where the 1 Pedologists are soil scientists who are primarily interested in the study mean soil temperature in June, July and August differs by less of soils as natural bodies. They should be distinguished from than 6 C from the mean soil temperature in December, Janu- edaphologists or agronomists, soil scientists who study soils as a fi medium for plant growth, and from soil ecologists, who study soils ary and February (Chapter 1). This criterion closely identi es all for the ecosystems services they provide. soils in tropical latitudes where the timing of planting crops 84 SOILS OF THE TROPICS

depends on local rainfall patterns rather than seasonal • Poorly drained soils: “typic” subgroups of “Aqu” temperatures. suborders. Soil moisture formative elements (syllables) are utilized The Soil Taxonomy system is based on quantitatively in the order, suborder, great group and subgroup categor- defined diagnostic horizons and attributes that are known ies. The depth at which aquic conditions occur is typically to exist in nature. Both are either easily observable and expressed in a spatial landscape arrangement of soils quantified in a soil profile or measured by standard known as a drainage catena, and is subdivided into well- laboratory techniques that have high reproducibility drained, moderately well-drained, somewhat poorly (USDA–NRCS 1996). Those observations that cannot be drained and poorly drained soils. Aquic conditions are quantified are not included (Buol et al. 2011). expressed by the following sequence of formative-element Table 4.1 provides a simplified definition of the orders, usage: suborders and main great groups found in the tropics. This • Well-drained soils: no aquic element present. table is an interpretation deemed sufficient for the purpose • Moderately well-drained soils: “aquic” used as subgroup of this book and does not include all the classification cri- identification. teria. The complete definitions are given in Soil Survey Staff • Somewhat poorly drained soils: “aeric” (aerated) sub- (1998, 1999) and more recent versions. groups of “Aqu” suborders.

Table 4.1 Simplified definition of orders, suborders and great groups of the Soil Taxonomy system, based on the Keys to Soil Taxonomy, 8th edition (Soil Survey Staff 1998), Soil Taxonomy, 2nd edition (Soil Survey Staff 1999) and Stanley Buol (personal communication, 2016). Soil orders are arranged in declining areal extent in the tropics; same for suborders within orders. For horizon designations, see Soil Survey Staff (2014). Order Suborder Great group (number of subgroups) Oxisols: Oxisols are deep, of sandy loam or finer texture (> 15% clay, belonging to the loamy or clayey types in Chapter 5), have strong granular structure and are present in all soil moisture regimes. Subsoil oxic horizons have apparent cation exchange capacity (CEC) values of 16 cmolc/kg clay or less determined at pH 7; 12 cmolc/kg clay or less as determined by the sum of bases displaced by NH4OAc plus KCl-extractable aluminum. Quartz dominates the sand fractions that contain < 10 percent weatherable minerals. Clays consist of mixtures of kaolinite, gibbsite, and aluminum and iron oxides, and silt content is low relative to most other soils. Although most Oxisols have nearly uniform texture with depth, and an A–B–C horizon sequence with kandic horizons (those with oxic properties) is allowed in soils with > 40 percent clay in the surface 18 cm. Many Oxisols are well-drained and have low fertility but some are quite naturally fertile, i.e. “eutr” great groups, and some are poorly drained, i.e. “aqu” suborder. Udox: Oxisols with udic soil moisture regime Hapludox: simple (15) Kandiudox: with kandic horizon (14)

Acrudox: very low effective CEC (ECEC) (< 1.5 cmolc/kg clay) and pHKCl > 5.0 in part of the oxic or kandic horizon (15) Eutrudox: high base saturation (> 35 percent at pH 7) in top 125 cm (15) Sombriudox: with sombric (dark subsurface) horizon (4) Ustox: Oxisols with ustic soil moisture regime Haplustox: simple (16) Kandiustox: with kandic horizon (13)

Acrustox: very low ECEC (< 1.5 cmolc/kg clay) and pHKCl > 5.0 in part of oxic or kandic horizon (15) Eutrustox: high base saturation (> 35 percent at pH 7) in top 125 cm (16) Sombriustox: with sombric (dark subsurface) horizon (4) Aquox: Wet Oxisols Haplaquox: simple (5)

Acraquox: very low ECEC (< 1.5 cmolc/kg clay) and pHKCl > 5.0 in part of oxic or kandic horizon (3) Eutraquox: high base saturation (> 35 percent at pH 7) in top 125 cm (5) Plinthaquox: plinthite forming a continuous phase within 125 cm (2) 4.1 SOIL TAXONOMY 85

Table 4.1 (cont.) Order Suborder Great group (number of subgroups) Perox: Oxisols with perudic soil moisture regime Haploperox: simple (14) Kandiperox: with kandic horizon (13)

Acroperox: very low ECEC (< 1.5 cmolc/kg clay) and pHKCl > 5.0 in part of oxic or kandic horizon (13) Eutroperox: high base saturation (> 35 percent at pH 7) in top 125 cm (16) Sombriperox: with sombric (dark subsurface) horizon (4) Torrox: Oxisols with aridic soil moisture regime Haplotorrox: simple (3)

Acrotorrox: very low ECEC (< 1.5 cmolc/kg clay) and pHKCl > 5.0 in part of oxic or kandic horizon (3) Eutrotorrox: high base saturation (> 35 percent at pH 7) in top 125 cm (3) Ultisols: Soils with an argillic or kandic horizon (clay content is at least 1.2 times that of the topsoil) and low base saturation (< 35 percent at pH 8.2 to 1.8 m depth). Most Ultisols are formed over acid igneous rocks or sediments derived from such rocks and have an A–E–Bt–C horizon sequence. Clay minerals consist mainly of kaolinite, gibbsite and aluminum-interlayer clays. Good, but less desirable physical properties than Oxisols, and relatively low native fertility. Previously known as Red Yellow Podzolic soils. Udults: Ultisols with udic soil moisture regime Hapludults: simple (13) Paleudults: thick argillic horizon (19) Rhodudults: deep red, high in iron oxides (3) Kandiudults: with thick kandic horizon (19) Kanhapludults: with kandic horizon (13) Plinthudults: with plinthite (1) Ustults: Ultisols with ustic soil moisture regime Haplustults: simple (8) Paleustults: thick argillic horizon (1) Rhodustults: deep red, high in iron oxides (3) Kandiustults: with thick kandic horizon (11) Kanhaplustults: with thin kandic horizon (12) Plinthustults: with plinthite (2) Aquults: Wet Ultisols Epiaquults: with perched water table (saturated from the top) (7) Endoaquults: saturated by groundwater from the bottom (4) Paleaquults: thick argillic horizon (9) Kandiaquults: with thick kandic horizon (9) Kanhaplaquults: with thin kandic horizon (6) Plinthaquults: with plinthite (2) Albaquults: abrupt textural change and dense argillic horizon (4) Umbraquults: dark epipedon (2) (cont.) 86 SOILS OF THE TROPICS

Table 4.1 (cont.) Order Suborder Great group (number of subgroups) Humults: Ultisols with high organic carbon (> 120 t/ha in top 100 cm) with udic or ustic soil moisture regimes. Humults key out before Udults and Ustults. Haplohumults: simple (8) Palehumults: thick argillic horizon (7) Kandihumults: with thick kandic horizon (9) Kanhaplohumults: with thin kandic horizon (8) Plinthohumults: with plinthite (1) Sombrihumults: with sombric (dark subsurface) horizon (1) Inceptisols: Soils with a cambic horizon but no other diagnostic horizons (A–B–C horizon sequence). They are present in all soil moisture regimes except aridic, and are most prevalent on sloping landscapes and relatively recent colluvial sediments. Udepts: Inceptisols with udic soil moisture regime Dystrudepts: < 60 percent base saturation at pH 7 in subsoil (21) Eutrudepts: > 60 percent base saturation at pH 7 in subsoil (20) Sulfudepts: sulfuric horizon (cat clays) (1) Durudepts: with duripan (silica-cemented) subsurface horizon (5) Ustepts: Inceptisols with ustic soil moisture regime Haplustepts: simple (23) Dystrustepts: < 60 percent base saturation at pH 7 in subsoil (8)

Calciustepts: calcic (CaCO3 accumulation) subsurface horizon (8) Durustepts: with duripan (silica-cemented) subsurface horizon (1) Aquepts: Wet Inceptisols Epiaquepts: with perched water table (saturated from the top) (7) Endoaquepts: saturated groundwater table from the bottom (9) Sulfaquepts: sulfuric horizon (cat clays) (3) Petraquepts: with plinthite or other hard horizons (4) Humaquepts: dark, humus-rich epipedon (7) Halaquepts: saline (5) Vermaquepts: presence of bioturbation (wormholes, casts, animal burrows) (2) Entisols: Soils of such slight development that only an ochric (yellowish) epipedon or simple man-made horizons have formed. Most Entisols are present on steep slopes, flood plains, or on inert sandy parent material, and have A–CorA–C–R horizon sequences. Orthents: Entisols on erosional surfaces Udorthents: with udic soil moisture regime (6) Ustorthents: with ustic soil moisture regime (14) Torriorthents: with aridic soil moisture regime (5) Fluvents: Alluvial Entisols that formed in water-deposited sediments, mainly on flood plains, fans and deltas. Profiles may contain stratified layers due to multiple sedimentation events. Udifluvents: with udic soil moisture regime (8) Ustifluvents: with ustic soil moisture regime (10) Torrifluvents: with aridic soil moisture regime (10) 4.1 SOIL TAXONOMY 87

Table 4.1 (cont.) Order Suborder Great group (number of subgroups) Aquents: Wet Entisols Epiaquepts: with perched water table (saturated from the top) (4) Endoaquents: saturated by groundwater from the bottom (7) Fluvaquents: wet alluvial Entisols (8) Psammaquents: sandy (6) Sulfaquents: with sulfuric horizon (cat clays) (4) Hydraquents: in tidal marshes, permanently water-saturated (4) Psamments: Sandy Entisols Udipsamments: with udic soil moisture regime (7) Ustipsamments: with ustic soil moisture regime (7) Torripsamments: with aridic soil moisture regime (6) Quartzipsamments: > 90 percent quartz sand (12) Alfisols: Soils with an argillic or kandic horizon with more than 35 percent base saturation at pH 8.2 to 1.8 m depth. Most Alfisols are formed over basic rocks or sediments and have A–E–Bt–C horizon sequences similar to Ultisols except for higher native fertility. Ustalfs:Alfisols with ustic soil moisture regime Haplustalfs: simple (24) Paleustalfs: thick argillic horizon (21) Rhodustalfs: deep red, high in iron oxides (4) Kandiustalfs: with thick kandic horizon (10) Kanhaplustalfs: with thin kandic horizon (6) Plinthustalfs: with plinthite (1) Natrustalfs: with natric horizon (> 15 percent sodium saturation of ECEC) (15) Durustalfs: with duripan (silica-cemented) subsurface horizon (1) Udalfs:Alfisols with udic soil moisture regime Hapludalfs: simple (25) Paleudalfs: thick argillic horizon (20) Rhodudalfs: deep red, high in iron oxides (1) Ferrudalfs: iron coats on argillic horizon (2) Kandiudalfs: with thick kandic horizon (11) Kanhapludalfs: with thin kandic horizon (5) Natrudalfs: with natric horizon (> 15 percent sodium saturation of ECEC) (4) Glossudalfs: with light-colored “tongues” in argillic horizon (12) Aqualfs: Wet Alfisols Epiaqualfs: with perched water table (saturated from the top) (13) Endoaqualfs: saturated by groundwater from the bottom (11) Glossaqualfs: with light-colored “tongues” in argillic horizon (5) Plinthaqualfs: with plinthite (1) Duraqualfs: with duripan (silica-cemented) subsurface horizon (1) Kandiaqualfs: with kandic horizon (7) Albaqualfs: abrupt textural change and dense argillic horizon (9) Natraqualfs: with natric horizon (> 15 percent sodium saturation of ECEC) (7) Vermaqualfs: bioturbation (wormholes, casts, animal burrows) (2) 88 SOILS OF THE TROPICS

