Soil Diversity in the Tropics D . D . RICHTER and L . I . BABBAR
I . Introduction and Objectives .......................... 316 I1. Changing Perspectives about Soil Taxonomy . . . . . . . . . . . . . . . 320
111. The Development of Misconceptions about “Tropical Soil” A . The Enormous Challenge of Mapping Soils on the 5-Billion
Hectare Tropical Landscape B . Interdisciplinary Miscommunication about Soils in the Tropics C . The Tower of Babel Effect of Too Many Taxonomies and Nomenclatures ........................... D . Emphasis on Factors Rather than on Effects of Soil Formation: The 1938 Soil Classification IV . Advances in Soil Taxonomy and the Creation of the World SoilMap ...................................... A . Soil Taxonomy: A New Scientific Paradigm . . . . . . . . . . . B. FAO/UNESCO Soil Map of the World V . Diversity of Soil Taxa in the Tropics .................... A . General Soil Taxonomic Variation in Tropical Africa. America and Asia ........................... B . Ferralsols: Modern Inheritors of the Latisol Concept C . Acrisols: the Underestimated Soil Order . . . . . . . . . . . . D . Lithosols. Arenosols and Luvisols: From Extremely Fertile to Infertile, 500 Million Hectares Each E . Regosols, Yermosols and Cambisols: Weakly Developed Soils F . The Other 1 Billion Hectares: Extreme Variation VI . How Much Area in the Tropics Is Covered by Oxisols?: Results of the First Soil Surveys of the Amazon Basin . . . . . . . . . . . . . . . A . Background ............................... B . Some Common Properties of Amazonian Soils . . . . . . . . C. The New Soils Map ofthe Brazilian Amazon . . . . . . . . . D . How Much Area in the Tropics Is Covered by Oxisols? . . E . A Realistic Concept of Soil Diversity in the Humid Tropics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Copyright 0 1991 Academic Press Limited All rights of reprodrrrtion in any form reseri.ed
ADVANCES IN ECOLOGICAL RESEARCH VOL . 21 ISBN 0-12-013921-9
D . D . RICHTER A N D L. I. BABBAR
VII. Meso- and Local-scale Soil Variation in the Tropics . . . . . . . . . . VIII. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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I. INTRODUCTION AND OBJECTIVES Concepts of diversity are central to our understanding of tropical plant and animal species, communities, and ecosystems. The concept of diversity as “richness in species” (Whittaker, 1977) is also important to understanding soils in tropical environments. Soil diversity in the tropics is affected by an extremely wide variety of parent materials, climates, biotic interactions, landforms and geomorphic elements, and soil ages. Many of these ecological factors that affect soils vary more widely in the tropics than in the temperate zone (Sanchez, 1976; Richter et a l . , 1985), and tropical landscapes commonly contain soils that range in age from amongst the youngest (e.g. Entisols and Inceptisols) to the oldest (e.g. Ultisols and Oxisols). Conceptual models of soil formation (Simonson, 1959; Jenny, 1941, 1980) also predict that environments present in tropical regions result in extreme heterogeneity in soil-ecological processes, soil properties, and consequently soil taxa. Such soil heterogeneity is expressed on regional landscape to microsite scales (Drosdoff, 1978; Buol et al., 1989). Despite ample evidence for the heterogeneity of soils in the tropics, the popular concept of a “tropical soil” often suggests that between the tropics of Cancer and Capricorn, ihere are special groups of soils with specific properties and problems. Early European natural historians and scientists, impressed by the prolific forest vegetation of the humid tropics, often concluded that tropical soils were extremely rich and fertile. Alfred Russel Wallace (1853) in Travels on the Amazon and Rio Negro exclaimed about Amazonia, “For richness of vegetable productions and universal fertility of soil, it is unequalled on the globe ..... Partly in reaction to this early exuberance about the fertility of tropical soils, many subsequent observers have fervently claimed that after native vegetation is cleared, “tropical soil” was extremely fragile and incapable of sustaining land development and management. Soils that support tropical rainforests have been commonly described with words such as “deserts covered by trees” (Goodland and Irwin, 1975). In this paper we emphasize that soils in the tropics can no longer be generalized as being universally fertile or dangerously fragile; they can, however, be generalized by their remarkable diversity.