Table 4.1 (cont.) Order Suborder Great group (number of subgroups) Aridisols: Soils of aridic moisture regimes with horizon differentiation (A–Bt–Bk or Bw–C horizon sequence). Mainly desert soils but also saline and alkali soils in other regions. Argids: Aridisols with argillic horizon Haplargids: simple (14) Paleargids: thick argillic horizon (11) Petroargids: with duripan, petrocalcic or petrogypsic horizons (6)

Calciargids: with calcic horizon (CaCO3 accumulation) (13) Natrargids: with natric horizon (> 15 percent sodium saturation of ECEC) (14)

Gypsiargids: with gypsic horizon (CaSO4 accumulation) (5) Cambids: Aridisols with cambic horizon Haplocambids: simple (21) Aquicambids: irrigated or naturally saturated layer within 1 m for more than 1 month each year (7) Petrocambids: with petrocalcic or petrogypsic horizons (4) Anthracambids: with anthropic horizon (1)

Calcids: Aridisols with calcic horizon (CaCO3 accumulation) Haplocalcids: simple (6) Petrocalcids: with petrocalcic horizon (8)

Gypsids: Aridisols with gypsic horizon (CaSO4 accumulation) Haplogypsids: simple (1) Argigypsids: with argillic horizon (8) Calcigypsids: with calcic horizon (6) Natrigypsids: with natric horizon (7) Petrogypsids: with petrogypsic or petrocalcic horizon (5) Durids: Aridisols with duripans (silica-cemented) Haplodurids: simple (7) Argidurids: with argillic horizon (6) Natridurids: with natric horizon (8) Salids: Aridisols with salic (saline) horizon Haplosalids: simple (5) Aquisalids: wet (3) Vertisols: Heavy, cracking clayey soils with more than 30 percent clay in all horizons, > 50 percent 2:1 minerals in clay fraction and cracks that open and close periodically with changes in soil moisture. Common horizon sequence is A–Bss–C. Some have gilgai microrelief and slickensides. Usterts: Vertisols with ustic soil moisture regime Haplusterts: simple (16)

Dystrusterts: pH CaCl2 < 4.5 (8)

Calciusterts: with calcic horizon (CaCO3 accumulation) (8) Salusterts: with salic horizon (8)

Gypsiusterts: with gypsic horizon (CaSO4 accumulation) (9) Table 4.1 (cont.) Order Suborder Great group (number of subgroups) Torrerts: Vertisols with aridic soil moisture regime Haplotorrerts: simple (6)

Calcitorrerts: with calcic horizon (CaCO3 accumulation) (5) Salitorrerts: with salic horizon (5)

Gypsitorrerts: with gypsic horizon (CaSO4 accumulation) (2) Aquerts: Wet Vertisols Epiaquerts: with perched water table (saturated from the top) (9) Endoaquerts: saturated by groundwater from the bottom (9)

Dystraquerts: pHCaCl2 < 4.5 (8) Salaquerts: with salic horizon (5) Natraquerts: with natric horizon (1)

Calciaquerts: with calcic horizon (CaCO3 accumulation) (2) Duraquerts: with duripan (silica-cemented) subsurface horizon (5) Uderts: Vertisols with udic soil moisture regime Hapluderts: simple (7)

Dystruderts: pHCaCl2 < 4.5 (6) Andisols: Soils developing in volcanic materials (volcanic ash, pumice, cinders and lava) with andic properties (colloidal fraction dominated by allophane, imogolite, ferrihydrite or aluminum–humus complexes; low bulk density and high phosphorus retention). Usually very fertile except for phosphorus. Vitrands: Andisols with high content of coarse volcanic glass Ustivitrands: with ustic soil moisture regime (6) Udivitrands: with udic soil moisture regime (7) Udands: Andisols with udic soil moisture regime Hapludands: simple (20) Hydrudands: more than 100 percent water retention at –1500 kPa (8) Melanudands: melanic epipedon (dark, high organic carbon) (15) Fulvudands: melanic horizon (11) Durudands: with cemented horizon (5) Placudands: with placic horizon (iron-cemented pan) (5) Ustands: Andisols with ustic soil moisture regime Haplustands: simple (13) Durustands: with cemented horizon (4) Aquands: Wet Andisols Epiaquands: with perched water table (saturated from the top) (6) Endoaquands: saturated by a groundwater table from the bottom (7) Melanaquands: melanic epipedon (dark, high organic carbon) (7) Vitraquands: high in volcanic glass (5) Duraquands: with cemented horizon (4) Placaquands: with placic horizon (iron-cemented pan) (6) Torrands: Andisols with aridic soil moisture regime Haplotorrands: simple (4) Vitritorrands: high in volcanic glass (5) Duritorrands: with petrocalcic horizon (3) (cont.) 90 SOILS OF THE TROPICS

Table 4.1 (cont.) Order Suborder Great group (number of subgroups) Mollisols: Soils with > 50 percent base saturation at pH 7 to a depth of 1.8 m and a mollic epipedon (> 0.6 percent organic C, not hard when dry). Horizon sequences include A–Bw Bt, or Bk–C. Very fertile soils typical of the best agricultural lands in the temperate region. Udolls: Mollisols with udic soil moisture regime Hapludolls: simple (16) Vermudolls: bioturbation (wormholes, casts, animal burrows) (3) Argiudolls: with argillic horizon (19) Paleudolls: with thick argillic horizon (7)

Calciudolls: with calcic horizon (subsoil CaCO3 accumulation) (5) Natrudolls: with natric horizon (8) Ustolls: Mollisols with ustic soil moisture regime Haplustolls: simple (28) Vermustolls: bioturbation (wormholes, casts, animal burrows) (5) Argiustolls: with argillic horizon (19) Paleustolls: with thick argillic horizon (12)

Calciustolls: with calcic horizon (subsoil CaCO3 accumulation) (18) Natrustolls: with natric horizon (12) Durustolls: with duripan horizon (6) Aquolls: Wet Mollisols Epiaquolls: with perched water table (saturated from the top) (10) Endoaquolls: saturated by groundwater from the bottom (11) Argiaquolls: with argillic horizon (5) Natraquolls: with natric horizon (2)

Calciaquolls: with calcic horizon (subsoil CaCO3 accumulation) (3) Duraquolls: with duripan (silica-cemented) subsurface horizon (4) Rendolls: < 50 cm deep over calcareous materials (limestone, talc, shells) Haprendolls: simple (5) Histosols: Organic soils (> 12 percent organic C to > 60 cm). Low bulk density, low fertility, wet (saturated > 30 days each year or artificially drained), subsides when drained. Saprists: Highly decomposed organic material (< 50 percent fiber) Haplosaprists: simple (8) Sulfosaprists: sulfuric horizon within top 50 cm (1) Sulfisaprists: sulfuric horizon within top 100 cm (2) Spodosols: Soils with a spodic horizon (of iron and/or organic matter accumulation), usually developed on sandy materials. Common profile sequence A–E–Bh or Bs–C. Equivalent to Podzols in all other classification systems. Typical soils of northern temperate and boreal regions. Present in the tropics. Humods: > 6 percent organic C in spodic horizon Haplohumods: simple (4) Placohumods: with placic horizon (iron-cemented pan) (2) Durihumods: with cemented subsurface horizon (2) 4.1 SOIL TAXONOMY 91

Table 4.1 (cont.) Order Suborder Great group (number of subgroups) Orthods: Simple Spodosols Haplorthods: simple (14) Alorthods: spodic horizon low in iron (9) Placorthods: with placic horizon (iron-cemented pan) (1) Durorthods: with cemented subsurface horizon (2) Aquods: Wet Spodosols Epiaquods: with perched water table (saturated from the top) (7) Endoaquods: saturated by groundwater (6) Alaquods: spodic horizon low in iron (12) Placaquods: with placic horizon (iron-cemented pan) (2) Duraquods: with cemented subsurface horizon (3) Gelisols: Soils with permafrost (likely around some tropical glaciers)

Table 4.2 The classification of a soil as fine, kaolinitic, isohyperthermic, Rhodic Acrudox contains the following information:

‘ox” = Oxisol order: a soil with low activity, a clay oxic horizon (apparent CEC at pH 7 < 16 cmolc/kg clay), < 10 percent weatherable minerals in coarse silt and sand fractions, and more than 30 cm thick, sandy loam (> 15 percent clay) or finer texture “ud” = Udox suborder: the soil has a udic soil moisture regime (the subsoil is not dry for more than 90 cumulative days during normal years)

“Acr” = Acrudox great group: the soil has very low ECEC (< 1.5 cmolc/kg clay) and pHKCl > 5.0 “Rhodic” = Rhodic subgroup: the soil has a deep red color (2.5 YR or redder on the Munsell Soil Color Chart), denoting high iron oxide content   “isohyperthermic” = soil temperature regime (> 22 C mean annual with < 6 C seasonal variation), i.e. hot tropics “kaolinitic” = the dominant clay mineral “fine” =35–60 percent clay

A compete classification of a soil through the family orders. Each syllable is laden with quantitative information, category is demonstrated in Table 4.2. Diagnostic horizons as shown in Tables 4.1 and 4.2. and attributes appear as syllables – formative elements – in Although cumbersome at the beginning, Soil Taxonomy each soil name. The formative element of the highest terminology packs quite a bit of quantitative information category, the soil order, is the ending of a great group proper into the name by which a soil is classified. The inventors of noun. For example, in Table 4.2 “ox” for Oxisols (all other Soil Taxonomy actually hired a linguist (Heller 1963) to help order elements are written as plural, i.e. “alfs” for Alfisols, them design the system, using mostly Latin and Greek roots. “ults” for Ultisols, etc., indicating a range of soil properties The use of Soil Taxonomy has made a major contribution are included in that order. “Ox” is used instead of “oxs” to the understanding of soils in tropical latitudes because of the pronunciation). by essentially eliminating the genetic bias. All tropical and The order formative element is preceded by the suborder temperate soils are now separated at the lowest categorical formative element, in this example “ud,” which is preceded level, the soil family, on the basis of temperature regimes. by the capitalized great group formative element, “Acr”. The Thereby, all the chemical, physical and morphological soil capitalized subgroup name precedes the great group name properties identified by the higher categories can be com- as an adjective. The family level consists of three adjectives pared. For example, many soils of the Amazon Basin and describing texture, mineralogy and temperature regime; the southeastern United States are grouped in the same they precede the subgroup and great group words. Add- categories down to the family level, where they are sep- itional families, not in this example, are used in some arated by their designated soil temperature regimes: 92 SOILS OF THE TROPICS

“isohyperthermic” (Amazon) versus “thermic” and “hyper- third and fourth categorical levels. The revised legend thermic” (southeastern United States). dropped soil temperature and moisture regimes because of the difficulty in obtaining reliable data from many parts of the world. The World Soil Map was digitized in 1995 and is 4.2 World Reference Base for Soil available online (FAO and UNESCO 1995). The FAO legend was transformed into the WRB (Bridges Resources et al. 1998, Deckers et al. 1998), a soil classification system with more rigorous quantification. Driessen at al. (2001) provide In 1960, the Food and Agriculture Organization of the extensive material. The WRB has two categories called refer- United Nations (FAO) embarked on the development of a ence groups and subgroups. They correspond closely to the soil map of the world, which was eventually published at order categories of Soil Taxonomy but inadequately identify the scale of 1:5 million for various continents between 1971 many suborders and great groups identified by soil moisture and 1981 (FAO and UNESCO 1971, 1974, 1975, 1977a, regimes in the Soil Taxonomy system. Most soil characteris- 1977b, 1978, 1979). A legend was developed by Dudal tics identified in the soil families of Soil Taxonomy are not (1968, 1970) to correlate all units of the various soil maps identified by the WRB. Maps of Africa and Latin America and in the world and to obtain a worldwide inventory of soil the Caribbean have been published as beautiful atlases by the resources with a common legend. The definitions were European Commission at the scale of 1:3 000 000 (Gardi et al. based on diagnostic horizons and some quantifiable criteria, 2013, Jones et al. 2013). The map in the African Atlas consists similar but often less rigorous than those of the Soil Tax- of 30 reference soil groups and 142 subgroups (Jones et al. onomy system. The nomenclature was drawn from a 2013). They are described in Table 4.3, which includes an number of national systems in a successful exercise of approximate translation into Soil Taxonomy. international diplomacy led by Rudolf Dudal of the FAO. The World Soil Map was a major achievement as no information was available for many countries before. How- 4.3 A National Soil Classification ever, its quality was highly variable, particularly in remote areas of the tropics. A revised legend was published in System: Cuba 1988 with two categories (FAO and UNESCO 1988). It con- tains 28 major soil groupings, which are subdivided into The Cuban soil classification system is presented as an 153 soil units, closely related to diagnostic criteria and hori- example of a well-developed national one. I could have chosen zons of Soil Taxonomy, but with different names. Guidelines the Australian, Brazilian or South African systems, which were provided for the design of subunits and phases as the have gone through a similar development, but I chose Cuba

Table 4.3 Main reference soil groups of the WRB with main characteristics and approximate Soil Taxonomy equivalents. Assembled from Bridges et al. (1998), with my updates. The key in IUSS Working Group on WRB (2014) and subsequent versions must be used for complete classification. Reference Main characteristics Approximate Soil Worldwide areal soil groups Taxonomy extent (million equivalents hectares)

Acrisols Soils with a horizon of low activity clays (< 24 cmolc/kg of Most Ultisols, kandic great 1000 clay determined at pH 7) and base saturation of < 50 groups percent, acid and nutrient poor Alisols Soils with subsurface accumulation of high-activity clays rich Non-kandic Ultisols 100 in exchangeable aluminum Andosols Young soils in volcanic deposits Andisols 110 Arenosols Deep, sandy soils featuring weak or no soil development. Psamments, sandy 900 Mainly in the Sahel, Kalahari and Australia Haplustepts Calcisols Soils with accumulation of secondary calcium carbonates. Calcids, and calcic 800 Mainly in the Sahara, middle and Central Asia, east and subgroups of Alfisols and western United States, coastal southwest Africa Aridisols Cambisols Weakly to moderately developed soils Ustepts, Udepts 1500 4.3 A NATIONAL SOIL CLASSIFICATION SYSTEM: CUBA 93

Table 4.3 (cont.) Reference Main characteristics Approximate Soil Worldwide areal soil groups Taxonomy extent (million equivalents hectares) Chernozems Soils with a thick, black topsoil, rich in organic matter with a Equivalent to Mollisols, 230 calcareous subsoil mainly Udolls in the temperate zone Cryosols Soils with permafrost within 1 m depth Gelisols 1770 Durisols Soils with accumulation of secondary silica Durids ? Ferralsols Deep, strongly weathered soils with chemically poor, but Oxisols (except Aquox) 750 physically stable subsoil Fluvisols Young soils in alluvial deposits Fluvents 350 Gleysols Soils temporarily or permanently wet near the surface Almost all “Aqu” 720 suborders and subgroups Gypsisols Soils with accumulation of secondary gypsum Gypsids, Gypsiargids 90 Histosols Soils composed of organic materials Histosols 275 Kastanozems Soils with a thick, dark brown topsoil, rich in organic matter Ustolls, Cryolls 465 with a calcareous or gypsum-rich subsoil Leptosols Very shallow soils over hard rock or gravel Some Orthents, Rendolls, 1655 lithic subgroups Lixisols Soils with subsurface accumulation of low-activity clays and Kandi and Kanhap great 435 high base saturation. Common in Mexico, Cuba, northeast groups of Alfisols Brazil, the Sahel, Mozambique, South Africa, India Luvisols Soils with subsurface accumulation of high-activity clays. Alfisols (in part) 650 Mainly in temperate zone Nitisols Deep, dark red, clayey soils with an argillic horizon. Very Rhodic or Eutric Ultisols 200 fertile. Brazil, Cuba, East Africa. and Alfisols Pale and Kandi great groups of Alfisols and Ultisols, and some Oxisols Phaeozems Soils with a thick, dark topsoil, rich in organic matter and Udolls, Aquolls 190 evidence of removal of carbonates. Pampas of Argentina, midwestern United States Planosols Soils with bleached, temporarily water-saturated topsoil on Albaquults, Albaqualfs 130 a slowly permeable subsoil. Central South America, northeast Brazil, eastern Australia Plinthosols Wet soils with an irreversibly hardening mixture of iron, Plinthaquox, 60 clay and quartz in subsoil Plinthaquults, Plinthaqualfs Podzols Acid soils with subsoil with illuvial iron–aluminum–organic Spodosols 485 compounds Regosols Soils with very limited development Some Orthents 258 Solonchaks Strongly saline soils Salids, Halaquepts 260 – 340 Solonetz Soils with subsurface clay accumulation rich in sodium Natrustalfs, Natrustolls Vertisols Heavy, cracking clayey soils Vertisols (all) 149 94 SOILS OF THE TROPICS

Table 4.4 Simplified definition of soil groupings (orders), and genetic type (suborders) of the Cuban soil classification system (Instituto de Suelos 1999) with correlations to the Soil Taxonomy and WRB systems provided by Dr. Alberto Hernández, with thanks. The names of the groupings and genetic type are kept in the Spanish originals. Grouping Genetic type Hectares Soil Taxonomy WRB Suelos Alíticos (Allitic soils) Soils with allitic (argillic) B horizon with > 50 percent aluminum saturation in this horizon. Alítico de Baja Actividad 193 100 Rhodic Rhodic, Alumic Arcillosa, Rojo Kandiustalf Acrisol Alítico de Baja Actividad 89 000 Rhodic–Xanthic Rhodic-Xanthic, Arcillosa, Rojo Amarillento Rhodustalf Alumic Acrisol Alítico de Baja Actividad 38 000 Xanthic Xanthic, Alumic Arcillosa Amarillento Rhodustalf Acrisol Alítico de Alta Actividad 320 000 Typic Rhodudalf Rhodic–Xanthic, Arcillosa Rojo Amarillento Alumic Alisol Alítico de Alta Actividad 31 600 Xanthic Xanthic, Alumic Arcillosa Rhodudalf Alisol Area total 671 700 Alfisols Acrisols

Suelos Ferríticos (Ferritic soils) Soils with ferric B horizon with > 50 percent Fe2O3, low in silica and bases. CEC < 12 cmolc/ kg clay. Derived from ultrabasic rocks (mainly serpentinite). Ferrítico Rojo Oscuro 187 500 Rhodic Eutrudox Ferritic, Rhodic, Eutric Ferralsol Ferrítico Amarillento 3 300 Xanthic Eutrudox Ferritic, Xanthic, Eutric Ferralsol Area total 190 800 Oxisols Ferralsols

Suelos Ferralíticos (Ferralitic Soils with ferralitic B horizon with 1:1 clays, < 60 percent Fe2O3, and SiO2/Al2O3 < 2.3 in clay soils) fraction. CEC < 12 cmolc/kg clay. Derived from hard calcareous rocks. Ferralítico Rojo 553 900 Rhodic Eutrustox Ferralic, Rhodic, Eutric, Clayey Nitisol Ferralítico Rojo Lixiviado 71 600 Typic Rhodustalf Ferralic, Rhodic, Lixic, Eutric Nitisol Ferralítico Amarillento 55 200 Xanthic Ferralic, Xanthic, Lixiviado Rhodustalf Lixic, Eutric Nitisol Area total 680 700 Oxisols, Nitisols Alfisols Suelos Ferrálicos (Ferralic soils) Soils with ferralic B horizon with 2:1 and 1:1 clay minerals, 10–40 percent 2:1 minerals, SiO2/Al2O3 < 2.3 in clay fraction. CEC: 20–30 cmolc/kg clay. Incomplete ferralitization processes. Derived from hard calcareous rocks. Ferrálico Rojo 140 000 Oxic–Rhodic Ferralic, Rhodic, Haplustept Eutric Cambisol Ferrálico Amarillento 25 000 Oxic–Xanthic Ferralic, Xanthic, Haplustept Eutric Cambisol Area total 165 000 Inceptisols Cambisols

Suelos Fersialíticos Soils with fersialic B horizon with < 50 percent 2:1 minerals and > 3 percent free Fe in the clay (Fersialitic soils) fraction. CEC: > 20 cmolc/kg clay. Incomplete ferralitization processes. Derived from basalt, serpentinite and other materials high in Fe. Fersialítico Pardo Rojizo 230 000 Oxic Haplustept Chromic, Eutric Cambisol Fersialítico Rojo 65 200 Oxic Haplustept Rhodic, Eutric Cambisol Area total 295 200 Inceptisols Cambisols 4.3 A NATIONAL SOIL CLASSIFICATION SYSTEM: CUBA 95

Table 4.4 (cont.) Grouping Genetic type Hectares Soil Taxonomy WRB

Suelos Pardos Sialíticos Soils with sialitic B horizon with smectitic mineralogy. CEC: > 30 cmolc/kg clay and free Fe2O5 (Brown sialitic soils) < 3 percent. No argillic horizon. Pardo (brown) 3 454 800 Typic Haplustept Eutric Cambisol Pardo Grisáceo 152 000 Typic Dystrustept Dystric Cambisol Area total 3 606 800 Inceptisols Cambisols Suelos Húmicos Sialíticos A horizon high in organic matter, smectitic mineralogy. Developed from limestone. (Humic sialitic soils) Húmico Calcimórfico 230 000 Typic Haplustoll Calcaric, Clayey Phaeozem Rendzina 344 400 Lithic Haplustoll Rendzic, Calcaric Phaeozem Area total 574 400 Mollisols Phaeozems Suelos Vertisoles (Vertisols) Soils defined as having a vertic horizon in other classification systems. Vertisol Pélico 820 000 Typic Hasplustert Pellic Vertisol Vertisol Crómico 86 000 Chromic Chromic Vertisol Haplustert Area total 906 000 Vertisols Vertisols Suelos Halomórficos (High Saline or sodic soils as defined in other systems. sodium soils) Salino 9 600 Salic Epiaquent Gleyic, clayey Solonchak Sódico 12 000 Typic Halaquept Stagnic Solonetz Area total 21 600 Entisols, Solonchaks, Inceptisols Solonetz Suelos Hidromórficos Poorly drained soils. (Hydromorphic soils) Gley Vértico 680 000 Ustic Endoaquert Eutric, Vertic Gleysol Gley Húmico 640 000 Typic Eutric, clayey, humic Endoaquept Gleysol Gley Nodular Ferruginoso 625 400 Typic Plinthaqualf Pisoplinthic Gleysol Típico Gley Nodular Ferruginoso 392 600 Petroferric Petroferric Gleysol Petroférrico Plinthaqualf Area total 2 338 000 Alfisols Gleysols Fluvisolos (Fluvisols) Alluvial soils as defined in the WRB. Fluvisoles 37 500 Typic Ustifluvent Eutric Fluvisol Area total 37 500 Entisols Fluvisols Histosoles (Histosols) Organic soils as defined in other classification systems. Histosol Fíbrico 520 000 Typic Haplofibrist Fibric Histosol Histosol Nésico 352 000 Typic Mesic Histosol Haplohemist Histosol Sáprico 34 200 Typic Sapric Histosol Haplosaprist Area total 906 200 Histosols Histosols (cont.) 96 SOILS OF THE TROPICS