SOIL DIVERSITY IN THE TROPICS
“Tropical soil” is commonly considered to be a deep, red, friable soil that is acid, well drained, and lacking distinct horizons. This soil is often associated with the Oxisol soil order of the Soil Taxonomy system (Soil Survey Staff, 1975, 1978a; Buol and Eswaran, 1988). Oxisols are similar although by no means identical to Latosols in the 1938 USDA soil classification system (Baldwin et al., 1938; Kellogg, 1949; Cline, 1975), Latossolos of the Brazilian classification system (Carmargo et al., 1986), Ferralsols in the FAO/UNESCO (1974) system, and Ferrallitic soils in the French system (Duchaufour, 1982). Such soils are often inferred to dominate tropical landscapes. Laterization is said to control such soils, an old and persistent idea that suggests that the intense leaching of the humid tropics has depleted the typical soil profile of nearly all nutrients, destroyed all clay minerals by dedication, and thereby concentrated sparingly soluble metal oxides of iron and aluminium. Laterization has also been suggested potentially to turn soils into a hardened mass of laterite following clearing of original vegetation (McNeil, 1964). In the past, laterite was considered to be a soil type toward which nearly all “tropical soil” was evolving. Such ideas about soils in the tropics retain a curious but persistent currency, despite highly persuasive papers and books that describe the diversity of soil conditions in this large and heterogenous region (Sanchez and Buol, 1975; Sanchez, 1976; Drosdoff, 1978; Buol et al., 1989; Greenlands, 1981). Over the last century, concepts of tropical soils have tended to oversimplify their taxonomy (e.g., McNeil, 1964), and to emphasize a relatively straightforward relationship between regional climate and soil (Fig. 1). Despite this frequent oversimplification, the technical scientific literature about soils in the tropics has actually contained a variety of perspectives about soil formation and climate influences (Hilgard, 1906; Sibirtev, 1914; Joffe, 1936; Jenny, 1941; Kellogg, 1949; Mohr and Van Baren, 1954; Sanchez, 1976). Climate factors are obviously important to soil development, but so are parent materials, landforms, hydrology, biological organisms, and soil age. Popular notions about “tropical soil” typically diminish the complexity of the interactions among these environmental factors. Soils are highly dynamic biogeochemical systems, influenced by multiple interacting factors. We make no pretence that our critique of the concept of a “tropical soil” is a new idea. A long and perceptive series of publications emphasize the diversity of soils in the tropics. Such a perspective has been expressed regularly throughout the twentieth century. Hilgard’s (1906) classical soil-ecology treatise, Soils,described a variety of the soil conditions in the tropical world and emphasized the notable absence of reliable data about soils in the tropics. Laterite has often been viewed as the representative “tropical soil” (Sibirtzev, 1914), but even Hilgard
D. D. RICHTER AND L . I . BABBAR
( . I
P a 4
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Increasing moisture -+
5 g E
' + 8
W Y, W
. Un* I I",
4 'GREY-BROWN PODSOLICSOILS % f: % k' LATERlTlC RED/ f $ 5 YELLOWSOILS Y LATERITES 5 w
Fig. 1 Historic diagrams that emphasize the influence of climate on vegetation and on the formation of "zonal" or "normal" soils (Sibirtzev, 1914; Marbut, 1935). Such diagrams relating climate, vegetation, and soils are directly inherited from nineteenth ideas of Dokuschev and Sibirtzev (Sibirtzev, 1914), and were used in this fashion in influential ecology and soil texts, including Lutz and Chandler (1946), Oosting (1956), Odum (1971), and Whittaker (1975).