Table 4.4 (cont.) Grouping Genetic type Hectares Soil Taxonomy WRB Suelos Poco Evolucionados Soils with slight development, either sands or shallow soils. (Soils with slight development) Arenosol 103 000 Typic Eutric Arenosol Quartzipsamment Lithosol 75 300 Lithic Ustorthent Lithic, skeletic Lithosol Protorrendzina 180 000 Lithic Ustorthent Lithic, rendzic Leptosol Area total 358 300 Entisols Lithosols, Leptosols Antrosoles (Anthrosols) Human-made anthropic horizon, the first one due to secondary salinity and the second made for urban organic farming. Salino Antrópico 200 000 No equivalent Salic Anthrosol Recultivado Antrópico 1 000 No equivalent Hortic Anthrosol Area total 201 000 Entisols Anthrosols Cuba TOTAL SOILS 10 953 200 ROCKS 80 800 Total area 110 346 000

since it is in my DNA and Cuban soils are in many ways Table 4.5 Extent of Cuban soils in terms of Soil atypical of larger countries of the tropics. As in the other Taxonomy orders (based on Table 4.4) and countries, classification started with a morphological compared with the tropics as a whole approach, naming soil series (Bennett and Allison 1928), (based on Table 4.6) then going through a genetic phase, and finally incorpor- Soil Cuba Tropics ating diagnostic horizons, based on quantitative criteria and adapting them to the national conditions. The history of orders Cuban soil classification has been published by Hernández Area Area (%) Area (%) et al. (2006). (thousand The 1999 system uses Soil Taxonomy approaches, includ- fi hectares) ing the quantitative de nitions of diagnostic horizons: fer- ritic, ferralitic, ferralic, fersialitic and sialitic. These Inceptisols 4718.8 43.2 15.7 diagnostic horizons represent a gradient of decreasing iron Alfisols 1816.5 16.6 12.4 oxide content and increasing smectite content in order to untangle the wide variety of red soils. The Cuban system Vertisols 1586.0 14.5 3.9 used diagnostic horizons from Soil Taxonomy (vertic, salic, Histosols 906.2 8.2 0.8 natric, anthropic), sometimes with a different name (e.g. Oxisols 744.7 6.8 24.8 alítico = argillic). It followed the WRB’s grouping of all poorly drained soils at the highest categorical level, and kept Entisols 606.4 5.5 15.6 Vertisols and Histosols as in Soil Taxonomy and the WRB. Mollisols 574.4 5.2 0.9 Given its use of quantitative criteria, the Cuban system is Ultisols 0 0 19.6 readily translatable into Soil Taxonomy. Table 4.5 shows Aridisols 0 0 4.8 how atypical Cuba is in relation to the tropics as a whole. Because it is basically an uplifted limestone plateau, with Andisols 0 0 1.2 three relatively small mountain ranges, Cuba has a low Spodosols 0 0 0.2 percentage of the two most extensive soils in the tropics Gelisols 0 0 0.0 (Oxisols and no Ultisols), while having a greater percentage of Inceptisols, Alfisols, Vertisols, Histosols and Mollisols Total 10 953.0 100 100 than the tropics as a whole. 4.4 MAIN SOILS OF THE TROPICS 97

4.4 Main Soils of the Tropics not formed in situ, but have developed on transported, pre- weathered sediments. Some have stone lines (Ruhe 1959, Ruhe and Cady 1954), indicating different depositions. The geographic distribution of soils in tropical, temperate Most Oxisols are present on old geomorphic surfaces and boreal regions is shown in Table 4.6 and at the order such as in the Guyana and Brazilian shields of South Amer- level in a map of the tropics at the beginning of this chapter ica and similar surfaces in the interior of Africa, but are (Fig. 4.1). also found on recent alluvial surfaces if the material was Soils classified in all 12 soil orders of Soil Taxonomy exist pre-weathered before deposition (Buol et al. 2011). Oxisols in the tropics. I estimate that 42 suborders, 198 great groups also occur in areas of the Caribbean, coastal West Africa, and 1552 subgroups identify soils in the tropics. Once again Southeast Asia, Australia and the Pacific islands. Many trop- this shows the great diversity of soils in tropical latitudes. ical countries have no significant extent of Oxisols. Since An overview of the main soil orders and suborders follows. Oxisols are composed of minerals resistant to alteration They are arranged from the most to the least extensive in by present factors of soil formation they are found under the tropics. a wide range of tropical vegetation, from rainforests The following describe the soil orders as they occur in to deserts, but their greatest extent is in the humid and the tropics, including their areal extent in the tropics subhumid tropics. In terms of soil fertility there is one big exception to the 4.4.1 Oxisols (962 million hectares) above generalizations: the eutric great groups (Eutrustox, Oxisols, the stereotype of tropical soils, are the most exten- Eutrudox, Eutrotorrox, Eutroperox, Eutraquox) and sub- sive soil order, covering approximately 25 percent of the groups developed from basalt and sediment derived from tropical land area. They are soils with oxic horizons other basic rocks (Moura and Buol 1972), because they have

(< 16 cmolc/kg clay at pH 7 and < 12 cmolc/kg ECEC of clay); high base saturation and no aluminum toxicity. This makes and with less than 10 percent weatherable minerals in sand them among the best soils of the tropics, as they combine great fractions, consisting of mixtures of kaolinite, iron and alu- physical and chemical properties. Moura and Buol (1976) attri- minum oxides and quartz. They are variable-charge soils, bute the retention of bases to the micropores within the strong also known as low-activity clay soils (Chapter 8). Oxisols are granular macroaggregates that retain water at tensions usually deep, well-drained, red or yellowish soils, acid with greater than permanent wilting, and thus do not have liquid very low fertility, but contrary to conventional wisdom they exchange with leaching water and retain basic cations derived are not low in soil organic carbon (SOC) content (Sanchez from basaltic or serpentinitic parent materials. and Buol 1975, Eswaran et al. 1993). Low-activity silicate These soils, known as Terra Roxa Legítima and Latosol Roxo clays and iron and aluminum oxides dominate their chem- in Brazil, and Red Ferralitic soils Matanzas clay (Rhodic istry, giving them pH-dependent charge. Oxisols, by defin- Eutrustox) in Cuba are considered the best soils of those ition, cannot have sandy texture (< 15 percent clay); they are countries. The WRB distinguishes the conventional Oxisols either clayey or loamy and therefore have considerable (Ferralsols) from these highly fertile ones that fall under the phosphorus sorption capacity (Chapter 14). Their physical Nitisol reference group (Deckers et al. 1998) – a useful dis- properties are generally excellent due to a strong granular tinction. About half of the red soils previously known as structure, although plant available water-holding capacity is Latosols in Brazil classify as Oxisols, the rest as Ultisols, often low, most likely due to a paucity of silt. They have Alfisols and Inceptisols. In the Brazilian classification fairly uniform properties with depth. system, Latossolos are Oxisols because they are defined as Oxisols form in three kinds of conditions, paraphrased having an oxic horizon (Embrapa 1999). from Buol and Eswaran (2000). First, on stable slopes, Oxi- The most extensive areas of Oxisols are nearly level to sols are formed from the slow weathering of acid igneous rolling landscapes that are ideally suited to highly mechanized rocks over long geologic time periods, for example, in cen- commercial farming practices. With the development of soil tral Puerto Rico (Beinroth 1982); they have a C horizon management technology in the late-twentieth century, large present and no evidence of stone lines or geological discon- areas of the poorest Oxisols in Brazil (in the Cerrado) are now tinuities. Second, where easily weatherable rocks like basalt productive grain-growing areas (Chapter 3). The major factor or serpentine exist, Oxisols with low aluminum saturation limiting similar agricultural production in other Oxisol areas is are produced rather rapidly, such as those found in the the limited availability of the infrastructure necessary for com- volcanic areas of East Africa and Hawaii as well as those mercial agriculture – long ignored because of limited popula- found over serpentine, like the Nipe clay of Cuba (Bennett tions in areas of these naturally infertile Oxisols. and Alison 1928, Instituto de Suelos 1999). Oxisols are divided into five suborders, 22 great groups and Third, and most importantly, Oxisols occur where geo- 211 subgroups, all of which occur in the tropics (Table 4.1). logically old, mid- to late-Tertiary or even Cretaceous There is a relatively small extent of Oxisols in the temperate and early Cenozoic materials have been subjected not only region, approximately 20 million hectares (Table 4.6). to long periods of weathering but have also been transported Books by Van Wambeke (1974), Boyer (1982) and Thomas and redeposited (Orme 2007). The majority of Oxisols were and Ayarza (1999) provide more in-depth information about 98 SOILS OF THE TROPICS

Table 4.6 Areal distribution of soil orders and suborders by major geographical region, with the percentage of the total for each region. t = trace. Areal estimates exclude areas not covered by soils (rocks, water bodies, ice). Adapted from Buol et al. (2011) and Soil Survey Staff (1999). Tropics: 0–23.28º; Temperate: 23.29–60º; Boreal: > 60º latitude. Order Suborder Tropical Temperate Boreal World

Area Area Area Area Area area Area area (million (%) (million (%) (million (%) (million (%) hectares) hectares) hectares) hectares) Oxisols 962 24.8 20 0.4 0 0 981 7.5 Udox 517 3 520 Ustox 309 1 310 Perox 101 15 116 Aquox 32 0 32 Torrox 3 t 3 Ultisols 760 19.6 341 7.3 3 0 1102 8.5 Ustults 363 22 0 387 Udults 265 290 t 553 Aquults 104 23 t 128 Humults 28 6 t 34 Xerults 0 1 1 2 Inceptisols 606 15.7 537 11.5 135 5.1 1275 9.8 Ustepts 280 137 6 424 Udepts 176 215 33 425 Aquepts 150 118 50 312 Cryepts 0 0 46 46 Xerepts 0 67 1 68 Entisols 603 15.6 1436 30.7 73 1.4 2113 16.2 Psamments 280 163 t 443 Orthents 210 1126 36 1372 Fluvents 102 146 37 286 Aquents 11 1 0 12 Alfisols 480 12.4 487 10.4 294 11.1 1263 9.7 Ustalfs 377 172 17 566 Udalfs 62 193 16 271 Aqualfs 41 37 5 84 Xeralfs 0 85 5 90 Cryalfs 0 0 251 252 Aridisols 186 4.8 1163 24.6 225 8.6 1578 12 Argids 57 404 78 540 Cambids 56 206 30 293 Calcids 45 440 1 487 Gypsids 23 43 2 68 4.4 MAIN SOILS OF THE TROPICS 99

Table 4.6 (cont.) Order Suborder Tropical Temperate Boreal World

Area Area Area Area Area area Area area (million (%) (million (%) (million (%) (million (%) hectares) hectares) hectares) hectares) Salids 5 63 20 89 Cryids 0 0 94 94 Durids 0 7 0 7 Vertisols 150 3.9 160 3.5 0 0 319 2.4 Usterts 117 59 t 177 Torrerts 24 65 0 89 Uderts 9 30 0 38 Aquerts t 6 0 5 Xererts 0 10 0 10 Andisols 45 1.2 20 0.5 0 0 90 0.7 Vitrands 20 8 0 28 Udands 19 9 0 28 Ustands 6 t 0 6 Xerands 0 3 0 3 Cryands 0 0 25 25 Mollisols 36 0.9 449 9.6 414 15.7 899 6.9 Ustolls 18 237 268 523 Rendolls 12 10 4 26 Udolls 5 106 15 126 Aquolls 1 8 3 11 Xerolls 0 87 5 92 Cryolls 0 0 116 116 Albolls 0 1 3 4 Histosols 32 0.8 12 0.3 109 4.1 153 1.2 Saprists 32 2 0 34 Hemists 0 10 89 99 Fibrists 0 0 20 20 Spodosols 6 0.2 59 1.3 270 10.3 336 2.6 Humods 3 3 t 6 Orthods 2 51 14 67 Aquods 1 5 10 17 Cryods 0 0 246 246 Gelisols 0 0 0 0 1126 42.7 1126 8.6 Histels 0 0 101 101 Orthels 0 0 390 390 Turbels 0 0 631 631 Total 3866 100 4684 100 2649 100 11 235 100 100 SOILS OF THE TROPICS

Oxisols, as well as a chapter in Buol et al. (2011) and a review article by Buol and Eswaran (2000). Although Oxisols are the stereotypical soil of the tropics, they cover just one-fourth of the region. Likewise, Mollisols, the stereotypical soil of the temperate region covers just about 10 percent of the area (Table 4.6). Both stereotypes are far from representative of their respective geographical regions. 4.4.2 Ultisols (760 million hectares) Ultisols are the second most extensive soil order, covering approximately 20 percent of the tropics. Ultisols are defined bythepresenceofanargillicor kandic horizon (a 20 percent relative increase in clay content from the A to the B horizon) and a low base saturation, decreasing with depth in the subsoil (< 35 percent base saturation by the sum of cations method, i.e. pH 8.2 at 1.8 m depth). They are generally deep, well-drained, red or yellow soils, acid, with low fertility but generally less so than Oxisols. Their chemistry is dominated by low-activity clays and they have pH-dependent charge. Poorly drained Aquults have gray-colored subsoils. Increasing acidity and potassium chloride-extractable aluminum with depth often causes alumi- num toxicity, limiting root elongation into the subsoil. Ultisols are distinguished from Oxisols by having the marked clay increase with depth. Horizons in Ultisols are more easily distinguished than in Oxisols. The 35 percent base saturation criterion deep in the subsoil is what separ- ates Ultisols from Alfisols. Many Ultisol topsoils have been limed and fertilized to have chemical properties equivalent to Alfisol topsoils; however, acidity and aluminum toxicity often hamper root elongation into the subsoil, thereby limiting available water supplies. Ultisols are mainly formed from geologically old, acidic parent materials that are relatively poor in bases, mainly crys- talline rocks and sediments. They occur in younger surfaces than Oxisols, in areas of undulating topography or downslope Fig. 4.2 Oxisol (fine, kaolinitic, isothermic Typic Haplustox) from the plateaus with Oxisols. They are most extensive in the known as Latosol Vermelho Amarelo (Red Yellow Latosol) at humid and subhumid tropics. Ultisols are the dominant soils of Embrapa’s Cerrado Research Center in Planaltina, Brazil. Note Southeast Asia, the Amazon Basin outside the Guyana and uniform horizons and depth. Brazilian shields, and the Caribbean coast of Central America. They are less extensive in Africa, but are found in coastal West Africa and parts of East Africa, and much of upland tropical Asia. Ultisols are the main soils in the southeastern United resemblance to their parent material. They occupy around States – as far north as Pennsylvania – and of southeastern 16 percent of the tropics and encompass a very diverse array China. The latter areas account for their presence in the tem- of soils. Inceptisols occur on steep slopes as well as in flood perateregionsasfaras35 north or south latitudes, away from plains, and in all tropical climates except the aridic soil recent glaciations and loess deposits. Additional information is moisture regime. provided by Tanaka et al.(1984)andWestet al. (1997). Aquepts are extensively used for wetland rice production In Soil Taxonomy, Ultisols are subdivided into four sub- (Fig. 4.4). They are generally very fertile except for two orders, 26 great groups and 182 subgroups that are likely to kinds: the saline soils (Halaquepts, or Solonetz in the WRB) occur in the tropics (Table 4.1). They are also known as Red and the acid sulfate soils or cat clays (classified as Sulfa- Yellow Podzolics, and as Acrisols and Dystric Nitisols in quepts, Sulfudepts, and Sulfaquents in the Entisol order) the WRB. shown in Fig. 4.5. Some well-drained Inceptisols, Dystrudepts and Dystrus- 4.4.3 Inceptisols (606 million hectares) tepts are acid with low soil fertility and behave similarly to Inceptisols are soils with a cambic horizon (slightly different nearby Oxisols and Ultisols, and have pH-dependent charge. from the A horizon) but no other diagnostic horizons, a Other Inceptisols have permanent charge. High-base-status simple A–B–C horizon sequence, and retaining a strong Eutrudepts and Haplustepts are excellent soils. 4.4 MAIN SOILS OF THE TROPICS 101

Fig. 4.4 An Aquept from Nueva Ecija, Philippines. Aquepts are used extensively for lowland rice production.

Fig. 4.3 A coarse, loamy, kaolinitic, isohyperthermic Typic Paleudult, the Yurimaguas series of the Peruvian Amazon. The Fluvents (Fig. 4.7), and most Aquents are soils with high Oxisol in Figure 4.2 and this Ultisol are from an area where long- fertility. Together with most Inceptisols of alluvial origin, term research has been conducted and which is described in this is where much of the green revolutions in the deltas and fl several chapters of this book. ood plains of Asia and Latin America has taken root. At the other extreme, the shallow family of Orthents present root restrictions, and sandy Entisols (Psamments) shown in Fig. 4.8 are often deep but very infertile. Inceptisols found in the tropics are subdivided in three Entisols are divided into four suborders, 16 great groups suborders according to soil moisture regime (Aquepts, and 118 subgroups that are likely to occur in the tropics Udepts and Ustepts), 15 great groups and 124 subgroups (Table 4.1). Entisols fall under the following soil units of the (Table 4.1). Inceptisols fall under the following soil units of WRB: Fluvisols (Fluvents), Regosols and Leptosols (Orthents), the WRB: Cambisols and some Gleysols (Table 4.3). and Arenosols (Psamments) as shown in Table 4.3. Although most Entisols are fertile, some are infertile like the Quart- 4.4.4 Entisols (603 million hectares) zipsamment in Fig. 4.8. Entisols are soils of such slight development that only an ochric epipedon (yellowish topsoil) or simple human-made 4.4.5 Alfisols (480 million hectares) horizons have formed. They occupy around 16 percent of Alfisols are the fifth most common soils, covering 12 percent the tropics and are located in all soil moisture and tempera- of the tropics. They are very important soils in this region, ture regimes, climatic zones and topographic positions. and although considered typical of the temperate zone, their Some are old soils, present on inert parent materials but areal extent in tropical and temperate regions are about the most of them are young (an extreme is shown in Fig. 4.6). same (Table 4.6). 102 SOILS OF THE TROPICS

Fig. 4.7 A Fluvent along the Shanusi River near Yurimaguas, Peru. Note the different strata produced by flooding and sedimentation episodes.

exchangeable bases (calcium, magnesium, potassium), and are therefore of intermediate to high fertility. Alfisols cover much of subhumid and semiarid tropical Africa, India, the Caribbean, northeast Brazil, the Chaco region of South America and eastern Australia. I believe that Alfisols are the most common soil order where smallholder farming Fig. 4.5 A special kind of wet Inceptisol - the Sulfaquepts (acid takes place in tropical Africa. sulfate soils or cat clays) Ransit series in the Bangkok Plain of Besides having in common an argillic horizon and high fi Thailand. base saturation, Al sols differ markedly in terms of how to manage them. Udalfs (Alfisols with a udic soil moisture regime) are excellent soils and are used for cacao production in coastal West Africa and Brazil. Aqualfs usually make excellent soils for wetland rice. But three other kinds of Alfisols pose particular chal- lenges in tropical Africa. Ustalfs, in large parts of subhumid West Africa, have gravelly topsoils that make tillage difficult, plus high-temperature and water-supply limitations to crop growth (Fig. 4.10). Many of them are underlain by plinthite or ironstone. Accelerated erosion in cultivated fields may expose plinthite. The International Institute for Tropical Agriculture in Ibadan, Nigeria, is located on such loamy, gravelly Alfisols, where Rattan Lal and colleagues have done excellent work on the physical dynamics of these soils (Lal et al. 1975). Sandy Alfisols in the semiarid Northern Guinea savanna and the Sahel pose different challenges (Pathak et al. 1987). They are very low in nitrogen and phosphorus Fig. 4.6 A 1-day-old Entisol, formed after a large earthquake and often exhibit surface sealing that minimizes rainfall produced a landslide in the Cordillera Blanca of Peru in 1970. infiltration. In the highlands and plateaus of East and southern Africa, originally very fertile Alfisols (Rhodudalfs, Rhodus- Alfisols are soils with an argillic, natric or kandic hori- talfs and Haplustalfs) show extensive depletion of soil nutri- zon (Fig. 4.9). Base saturation typically increases with depth, ents after long-term food crop production. reaching more than 35 percent base saturation (pH 8.2) at At the drier end, some Alfisols exhibit a sharp textural 1.8 m depth. Alfisols are similar to Ultisols but subsoils break between a sandy topsoil and dense, clayey subsoil. They seldom have aluminum-toxic conditions and are high in are commonly known as Planosols (Albaqualfs). These soils 4.4 MAIN SOILS OF THE TROPICS 103

Fig. 4.8 A deep, coarse, sandy Entisol (Quartzipsamment) in Fig. 4.9 Kikuyu Red Loam, a Rhodic Nitisol (Rhodudalf ) in the an upland position near Iquitos, Peru. highlands of Kenya. The soil is about 3 m deep. Note the pronounced argillic horizon just below the A horizon. are extensive in parts of Bolivia, Paraguay and Brazil in the Pantanal and Chaco regions as well as in northeastern occupy 5 percent of the tropics, including parts of the Australia (Fig. 4.11) and parts of South Africa (Deckers et al. Sahara, the horn of Africa (Somalia, Ethiopia and Kenya), 1998). eastern and southern parts of the Kalahari Desert, the Some Alfisols in semiarid regions exhibit silica-cemented coastal deserts of South America and southeast Africa, subsoil layers (Durustalfs, Duraqualfs) and some are sodic much of northern Mexico, the Maracaibo Basin and the (Natrustalfs, Natraqualfs, Solonchaks in the WRB) (Sumner Arabian Peninsula. In contrast with the stereotypical sand- and Naidu 1998). Therefore, the Alfisol order represents an dune desert scenes (which are Psamments), Aridisols have extremely wide range in soil conditions. scattered tree vegetation of the Gobi type of deserts. Ari- Most of the tropical red Alfisols were previously mapped disols with adequate topography and depth can become as Latosols. Alfisols of the tropics are divided into three productive soils when irrigated. Engineered land leveling suborders, 25 great groups and 218 subgroups that can to facilitate surface irrigation often exposes finer-textured occur in the tropics (Table 4.1). Alfisols fall under the argillic horizons or carbonate-rich calcic horizons that following soil units of the WRB: Nitisols, Lixisols, Luvisols, bedevil uniform infiltration. Sprinkler irrigation systems Planosols, Calcisols and Durisols (Table 4.3). are preferable. Many saline and sodic soils located outside deserts are 4.4.6 Aridisols (186 million hectares) included in the Aridisol order because for a plant these soils Aridisols are desert soils with an aridic soil moisture are arid, as osmotic tension dehydrates plant cells. regime and horizon differentiation (Fig. 4.12). They Figure 4.13 shows an example. 104 SOILS OF THE TROPICS

Fig. 4.10 An Ustalf near Ibadan, Nigeria, with a sandy, gravelly Fig. 4.11 A Planosol (Albaqualf ) in Queensland, Australia. topsoil that is commonly found in subhumid West Africa. Note the abrupt textural change.

Aridisols have not been intensively studied for manage- ment purposes in the tropics, but many communities the same process (Fig. 4.17) (Dudal 1963, 1965, Latham and depend on them for their livelihoods, particularly pastoral- Ahn 1987). ists. Key references include Bonnet (1960), Beinroth and Vertisols have difficult physical properties for agricul- Osman (1981), Ayoub et al. (1996) and Sumner and Naidu ture and particularly for civil engineering. The shrink–swell (1998) for saline and sodic soils. cycles in Vertisols of the Ethiopian highlands often bring Aridisols are divided into six suborders, 22 great groups stones from a stony layer underlying in the subsoil to the and 173 subgroups that can occur in the tropics (Table 4.1). surface via the cracks (Nyssen et al. 2002). Aridisols fall under the following soil units of the WRB: Vertisols have generally high fertility, but some are defi- Calcisols, Durisols, Gypsisols, Solonchaks and Solonetz cient in phosphorus. In spite of their dark color and clayey (Table 4.3). textures they do not have high contents of soil organic matter (SOM). The dominance of smectite (2:1 clay minerals) 4.4.7 Vertisols (150 million hectares) makes them very sticky when wet – a sure bet for vehicles Vertisols are dark soils with more than 30 percent clay getting stuck in wet dirt roads – in comparison with the less and more than 50 percent of 2:1 minerals in the clay frac- sticky kaolinite (1:1 clay minerals), typical of the red-colored tion (Fig. 4.14). They crack, shrink and swell with changes Oxisols, Ultisols and some Alfisols. in soil moisture (Fig. 4.15). Some show “gilgai,” or micro- Vertisols are the dominant soils of central India, north- relief (small mounds and depressions), resulting from differ- central Australia and much of the Ethiopian highlands. They ential shrinking and swelling (Fig 4.16). Others show cover around 4 percent of the tropics and are locally important “slickensides,” e.g. shiny surfaces on soil aggregates, due to in plains and valley bottoms of subhumid and semiarid 4.4 MAIN SOILS OF THE TROPICS 105

Fig. 4.13 A Natrargid (Solonchak) at the shores of Lake Elementaita in Kenya, where the water contains high quantities of soda (sodium bicarbonate).