SOIL DIVERSITY IN THE TROPICS
(1906) was puzzled by claims that laterite soils were supposed to be the most characteristic soils of the tropics. In the 1930s, Hardy (1933) argued that laterite occupied a relatively minor area of the tropics, and based on South American and African experience, De Camargo and Vageler (1937) vigorously contended that despite a so-called “logical conclusion” that laterite should occur throughout intertropical regions, there was nearly an “absolute absence of real laterite in the whole region.” De Camargo and Vageler (1937) criticized soil maps that were based on extrapolations from broad patterns of temperature, rainfall, and geology, which they claimed overestimated the occurrence of laterite, and also underestimated the content of soil organic matter. In the 1940s, Pendleton and Sharasuvana (1942, 1946) also criticized generalizations made about so-called “laterite tropical soils”, especially those descriptions about laterite and latosols “found in leading works on soils”. Some more recent and notable publications that describe many aspects of tropical soil diversity include Nye (1954), Nye and Greenland (1960), Travernier and Sys (1965), Sombroek (1966), D’Hoore (1968), Buol (1973), FAO-UNESCO (1974, 1988), Bornemisza and Alvarado (1974), Sanchez and Buol (1975), Soil Survey Staff (1975, 1986, 1987a, b), Sanchez (1976), Lepsch et al. (1977), Van Wambeke (1978), Drosdoff (1978), Cole and Johnson (1978), Perez et al. (1979), Greenland (1981), El-Swaify er a l . (1982), Bleeker (1983), Burnham (1984), Cochrane et al. (1985), Lathwell and Grove (1986), Vitousek and Sanford (1986), La1 (1987), and Buol et al. (1989). Although there have been many soil surveys and major conceptual developments since Hilgard’s time (1906), similar observations can still be made today; that there is a diversity of soil conditions in the tropics, yet a notable absence of reliable soil data in many tropical regions. Many landscapes of tropical America and Asia have now been systematically surveyed at a relatively small mapping scale (<1:1 million), yet major concentrations of unsurveyed soils in the tropics remain, especially in tropical Africa. In recent decades, soil science has lost many of its parochial perspectives and temperate-zone biases. Increasing numbers of soil scientists and ecologists from tropical nations have provided substantial pressure for making soil concepts more realistically global in application. Nonetheless, notable similarities exist between the criticisms of “tropical soil” in the 1930s and 1940s (Hardy, 1933; De Camargo and Vageler, 1937: Pendleton and Sharasuvana, 1942, 1946) and those made in more recent years (Sanchez, 1976; Drosdoff 1978; Buol et a l . , 1989). We are impressed by the persistence of misconceptions about “tropical soil”, but also with the rapid scientific developments made in recent years. Even a book such as Soils of the Humid Tropics (National Academy of Science, 1972), an excellent “summary of present knowledge” written in
D . D . RICHTER AND L. I . BABBAR
the early 1970s by widely respected scientists, now gives the reader a clear sense of how rapidly soil science and ecology of tropical ecosystems have progressed in recent years. The objectives of this paper are to evaluate common misconceptions about “tropical soil” that are of ecological significance, to consider reasons why such misconceptions have persisted over many years, and to evaluate quantitatively a hypothesis originally made by Sanchez and Buol (1975) that soils in the tropics are noted for their marked taxonomic diversity. Heterogeneity of soils throughout the tropics is quantified by estimating areal extents of soil taxa from recent soil maps of tropical America, Africa, and Asia. Soil diversity in the humid tropics is evaluated by examining the first comprehensive soil maps of the Brazilian Amazon River basin, a 500 million hectare region where systematic soil surveys have only recently been completed. Geographic Information Systems (GIS) are used to compare the FAO/UNESCO (1971, 1974) and the more recent Brazilian soil maps of Amazonia (EMBRAPA, 1981; Carmargo et al., 1986).