Fig. 4.12 An Aridisol in a coastal desert of Peru.

Fig. 4.14 A Vertisol (Ustert) in the subhumid Guanacaste tropical Africa. Often they occur in red-black (Alfisol–Vertisol) Peninsula of Costa Rica. catenas in the landscape. Vertisol savannas often harbor high wild-animal biomass, including the Maasai Mara, Serengeti and Luangwa national parks in Kenya, Tanzania and Zambia. Their richness in calcium and phosphorus supports nutritious classification systems including the WRB. Additional infor- savanna vegetation that is needed to support the bone struc- mation on Vertisols can be found in Dudal (1963, 1965) and ture of large mammals. This is quite in contrast to the savanna Latham and Ahn (1987). vegetation on Oxisols in South America where native vegeta- tion is deficient in calcium and phosphorus and no large 4.4.8 Andisols (45 million hectares) mammals are present (Sanchez and Buol 1975). Andisols occupy only 1.2 percent of the tropics, but where Vertisols are easy to identify. They are known by a var- they occur they are generally regarded as very fertile soils. iety of local names such as “black cotton soils” (India, They are formed in volcanic parent materials (volcanic ash, Africa), cracking clays (Australia), adobe (Philippines), pumice, cinders and lava). They have “andic” properties: the makande (Malawi), vleigrond (South Africa), sonsosuite (Nicar- colloidal fraction dominated by amorphous or poorly crystal- agua), margalite soils (Indonesia), densinegra soils (Angola) lized minerals (allophane, imogolite, ferrihydrite or and others (Deckers et al. 1998). aluminum–humus complexes), low bulk density and high In Soil Taxonomy, Vertisols are divided into four sub- phosphorus sorption. The term colloidal is used because orders, 18 great groups and 119 subgroups that can be found many Andisols do not have texture (particle size distribution) in the tropics. The term Vertisol is used in most other in the conventional sense since the minerals are amorphous. 106 SOILS OF THE TROPICS

Fig. 4.15 Surface view of cracks in a Vertisol from Laguna Province, Philippines, during a dry period.

Fig. 4.17 Slickensides in a horizon of a Vertisol from Queensland, Australia.

returning to the site 1 year later it looked like nothing had happened; the crops benefited from an additional input of rapidly weathering ash. Figure 4.19 shows fully developed Fig. 4.16 Gilgai topography due to differential shrinking and Andisol nearby, and Fig. 4.20 a multiple profile due to sub- swelling in a Vertisol of Queensland, Australia. sequent volcanic ash depositions. Andisols are associated with fertile red soils of other orders. They are locally important in both tropical and tem- Most Andisols feel soft and greasy to the touch except perate regions and usually support intensive agriculture in Vitrands and Vitric subgroups containing large amounts of the tropics. Tropical Andisols are most extensive in the coarse cinders. Andisols are usually very fertile except for Pacific Rim around active or dormant volcanoes in Mexico, phosphorus, because they are releasing nutrients from Central America, Ecuador, Peru, Bolivia, the Philippines, easily weathered minerals. The amorphous minerals have Indonesia and many islands. They are also important in a very high capacity to sorb phosphate ions, actually the Rift Valley of East Africa and in Cameroon. opening additional fixation sites as phosphate is fixed In Soil Taxonomy, Andisols are divided into five sub- (Chapter 14). orders, 19 great groups and 141 subgroups that can be found Figure 4.18 shows an eruption I saw in the Philippines, in the tropics (Table 4.1). The term Andosol is used in most which destroyed nearby houses and crops. However, other classification systems including the WRB. 4.4 MAIN SOILS OF THE TROPICS 107

Fig. 4.18 Eruption of ash from Taal Volcano in the Philippines, 1965. Note ash deposit on the shore of this crater lake and a Fig. 4.19 A well-developed Andisol (probably a Melanudad) previous eruption in the air. near Tagaytay, Cavite, Philippines.

Figure 4.21 shows a shallow Mollisol over limestone, and Fig. 4.22 shows a more conventional Mollisol. Mollisols that may occur in the tropics are divided into four suborders and 20 great groups. These comprise 198 subgroups; most of them have been identified in temperate regions but can occur in the tropics (Table 4.1). Mollisols are known in other systems as Chernozems, Kastanozems and Phaeozems in the WRB and other soil classification systems.

4.4.10 Histosols (32 million ha) Histosols are organic or peat soils, and are located mainly in swamps, covering 0.8 percent of the tropics. Histosols in the tropics contain around 3 percent of the global soil carbon stocks and at least 20 percent of global peat carbon. The Fig. 4.20 Multiple mini soil profiles with black soil layers large deforestation taking place in Histosols of Indonesia is interspersed with white ash deposits near the Nevado del Ruiz resulting in very large carbon emissions to the atmosphere. volcano in Caldas, Colombia. Probably a Vitrand. Although rich in carbon, Histosols often have high carbon- to-nitrogen ratios, are quite acidic, and are deficient in some micronutrients. Only one suborder of Histosols is recognized in the A book by Alvarado et al. (2001) as well as Buol et al. tropics by Soil Taxonomy, the Saprists, having the most (2011), and a chapter on Andisols by Schlesinger et al. decomposed kind of organic materials (Fig. 4.23). Saprists (1998), provide additional background. are subdivided into three great groups and 11 subgroups (Table 4.1). The term Histosol, or organic soils, is used in fi 4.4.9 Mollisols (36 million hectares) other classi cation systems. Additional information is pro- vided by Andriesse (1988). Mollisols are the stereotypical temperate-region soils found in the Midwest of the United States, the Russian steppes and the Pampas of Argentina. They are excellent soils, both 4.4.11 Spodosols (6 million hectares) in terms of fertility and physical properties. They cover Spodosols are soils with a spodic horizon (of iron and organic only 0.9 percent of the tropics but where they occur, as in matter accumulation), usually developed on sandy materials. the Cauca Valley of Colombia, they support highly product- Figure 4.24 shows an example from central Amazonia. These ive agriculture. Like Aridisols, Mollisols have not been soils are known as Podzols in most other classification intensively studied for management purposes in the systems and are the typical soils of northern temperate and tropics, but many communities depend on them for their boreal regions but also occur in subtropical Florida in the livelihoods. United States. They occur in the tropics in humid climates 108 SOILS OF THE TROPICS

Fig. 4.22 A Mollisol (Vertic Calciustoll), Santa Clara series in Central Cuba.

Fig. 4.21 A shallow Mollisol (Rendoll) over limestone in Mindanao, Philippines.

with sand as parent material. They are most extensive in the northwestern Amazon and in small areas of Indonesia. They give rise to “black-water” rivers due to the concentration of dissolved carbon compounds leached from them. Spodosols have very low native fertility and in the Amazon support a stunted forest called capinarema next to the Oxisols and Ulti- sols that support typical moist tropical forests. Unfortunately, Spodosols have received inordinate atten- tion by ecologists who discovered a nutrient cycling process that bypasses the topsoil in the San Carlos de Rio Negro region of the Venezuelan Amazon. This type of nutrient cycling is very different from processes found in the domin- ant Oxisols and Ultisols. Jordan (1985, 1989) erroneously attempted to generalize these findings to the rest of the humid tropics. Spodosols are found in only 0.2 percent of the tropics. Fig. 4.23 A Histosol (probably a Saprist) in the highlands of They are subdivided into three suborders, 12 great groups Ecuador. and 64 subgroups that can occur in the tropics (Table 4.1). 4.5 SOILS AT THE LANDSCAPE SCALE 109

others occur, the ones chosen are considered to be represen- tative of large areas. The relationship between geomorphic surfaces and soils is of great value in predicting where certain soils will occur. Soil–landscape associations have been recognized and studied for a long time in the tropics. Milne (1935) developed the soil catena concept in East Africa, which has been used worldwide. 4.5.1 Udic Environments Soil-forming processes proceed at faster rates in hot, humid, tropical climates than in others because of the almost con- stant downward water movement, the large amounts of biomass added to the soil and the constantly high tempera- tures. However, much of the parent material consists of rather inert rocks and sediment, which have been exposed to weathering for many eons of time. Therefore, unlike the parent materials more recently exposed by continental glaciation in northern temperate latitudes, the soils formed in udic, isohyperthermic environments are less likely to express soil properties commonly associated with present soil-forming factors. The predominant soils of udic, isohy- perthermic regimes are Oxisols, Ultisols, Alfisols and Inceptisols. Oxisol-Dominated Landscapes. Oxisols are associated with very old, stable land surfaces such as peneplains. Ulti- sols and Inceptisols occupy the younger land surfaces. Oxi- sols, Ultisols and Inceptisols are frequently intermixed in udic environments but occur in predictable positions in the landscape (Lepsch and Buol 1974, Beinroth et al. 1974, Anjos et al. 1998). Oxisols occupy the older land surfaces, which may be remnants of a previous peneplain. Ultisols often occupy the slopes below areas where Oxisols are found. Their argillic horizons apparently formed after the original Fig. 4.24 A Spodosol in central Amazonia, near Manaus, peneplain was truncated by erosion and clay moved down Brazil. Note spodic horizon of iron and organic carbon the slope, as clay films and shear planes indicate. Inceptisols accumulation at the bottom. occupy the steeper slopes, developing on recently exposed rock. This is illustrated in Fig. 4.25 from the island of Kauai, Hawaii. In the state of São Paulo, Brazil the formation of 4.4.12 Gelisols Ultisols on side slopes below Oxisols was attributed to sea- The newest soil order encompasses soils with permafrost in sonal lateral seepage of groundwater (Moniz and Buol 1982). their top 50–100 cm. They are the most extensive soils of Ultisol-Dominated Landscapes. In udic areas where boreal regions and are found in the coldest areas of the the sediments are too young for Oxisols to form, as in the temperate regions. Gelisols have been reported in the trop- upper Amazon Basin of Peru and Colombia, outside of the fl ical Andes of northern Chile in USDA National Resource in uence of the Guyana and Brazilian shields, Ultisols are fi Conservation Service maps, and are also likely to occur near found to be in association with Al sols, Inceptisols and mountainous peaks with tropical glaciers, located elsewhere Entisols. Studies by Sanchez and Buol (1974) and Tyler et al. in the Andes, East Africa (Mount Kenya, Mount Kilimanjaro, (1978) show the following landscape relationships (Fig. 4.26). Mount Elgon and the Rwenzori range) and in Papua New At the highest topographic positions, the predominant well- Guinea (Puncak Jaya and Mount Wilhelm). drained soils are Udults, which are very acid. In the oldest flat surfaces, the A horizons are sandy, and the argillic horizons are deep, forming Paleudults (shown in Fig. 4.3). Down the slope there is a drainage catena with increasing 4.5 Soils at the Landscape Scale wetness and a clayey, mottled horizon, which is a mixture of kaolinite and smectite. Aquults are found in intermediate This section describes some of the most frequently observed positions, and Aqualfs at the lower ones, due to enrichment soil associations in tropical landscapes. Although many of bases. Floodplains along the rivers consist of fertile 110 SOILS OF THE TROPICS

Fluvents, deposited from rivers that originate in the Andes, reports indicate that similar soils are quite extensive in the which are rich in bases (Hoag 1987). Amazon (Marbut and Manifold 1926, Sombroek 1966, Bena- The mottled grey and reddish horizon led earlier vides 1973). These soils are probably classified as Alisols in workers to call these Aquults and Aqualfs “groundwater WRB (Table 4.3). laterite.” Sanchez and Buol (1974) have shown that its min- eralogy is primarily 2:1 and 1:1 clay minerals. Although having the color pattern of plinthite, this material does not 4.5.2 Ustic Environments Ustic environments are characterized by major soil moisture indurate upon exposure and is obviously not plinthite. Some fluctuations during the year. It is reasonable to assume that many weathering processes occur at a substantially slower

(c) rate during the dry season than during the rainy season. fl INCEPT- ULTI- OXISOLS ULTI- INCEPTISOLS ULTI- OXISOLS Thus, some consequences of these uctuations can be ISOLS SOLS SOLS SOLS observed in many landscapes. SOIL CREEP Oxisol-Dominated Landscapes. Parts of the African and SHEAR PLANES CLAY FILMS Brazilian shields have multicycle landscapes with two or more erosion surfaces (Ruhe and Cady 1954, Feuer 1956). Figure 4.27 shows a landscape consisting of three erosion surfaces close to Brasília. The first erosion surface, believed to be the product of an ancient peneplain, marks the skyline at elevations of about 1000 to 1200 m. Locally known as – (b) chapadas, these surfaces have a slope of 0 3 percent. The soils ULTISOLS OXISOLSULTI- IN- ULTI- OXISOLS are predominantly deep Ustox, characterized by a strong SOLS CEPTI- SOLS SOLS granular structure, high permeability and extremely low fertility. This surface terminates abruptly at an escarpment of 100–150 m depth, followed by a second gently sloping erosion surface. The escarpments have little soil develop- ment in the weathered rock material, and some have out- crops of ironstone or other rocks. The second erosion surface is often several kilometers long with a 2–8 percent slope.

(a) Ustox are also the predominant soils, with inclusions of His- OXISOLS tosols and Aquox in poorly drained spots. The third erosion OXIC HORIZON surface consists of an 8–15 percent slope, parallel to narrow floodplains. These soils have a higher base status and are BEDROCK probably Ustalfs, as bases moved downslope. The first two surfaces are covered by typical savanna vegetation, indicative of low soil fertility. The third surface consists of semi decidu- Fig. 4.25 Formation of an Oxisol–Ultisol–Inceptisol landscape ous forests, reflecting a high base status. I have seen the same from an old land surface through dissection in Kauai, Hawaii. (a) landscape in the Northern Province of Zambia, probably part Undissected, (b) moderately dissected, (c) strongly dissected of what Ruhe and Cady reported in 1954. peneplain. Source: Beinroth et al. (1974). Reproduced with Similar landscapes have been described by Ruhe (1954) in permission by the Soil Science Society of America Central Africa. Many of them have stone lines indicating a buried soil below a more recent soil material, probably

Fig. 4.26 Ultisol–Alfisol–Entisol toposequence found around Yurimaguas, Peru. Highly weathered acid Udults appear in the flat surfaces, followed by increasing wetness, related to the presence of an impermeable gley layer. Adapted from NCSU (1973) 4.5 SOILS AT THE LANDSCAPE SCALE 111

FIRST EROSION SURFACE Chapadas 0–3% slope OR ROCK USTOX 1000 OUTCROPS LATERITE SECOND EROSION SURFACE 2–8% slope THIRD EROSION SURFACE 850 USTOX 8–15% slope USTALFS 400 Stream

2–10 km

Fig. 4.27 Soil–geomorphology relationships at Embrapa’s Cerrado Research Center near Brasília, Brazil. Dots represent igneous rock or ironstone outcrops. Adapted from Feuer (1956), Cline and Buol (1973) and personal observations

Fig. 4.28 Alfisol catena in Ibadan, Nigeria, with increasing plinthite concretions in the subsoil as drainage gets worse. The well-drained members are Ustalfs; the poorly drained ones Plinthaqualfs. Source: Nye (1954). Reproduced with permission by the Journal of Soil Science

transported from elsewhere, on top of it. Stone lines are above or below argillic horizons. Classic drainage catenas common in many landscapes throughout tropical Africa, have been described where the amount of plinthite including the more common Alfisol-dominated landscapes increases and becomes shallower as drainage becomes pro- (Ruhe 1959, Riquier 1969, Segalen 1969). gressively worse. Nye (1954) shows one example near Alfisol-Dominated Landscapes. Parts of subhumid and Ibadan, Nigeria on a 5 percent slope landscape underlain semiarid West Africa consists of Ustalfs with plinthite layers by granite–gneiss materials (Fig. 4.28). The soils have sandy 112 SOILS OF THE TROPICS

ANDISOLS USTULTS USTERTS 1400 UDEPTS HUMULTS 1000

750

Fig. 4.29 Typical Alfisol (red) and Vertisol (black) catena in 400 ustic areas of Kenya. The arrow indicates the direction of silica movement. 200 Depressional area

0 loam topsoils underlain by a clayey argillic horizon that has Temperature increases few iron nodules in the upper parts of the slope. Iron con- Annual rainfall decreases cretions increase in abundance as drainage becomes pro- Length of dry season increases Organic matter decreases gressively impeded. In the wetter sites, a partly cemented Amorphous minerals decrease mass of nodules (a petroferric contact) is found at depths of less than 1 m. The International Institute for Tropical Fig. 4.30 Soil associations along the slope and base of a Agriculture in Ibadan, Nigeria has this type of soil associ- volcano in Indonesia. Soils developed from volcanic ash ation (the top member of the catena is shown in Fig. 4.10). of uniform age. Adapted from Dudal and Soepraptohardjo The erosion hazard of such soils is considerable, even in (1960) gentle slopes, because of the textural change. Erosion of the sandy A horizon will render the soil practically worth- less, as it is very difficult to work the gravelly, clayey (Fig. 4.30) across mountain ranges where the volcanic B horizon. ash is believed to be of the same age and where enough Alfisol–Vertisol Landscapes. A very common occur- time has elapsed for weathering to occur (Dudal and Soe- rence in the ustic tropics are the red and black catenas. praptohardjo 1960). Red soils (mainly Ustalfs) occupy the better drained sites, At elevations above 750 m, Andisols dominate because and dark, cracking clays (Vertisols) the lower topographic the udic moisture regime and the isomesic (cold) soil tem- positions. The red soils are predominantly kaolinitic, and perature regime favors SOC accumulation and slows the the black soils, smectitic. Such relationships are common crystallization of allophane into kaolinite. Also, coarser par- in East Africa and India (Milne 1935, Greene 1947, Rad- ticles fall more quickly and thus closer to volcanic vents. wanski and Ollier 1959). The headquarters of the Inter- These soils are likely Vitrands. At elevations between national Crops Research Institute for the Semiarid Tropics 400 and 750 m, the predominant soils are brown Inceptisols, in Patancheru, Andhra Pradesh, India is located on one such probably Udepts. The change in color from black to brown is catena. Since the parent material is probably uniform, associated with lower organic matter, higher clay content movement of soluble silica and base levels are believed to and the presence of kaolinite. This indicates a crystallization constitute the main soil-forming process. The fact that smec- of allophane into kaolinite through many intermediate tite can be synthesized in the laboratory in a similar way forms. At lower elevations (200 to 400 m) the soils become lends credit to this hypothesis. This relationship is shown in red with corresponding increases in clay content and Fig. 4.29. In Sudan, similar Oxisol–Vertisol catenas exist. decreases in cation exchange capacity (Humults). In the lowland areas the moisture regime changes to ustic, and 4.5.3 Tropical Highlands the predominant soils become Ustults. In depressed areas, Volcanic Highlands. The distribution of soils in volcanic Vertisols may form either by direct transformation of allo- landscapes depends primarily on the age of the ash phane into montmorillonite or by the resilication process deposits and climate. In humid regions, volcanic ash mentioned in regard to the East African catena. weathers quickly into allophane, which forms complexes The above example is limited to areas where elevation with organic matter, resulting in Andisols. With further and moisture regime dominate the soil-forming processes. weathering, allophane is converted into kaolinite or hal- Even in areas with relatively uniform parent materials the loysite under well-drained conditions, and into smectite in relationships are more complicated. Tamura et al. (1953) poorly drained ones (Mohr et al. 1972). Although Andisols illustrated the distribution of soils on the island of Hawaii may be located in any topographic position when the ash according to topography, rainfall, parent material and age. is young, a common occurrence in volcanic regions of Figure 4.31 illustrates this relationship, with Andisols Southeast Asia is for Andisols to be at higher elevations, grading into Humults on the windward side of the island, grading into red soils (Ultisols, Alfisols and Oxisols) at which has a perudic and udic soil moisture regime. On the lower elevations. This has been observed in Indonesia leeward side, the soil moisture regime is ustic or aridic. 4.5 SOILS AT THE LANDSCAPE SCALE 113

Differences in the age of parent materials and rainfall pro- agriculture. The lower parts of these irrigated areas progres- duce soils ranging from Oxisols to Aridisols within short sively receive salt deposits from higher parts, converting distances. some Fluvents into Salids and poorly drained saline soils. A Cross Section of the Andes. Several widespread soil Alfisols tend to surround the Aridisols as rainfall increases. associations in highlands have been described by Zamora Other deserts also have considerable diversity in their (1972) for parts of the Peruvian Andes. A west-to-east soils. The stereotypical desert of sand dunes is only part of sequence is shown in Fig. 4.32. The westernmost ranges the picture. have aridic soil moisture regimes with shallow soils or none at all because the Pacific coast is a desert. In the 4.5.5 Tropical Alluvial Plains and Deltas major highland plateaus such as the Mantaro Valley, a River Valleys. The geomorphic relationships of river valleys system of valley terraces on calcareous parent materials are similar throughout the world. The systems of flood- produced the following soil association: At the edges of plains, levees, terraces and adjacent uplands in valleys and the valleys, soils shallow to bedrock predominate, particu- deltas are basically no different in the tropics than in the larly Rendolls when limestone is the parent material. In rest of the world. The principal difference is in soil proper- the higher terraces, Mollisols (Ustolls and Aquolls) are ties. In glaciated temperate areas, alluvium is generally rich most common, grading into Fluvents in the valley flood- in bases and weatherable minerals, with hydrated mica plains. At altitudes above 4000 m, the relatively flat alti- often as the dominant clay mineral. In the tropics, such rich plano or puna is dominated by Cryands in the well-drained alluvial deposits are found in watersheds with freshly sites, and by Histosols in the poorly drained areas. On the exposed sediments, such as those originating in the Andes eastern flank of the Andes with udic soil moisture and the Himalayas, or in areas with fresh volcanic ash regimes, shallow Entisols are found near the high peaks. deposits. Rivers originating in areas of old landscapes that Below, dystric or eutric Udepts occur, according to parent have undergone several erosion cycles will form alluvial material. At lower elevations, Ultisols occur together with deposits that are rich in quartz, kaolinite and iron oxides. Fluvents near the rivers. Many of them are quite infertile (Edelman and Van der Voorde 1963). 4.5.4 Tropical Deserts Soil differences in the upper watersheds of many rivers Tropical deserts contain bare rock, Aridisols, and, to a much provide marked contrasts in the chemistry of the rivers and lesser extent. sandy soils. In the coast of Peru, the following in alluvial soils downstream. This is rather dramatic in the relationships can be found. The bare mountains to the east Amazon Basin, where rivers are often classified as “white- are essentially soilless or have lithic subgroups of Entisols. water,”“black-water” or “clear-water” rivers (Schubart and In the valleys, moving sand dunes (Psamments) are found in Salati 1982, Hoag 1987). White-water rivers (beige, muddy) association with several Aridisols, including Cambids, Cal- originate in geologically young watersheds, high in sus- cids, Gypsids and Durids. In the river floodplains, Fluvents pended sediments (40–300 mg/liter) that are rich in basic occur, forming the basis for highly productive irrigated cations, producing fertile Fluvents and Fluvaquents.