11. CHANGING PERSPECTIVES ABOUT SOIL TAXONOMY In order to understand more fully soil diversity in the tropics, an understanding of soil taxonomy and classification is mandatory. Traditionally, soil taxonomy has been strongly guided by ideas about soil genesis and formation. In other words, environmental factors have traditionally been used to provide the structure of soil taxonomy. This soil taxonomic emphasis on the factors of soil formation is a distinctly nineteenth century Russian concept that has dominated soil and ecological sciences from the 1880s until at least the 1950s. Great soil groups (major soil types) such as Podzols, Chernozems, Tundra soils, and Latosols have been recognized by soil scientists and ecologists, from the time of Dokuschaev and his colleagues in Russia (Sibirtzev, 1914) to Whittaker (1975), the noted American ecosystem ecologist. Traditionally, most great soils groups were associated with major bioclimatic regions. Latosols, a soil considered to exist almost entirely in the “torrid and humid tropics” (Sibirtzev, 1914), were often conceived as the typical soil type that forms under hot and humid tropical climates (Fig. 1). Although Dokuschaev is widely recognized for developing the concept of five basic soil-forming factors (climate, parent material, biology, topography, and soil age), the predominant factor actually emphasized by Dokuschaev was climate (Fig. 1). This emphasis on climate has had major implications for how soils in the tropics have been popularly perceived.
SOIL DIVERSITY IN THE TROPICS
Classifying soils using soil genetic concepts was often a deductive process since representative and reliable soils data were often lacking, especially in the tropics. The absence of reliable soil composition data for representative soils in the tropics has been consistently noted by many critical thinkers, at least as far in the past as Hilgard (1906). Even until the 1980s, for example, systematical soil surveys and soil maps of the Amazon basin (approximately 10% of the tropics as a whole) were entirely non-existent. Comprehensive soil surveys are still non-existent for soils in the Zaire River basin in central Africa. Many soil maps in the tropics have thus been based on general climatic, vegetative, and physiographic information, rather than on observations of the soils themselves. Major soil taxonomic problems were created because soil classification was initially based on inferred factors of soil formation rather than on empirical soil data. Since genesis of nearly all soils has occurred over long periods of time (i.e. in the past), many soil forming factors can only be inferred and are not evident for specific soils (Smith, 1983). For good reasons, the historical emphasis on soil genesis and formation has recently been radically altered by the publication and use of Soil Taxonomy (Soil Survey Staff, 1975; 1987b). The Soil Taxonomy system and a number of other modern classification systems, e.g. the Brazilian classification system (Carmargo et al., 1986, have directed attention to quantifiable soil data and away from speculative factors of soil formation. This change in perspective represents a fundamental change in the scientific paradigm of soil taxonomy, a change that is described well by Kuhn’s (1970) definition of scientific revolution. The change in perspective has had significant implications for understanding of soils in general (Smith, 1983), but especially for soils in the tropics. Since relatively little quantitative data have been available to describe soil conditions in the tropics, soils in the tropical environment have been subject to much misleading speculation. The quantitative approach of the Soil Taxonomy system has put priority on detailed soil surveys and descriptions such as the recently completed Brazilian soil surveys of the Amazon basin (EMBRAPA, 1981). The Soil Taxonomy system has forced soil and ecological scientists to collect empirical data on the soils themselves, to re-examine old ideas about soils in the tropics, and to continue to develop new soil concepts (Soil Survey Staff, 1986, 1987a, b; Buol and Eswaran, 1988). The new Soil Taxonomy system is organized into eleven soil orders, all of which are well represented in the tropics. Soil orders are briefly described in Table 1 and are concepts discussed throughout the paper. In summary, the popular notion of “tropical soil” developed directly from the traditional paradigm of a soil taxonomy that was based on soil-forming factors. The recent change in taxonomy to quantitative and
D . D. RICHTER AND L . I . BABBAR
Table 1 Eleven soil orders in soil taxonomy describe all of the earth’s soils; all eleven orders can be found in the tropics (Soil Survey Staff, 1987a,b; Leamy et al., 1988) Order
Soils with clay-enriched subsoils (argillic or kandic) with moderate to high pH and base saturation (35%).