Annual rainfall (min) 5000–3750 3750–1125 1125–375 < 375 Soil moisture regime Udic Ustic Aridic Basalt Parent material Basalt or ashAsh Basalt Basalt Ash or ash Age Old OldYoung Old Old Young -

Rain, bearing winds ANDISOLS

ANDISOLS HUMULTS

HUMULTS

USTOX

ENTISOLS (Beach and alluvial) ANDISOLS Ocean ARIDISOLS

Fig. 4.31 Soil associations on the island of Hawaii, affected by rainfall, topography, parent material and age. Adapted from Tamura et al. (1953) 114 SOILS OF THE TROPICS

LITHIC RENDOLLS USTOLLS USTOLLS GROUPS and and AND CRYANDS (EUTRIC) LITHIC AQUOLLS AQUOLLS GROUPS Snow Snow line

5000 AQUEPTS

Snow HISTOSOLS CRYANDS FLUVENTS

4000 Puna or Paramo

3000 Interandean Valley Elevation (m)

2000

1000 West East Amazon Pacific 300 km coastal jungle desert

Fig. 4.32 Major soil associations in a transect of the Peruvian Andes. Adapted from Zamora (1972) and his unpublished data

Black-water rivers originate from sandy watersheds, dominated by Spodosols and Psamments that result in coffee-colored water with a high content of dissolved carbon and low in base cations (river water pH ranging from 3.8 to 4.7), and less than 5 mg per liter of suspended solids. They produce sandy, low-fertility alluvial soils. The most famous example is the Rio Negro, which originates in Spodosol/Psamment-dominated watersheds in the northwestern Amazon Basin. When the Solimôes (the Amazon proper, coming from Peru) and the Rio Negro meet near Manaus at the encontro das aguas, the muddy beige and black streams continue to flow side by side for several kilometers until they eventually mix to form the white-water lower Amazon River (Fig. 4.33). Smaller black- water rivers are found draining from Spodosol areas in Indonesia. Clear-water rivers originate in Oxisol- or Andisol-dominated watersheds that produce little sedimentation because of the Fig. 4.33 The “meeting of the waters,” about 20 km east of strong aggregated structure of these soils. The Tocantins River Manaus, Brazil. The black nutrient-poor water (top and left) in the central Amazon is one example. Schubart and Salati comes from the Rio Negro whose watershed has large areas of (1982) report that the suspended particle content of clear-water Spodosols and other sandy, infertile soils. The nutrient-laden rivers in the Amazon Basin is also less than 5 mg per liter and white waters (bottom and right) of the Solimôes River have their the pH varies widely from 4.5 to 7.8. headwaters in the Andes of Peru, Colombia and Ecuador. The The major tropical river systems (Amazon, Orinoco, two rivers join, and the Amazon flows for 1400 km, all the way Paraná, São Francisco, Magdalena, Ganges, Brahmaputra, to the Atlantic Ocean. Irrawaddy, Chao Phraya, Mekong, Congo, Nile, Niger, Volta) are white-water rivers during most of their courses. The two rivers that join at Khartoum, Sudan to form the proper Nile Consequently, high fertility in recent alluvial soils also show this difference. The White Nile, originating in cannot be taken for granted in the tropics, especially those Lake Victoria, is a muddy one, while the Blue Nile, originat- rivers originating from Oxisol or sandy soil watersheds. ing in the mountains of Ethiopia with many Andisols, is a Coastal Areas. Mangrove vegetation, coral reefs and bar- clear-water river. rier islands dominate the majority of tropical coastal areas. 4.7 SUMMARY AND CONCLUSIONS 115

Coastal climates vary from deserts to rainforests, resulting • The advent of the quantitative Soil Taxonomy system and in a wide variety of soils. But the abundance of pyrite-rich the World Reference Base for Soil Resources (WRB) now marine deposits in some tropical coastal areas results in a permits the grouping of soils of the tropics according to greater extension of acid sulfate soils (Sulfaquents, Sulfa- their properties rather than according to what genesis quepts, Sulfudepts) than in the temperate region. theory dictates. • Soil Taxonomy is based on quantitatively defined diagnos- tic horizons and attributes known to exist in nature. Both 4.6 Importance of Soil Diversity are either easily observable and quantified in a soil profile or measured by standard laboratory techniques that have to Ecologists and Modelers high reproducibility. Observations that cannot be quanti- fied are not included in Soil Taxonomy. While most agronomists value knowing which soils they are • Oxisols are the most abundant soils found in the tropics, dealing with in their research, ecologists have been slow to covering about 25 percent of the tropical land area. Oxisols learn their soils. Two prominent ecologists, Joshua Schimel are usually deep, well-drained, red or yellowish soils, acid, and Oliver Chadwick (Schimel and Chadwick 2013), urged with very low fertility, but contrary to conventional wisdom their brethren to embrace soil classification, indicating that they are not low in soil organic carbon (SOC) content. Low- calling the soils where they work, say “black soils,” is analo- activity clays and the iron and aluminum oxides dominate gous to describing the vegetation as “trees.” They empha- their chemistry, giving them pH-dependent charge. By def- sized that soil classification offers an insight into key inition, Oxisols cannot have sandy texture (< 15 percent processes and properties that are determinants of many clay). They are either clayey or loamy and therefore have important below-ground biochemical ecosystem processes, considerable phosphorus sorption capacity. Their physical particularly in the subsoil. properties are generally excellent due to a strong granular Crop modelers are rapidly developing ways to predict structure, although plant available water-holding capacity is whether or not the world will be able to feed itself by mid- often low, most likely due to a paucity of silt. century, in view of climate change. Such models seldom • Ultisols are the second most extensive soil order, covering consider soils. Digital soil maps now convert soil classes into around 20 percent of the tropics. Ultisols are defined by important soil properties such as clay content, organic the presence of an argillic horizon and low base satur- carbon, bulk density, pH, ECEC and electrical conductivity; ation. They are generally deep, well-drained, red or yellow as well as soil functions such as available soil water storage, soils, acid, with low fertility but less so than Oxisols. carbon density, phosphorus fixation and aluminum toxicity Increasing acidity with depth often causes aluminum tox- (Lagacharie et al. 2007, Sanchez et al. 2009). Since most of the icity, limiting root elongation into the subsoil. Ultisols can digital soil maps can be produced at the same spatial scales have sandy topsoils, and are usually grayish in color over as data from climate, vegetation, etc., it is possible to incorp- yellowish or reddish subsoils. Topsoil SOC content is vari- orate soils data into these models. able and correlated with clay content. Their physical prop- In fact, Folberth et al. (2016), utilizing version 1.2 of the erties are less favorable than Oxisols. Their chemistry is Harmonized World Soil Database (FAO/IIASA/ISRIC/ISS-CAS/JRC dominated by low-activity clays and they have pH- 2008), found that maize yield variability due the type of soil dependent charge. data used in a crop model was more important than the simu- • Inceptisols, the third most abundant soils of the tropics lated inter-annual variability in yield due to weather in systems (around 16 percent) are soils with a simple A–B–C horizon without fertilizer inputs, largely in the tropics. In systems with sequence, retaining a strong resemblance to their parent fertilization and irrigation, weather variability gradually material. Inceptisols comprise a very diverse array of soils. became more important than soil-related yield variability until They occur in steep slopes as well as in flood plains, and in it became overwhelming at high input levels. This shows the all tropical climates except the aridic soil moisture regime. importance of using soil information in crop models. Aquepts are extensively used for wetland rice production. They are generally very fertile except for two kinds: saline soils and acid sulfate soils or cat clays. Some well-drained Inceptisols are acid with low soil fertility and behave simi- 4.7 Summary and Conclusions larly to nearby Oxisols and Ultisols, and have pH-dependent charge. Other Inceptisols have permanent charge. • The only property common to all “tropical soils” is their • Entisols are the fourth most extensive soils of the tropics. uniform temperature regime. They are soils of such slight development that they have • Natural soil classification systems are divided into only an ochric epipedon (yellowish topsoil). They also systems based on soil genesis theory (what the soil should occupy around 16 percent of the tropics and are located be according to the factors of soil formation) and morpho- in all soil moisture and temperature regimes, climactic logical systems that are based on actual soil properties zones and topographic positions. Most Fluvents and (what the soils are). Aquents are soils with high fertility. 116 SOILS OF THE TROPICS

• Alfisols are the fifth most common soils, covering around of Indonesia is resulting in very large carbon emissions to 12 percent of the tropics. Alfisols are similar to Ultisols the atmosphere. Although rich in carbon, Histosols often but are high in exchangeable bases, and subsoils seldom have high carbon-to-nitrogen ratios, are quite acidic, and have aluminum-toxic conditions, and therefore these soils are deficient in some micronutrients. are of intermediate to high fertility. Alfisols are the most • Spodosols are soils with a spodic horizon (of iron and common soil order where smallholder farming takes organic matter accumulation), usually developed on place in tropical Africa. Alfisols differ markedly in terms sandy materials. They occur in 0.2 percent of the of how to manage them. Udalfs are excellent soils and are tropics. Spodosols have very low native fertility and, in used for cacao production. Aqualfs usually make excellent the Amazon, support a stunted forest. Unfortunately, soils for wetland rice. Ustalfs, in large parts of subhumid Spodosols have received inordinate attention by ecolo- West Africa, have gravelly topsoils, which makes tillage gists working in the San Carlos de Rio Negro region of difficult. Many of them are underlain by plinthite or the Venezuelan Amazon, where they discovered nutri- ironstone. In East and southern Africa, originally fertile ent cycling processes that bypass the topsoil, very dif- Alfisols show extensive depletion of soil nutrients from ferent from those found in the dominant Oxisols and decades of food crop production without nutrient Ultisols. Some of these scientists erroneously attempted additions. to generalize these findings to the rest of the humid • Aridisols are desert soils with an aridic soil moisture tropics. regime and horizon differentiation. They occupy 5 percent • At the landscape level, several well-defined soil relation- of the tropics. Aridisols with adequate topography and ships are found in tropical areas. In udic environments, depth can become productive soils when irrigated. sequences of Oxisol–Ultisol–Inceptisol are common in • Vertisols are dark soils with more than 30 percent clay areas with acidic parent materials. On more basic parent and > 50 percent of 2:1 minerals in the clay fraction. They materials Ultisol–Alfisol–Inceptisol sequences are found. crack, shrink and swell with changes in soil moisture. In ustic environments with very old parent materials Vertisols have difficult physical properties for agriculture Oxisol landscapes occur with rock or plinthite outcrops and particularly for civil engineering. They cover around at the breaks of the slopes separating erosion surfaces. On 4 percent of the tropics and are locally important in plains younger materials Alfisol–Vertisol landscapes are and valley bottoms of subhumid and semiarid tropical common. Africa. Vertisol savannas often harbor high wild-animal • In the tropical highlands, the influence of volcanic mater- biomass. Their richness in calcium and phosphorus sup- ials commonly results in Andisol–Inceptisol–Ultisol– ports savanna vegetation that is needed to support bone Vertisol landscape sequences. In non-volcanic highlands, structure in large mammals, quite in contrast to the a wide diversity of landscape–soil relationships are found. savanna vegetation growing on Oxisols in South America, Andisols are more extensive in the tropics than in the where native vegetation is calcium- and phosphorus- temperate region and are of major economic importance deficient, and no large mammals are present. in the highlands of tropical Asia, Africa and Latin America. • Andisols are formed in volcanic parent materials. The • Although the geomorphology of alluvial plains and deltas colloidal fraction is dominated by amorphous or poorly is no different in the tropics than in the temperate region, crystallized minerals (allophane, imogolite, ferrihydrite the high-fertility properties of alluvium are not always or aluminum–humus complexes), low bulk density and found in the tropics. In watersheds originating from high phosphorus retention. Most Andisols feel soft and young materials such as the Andes and the Himalayas, greasy to the touch. Andisols are usually very fertile alluvium is rich. In watersheds originating from highly except for phosphorus, because they are releasing nutri- weathered surfaces, alluvial materials are usually ents from easily weathered minerals. The amorphous infertile. minerals have a very high capacity to sorb phosphate ions, • Many tropical coastal and deltaic areas have extensive actually opening additional fixation sites as phosphate is regions of acid sulfate soils. When drained, these soils fixed. Even though they occupy only 1.2 percent of the are extremely acid with a pH below 3 and are difficult to tropics, they host densely populated farming populations. put into production. • Mollisols are the stereotypical temperate-region soils. • While most agronomists value knowing which soils they They are excellent soils both in terms of fertility and are dealing with in their research, ecologists have been physical properties. They cover only 0.9 percent of the slow to learn their soils. Two prominent ecologists urged tropics but where they occur, they support highly pro- their brethren to embrace soil classification, indicating ductive agriculture. that calling the soils where they work, say “black soils,” • Histosols are organic or peat soils, and are located mainly is analogous to describing the vegetation as “trees.” They in swamps, covering 0.8 percent of the tropics. Histosols emphasized that soil classification offers an insight into in the tropics contain around 3 percent of the global soil key processes and properties that are determinants of carbon stocks and at least 20 percent of global peat many important below-ground biochemical ecosystem carbon. The large deforestation taking place in Histosols processes, particularly in the subsoil. 4.7 SUMMARY AND CONCLUSIONS 117

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