Soils derived from volcanic ash with > 35 cm organic-rich surface soil horizon (andic).
Soils dry > 50% of the year with no organic rich surface horizon.
Soils with little if any soil profile development.
Soils with > 30% organic matter to a depth of > 40 cm. Soils with incipient soil profile development (more development than Entisol).
Soils with deep surface horizon with high organic matter and moderate to high pH and base saturation (mollic).
Soils with little texture difference between surface and subsoil horizons with low cation exchange capacity and high iron and aluminium oxide content (oxic).
Soils with subsoil horizon having high accumulations of organic matter, iron, and aluminium (spodic).
Soils with clay-enriched subsoils (argillic or kandic) with low pH and base saturation (35%).
Soils with > 30% clay in all horizons with large shrink-swell properties upon drying and wetting, respectively.
descriptive approaches marks a fundamental change that greatly impacts soil and ecological sciences. This change has yet to be fully incorporated into the discourse of soil scientists and ecologists. Our intention in this paper is to encourage the process of paradigm change in soil taxonomy, especially among soil and ecological scientists with interests in the tropics.
111. THE DEVELOPMENT OF MISCONCEPTIONS ABOUT “TROPICAL SOIL” Soil science developed rapidly in the second half of the nineteenth century, largely in temperate zone regions of Russia, western Europe,
SOIL DIVERSITY IN THE TROPICS
and North America. Traditional definitions and theoretical concepts about “what soil is” and “how soil develops” were derived almost entirely from this temperate-zone experience. Although many early studies of soils in the tropics to the warm temperate zone can be credited for their insights (e.g. Hilgard, 1860; Marbut and Manifold, 1926; Mohr, 1944), soil science and ecology benefited little from soil conditions in tropical environments. This early temperate-zone bias was a serious deficiency for the development of soil science and ecology, especially considering that the terrestrial tropics cover about 40% of the earth’s terrestrial surface. Ideas about tropical soils were for many years closely associated with laterite, a geologic material (not a soil) first described in southern India by Buchanan (1807). Buchanan (1807) described laterite as one of the most valuable materials for building. It is diffused in immense masses, without any appearance of stratification, and is placed over the granite that forihs the basis of Muluyulu. It is full of cavities and pores, and contains a very large quantity of iron in the form of red and yellow ochres. In the mass, while excluded from the air, it is soft, that any iron instrument readily cuts it, and is dug up in square masses with a pick-ax, and immediately cut into the shape wanted with a large trowel, or large knife. It is (sic) very soon after becomes hard as brick, and resists the air and water much better than any bricks that I have seen in India . . . (1)n several native dialects, it is called the brick-stone (Zticu cullu). Where, however, by washing away the soil, part of it has been exposed to the air, and has hardened into a rock, its color becomes black, and its pores and inequalities give it a kind of resemblance to the skin of a person affected with cutaneous disorders; hence in the Turnul language it is called Shuri cull, or itch stone. The most proper English name would be Laterite, from Lateritis, the appellation that may be given to it in science.
Buchanan obtained the term “laterite” from later, meaning “brick” in Latin. By the late nineteenth century, the term laterite was in general use throughout India with detailed descriptions recorded (Meldicott and Blanford, 1879). In fact, great interest in laterite was stimulated throughout the tropical world (Fox, 1923; Harrassowitz, 1926). By the end of the nineteenth century, laterite had also been identified in surficial formations in South America, Africa, and Australia (Sivarajasingham et al., 1962). Maps of laterite suggested very wide distribution (Fig. 2). In the popular mind, laterite became a soil type, and was no longer restricted to Buchanan’s (1807) narrow definition of a geologic material. Laterization became the soil process considered to be representative of “tropical soil,” and laterite became for many the material toward which soils in the tropics naturally evolved (Sibirtsev, 1914). These perceptions of “tropical soil” did not go uncontested. From the time of Buchahan’s first description, laterite created controversy among
D. D .
Fig. 2 Historic maps of Africa and India with areas of laterite suggested in black (after Prescott and Pendleton, 1952). Such maps reflect old notions of the wide extent of laterite across the tropics. Some authors (e.g. McNeil, 1964) have mapped laterite even more extensively than Prescott and Pendleton (1952).
soil scientists and geologists (Mohr and Van Baren, 1954; Sivarajasingham et al., 1962). Definitions of laterite ranged widely and little consistancy was followed. To many, all red soils in the tropics came to be considered as laterite or at least lateritic. Efforts were made to keep the definition of laterite as narrow as Buchanan’s, and to distinguish between laterite and lateritic on the basis of chemical composition. One example is that of Fermor (1911), who defined “laterite soils” as containing >90% oxides of Fe, Al, Ti, and Mn; “siliceous laterite soils” as containing between 50 and 90% metal oxides; and “lateritic soils” as containing 25-50% metal oxides. The terms “laterite” and “lateritic” received very wide use by soil and ecological scientists, following the
SOIL DIVERSITY IN THE TROPICS
adoption of these terms as names of soils in the 1938 soil taxonomic system of the US Department of Agriculture (Baldwin et al., 1938). In 1949, the concept “Latosol” was proposed by Kellogg (1949) as a major soil type and as an amendment to the 1938 USDA classification system. Kellogg’s (1949) intention for the term “Latosol” was to help avoid real and semantic problems that surrounded the terms laterite and lateritic. Kellogg (1949) very tentatively proposed the concept of Latosol, specifically because of his high regard for the taxonomic uncertainties presented by the diversity of soils in the tropics. Nonetheless, the Latosol proposal proved very controversial (Prescott and Pendleton, 1952; Mohr and Van Baren, 1954). Despite controversy and an absence of accepted definitions and terminology, a set of soil characteristics became closely associated with “tropical soil”, characteristics that are often synonymous with the process and products of laterization. These soil characteristics include: (1) extremely intense weathering and leaching; (2) low contents of soil organic matter; (3) destructive weathering of soil alumino-silicate clay minerals; (4) low nutrient retention capacity; (5) an ability to harden irreversibly upon exposure to sun and air; and (6) a homogeneity in physical and chemical properties, both horizontally over the landscape and vertically within the soil profile. These six soil characteristics have veracity when applied to certain soils found in the tropics, however, they cannot be used to generalize about the soil conditions found within the 5-billion hectare tropical landscape. Many soils in the humid tropics are, of course, highly leached and intensively weathered; some have relatively low contents of organic matter and alumino-silicate clay minerals, and thus low cation exchange capacity. Many highly weathered upland soils in the tropics have relatively little apparent distinctions between horizons, and are red and well drained. Relatively few soils, however, appear to have problems with irreversible hardening of laterite. Because these six characteristics are so often associated with “tropical soil”, each is briefly described and evaluated.
I . Intense Weathering and Leaching Over pedogenic time, soil leaching by intense and abundant rainfall at relatively high temperature removes relatively soluble soil constituents such as mineral nutrients and even silica. Leaching and dedication remove nutrients from soils, and concentrate the low solubility oxides of iron, aluminium, titanium, chromium, and nickel. The rate at which leaching and weathering can form Oxisols has been greatly overestimated in the past (Van Wambeke et af., 1983), an overestimate that developed mainly from ideas about soil formation rather than from data
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derived from soils themselves. Van Wambeke et al. (1983), for example, emphasize that because clay minerals of Oxisols (Table 1) are often dominated by kaolinite rather than gibbsite, the desilication of Oxisols even in the humid tropics may seldom reach the intensity required for gibbsite [Al(OH)3] domination of the soil clay fraction. In fact, although Oxisols represent soils in advanced stages of weathering, they are most commonly found in materials that have been transported and pre-weathered during earlier geologic periods (Buol et al., 1989). The materials that compose Oxisol soils may often have been subjected to several leaching and weathering cycles. Moreover, Oxisols that are not formed from transported, pre-weathered materials are typically derived from geologic materials that weather rapidly, e.g. basalts. In sum, although many soils in the tropics are weathered very intensively, soils in the tropics can best be generalized by their extremely wide range of weathering states. The pedogenic time and the intensity of weathering that are required to form Oxisols should not be underestimated.
2. Low Soil Organic Matter Accumulation of soil organic matter (SOM) results from the balance between carbon inputs and outputs to and from soil; i.e. between plant production and microbial respiration of plant products. Soil organic matter has long been suggested to be relatively low in “tropical soil”, due to the idea that at the relatively high temperatures of the tropics, microbial respiration and decomposition may be stimulated more than plant production (Jordan, 1985, 1988). However, there are relatively few data to support this contention, and the SOM of forested soils in the humid tropics may frequently exceed 16 kgC/m2 in the surface metre depth of soil (Post er a l . , 1982; Zinke et al., 1984), a substantial amount of soil C for mineral soils (Soil Survey Staff, 1975). The idea that soils of the tropics have relatively low SOM is criticized by a number of scientists in recent years (Anderson and Swift, 1984; Sanchez et al., 1982; Buol et al., 1989). In fact, Sanchez et al. (1982) indicate that SOM in Mollisols, Alfisols, and Ultisols (Table 1) in temperate climates differs little from comparable soils in the tropics, and that SOM in Oxisols of humid tropical climates is no lower than in similar textured soils on comparable landforms in temperate zones. Much is not understood concerning soil organic matter; for example, some Oxisols on upland landforms have very large acculmulations of SOM in deep, “humic” surface horizons, apparently indicating that soil clay minerals have stabilized and protected organic compounds against decomposition (I.F. Lepsch, personal communication). Detailed analyses of the very large soil data set assembled by Post et al. (1982) and updated in Zinke et al. (1984), reveal two points about
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SOM in soils of the tropics. First, SOM in soils is highly variable. Mean estimates of SOM shown in Fig. 3 for soils with > 24 "C and annual PET/PPT ratios > 0.5, have standard deviations that approach 100% of the mean, despite relatively large sample sizes ( n > 120). Second, there is little indication that SOM is lower in the tropics compared to ecosystems in the temperate zone that have comparable soil moisture regime, i.e., that have similar PET/PPT ratios in Fig. 3. Controlling for soil moisture regime, the largest and most comprehensive global soil collection indicates that SOM in the tropics appears similar in content to that in soils of subtropical, warm, and cool temperate climates. Although plant productivity and microbial respiration are strongly controlled by temperature and moisture regimes, the interactive effects of temperature and moisture on the balance between plant productivity
E 0 C
Annual PET/PPT ratio
Fig. 3 Soils with tropical climatic regimes appear no lower in carbon content than those with temperate climatic regimes, as estimated in the large soil data set of Post el a l . (1982) and Zinke et al. (1984). Temperature effects on soil carbon are most apparent only at low temperature regimes. Biotemperature is the Holdridge variable that is related to mean annual temperature (Post et a l . , 1982). In contrast to temperature effects, the data indicate that soil moisture regime (expressed by annual potential evapotranspiration/precipitation ratio) strongly influences accumulation of soil organic carbon. Soil texture also mediates soil carbon storage but its effects are not illustrated on this diagram.
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and microbial respiration (i.e. SOM) need much better evaluation. The mediating effect of soil texture also needs consideration in these evaluations, because clay minerals often can adsorb and protect organic compounds from microbial decomposition.
3. Weathering of Alumino-Silicate Clay Minerals Contents of alumino-silicate clay minerals in some soils of the tropics are low, due to intense leaching and weathering conditions that eventually decompose weatherable clay minerals that are inherited from parent material (primary minerals) or that have formed during soil development (secondary minerals). Destruction of alumino-silicate clays is often emphasized to be nearly complete in “tropical soil”. The rate of weathering and decomposition of clay minerals has often been overestimated, as kaolinite is the most common clay mineral in many soils of the humid tropics, and is frequently observed to dominate clay mineralogy of Oxisols (Van Wambeke et a l . , 1983). Recently, kaolinite-dominated subsoils have received detailed research attention (Moormann, 1985), and such work has recently led to the recognition of a new diagnostic soil horizon in Soil Taxonomy (Soil Survey Staff, 1987a). These are kandic horizons (kaolin-dominated) and are relatively common in the humid tropics, sub-tropics, and temperate zones. Kandic horizons will hopefully make it easier to distinguish Alfisols and Ultisols (Table 1) from Oxisols (Moormann, 1985). As more soil mapping is accomplished, kandic horizons of Ultisols (Moormann, 1985) may well replace oxic horizons of Oxisols in many areas of the humid tropics, especially in Africa (C. Sys, personal communication).
4. Low Cation Exchange Capacity A soil’s cation exchange capacity (CEC) is one of its most important attributes for controlling nutrient availability to plants and the chemistry of drainage waters. Negative charge in soil (soil CEC) also gives an ability to retain positively charged nutrient cations against leaching, and to control plant-availability of potentially toxic cations such as aluminium. Soil CEC is actually the net negative charge possessed by soils, and under acidic conditions soils may develop additional positive charge. Soils with such properties are known as variable charge soils. Although variable charge is a soil property found in soils from boreal to tropical climates, many soils in the tropics have significant variable charge and under acidic conditions may have difficulty retaining base cation nutrients (Uehara and Gillman, 1981). Variable charge means that the electrical charge possessed by soils is not constant and that at low pH, a soil’s positive charge may reduce net cation retention. Although most
SOIL DIVERSITY IN THE TROPICS
soils in the field appear to have net negative charge even at low pH (Cochrane et af ., 1985), of potentially great significance, however, are Acric Oxisols (Buol and Eswaran, 1989), soils with clays that have CEC that approaches zero (< 15 mmol,/kg of clay). Low CEC is a problem of many soils throughout the world. Whether due to low soil clay mineral content, low soil organic matter, or acidified variable charged surfaces, low net CEC gives a soil little ability to retain nutrient cations against leaching. Low CEC is thus a biologically significant soil property, and sustained productivity of low CEC soils is dependent on careful soil nutrient management. Soil management principles and technologies have been developed for low CEC soils, especially in southeastern Asia, southeastern USA, and elsewhere, and long-term management of such soils is possible with attention paid to soil organic matter, soil chemistry and fertility, and agronomic factors.
5. Irreversible Soil Hardening The popular notion of “tropical soil” has often contained the idea that once a tropical soil is exposed to the elements, it will irreversibly harden. Much attention has been devoted to the supposed threat that such laterite soil poses for agriculatural development in the tropics (e.g. McNeil, 1964; Sanchez, 1976). Recent soil surveys demonstrate clearly that soils with laterite (which is now called plinthite) occur in a relatively small portion of the tropics, exactly as predicted by De Carmargo and Vageler (1937). This is actually a major change in thinking compared to claims about laterite’s distribution that were made in the past (Fig. 2). Plinthite nodules or layers are found in a variety of soil taxa, in Alfisols, Ultisols, and Oxisols, and actually appear to be more prevalent in Alfisols and Ultisols than in Oxisols (Van Wambeke et af., 1988). Iron pans and other indurated layers also occur in a variety of soils in the tropics. It is unfortunate that scientists and other observers have spent such attention on laterite hardening as a potential problem in the tropics. Given the other soil physical problems that have nothing to do with laterite, there is no question that soil physical properties are critical to sustaining the potential productivity of soils following the clearing of native vegetation (Lal, 1987). Important physical properties of soils that are susceptible to land use degradation (unless adequate vegetation is maintained) include soil structure, porosity, and infiltration capacity. As environmental problems, each is much more significant than the problem of laterite to the long-term management of soils in the tropics.
6. Homogeneity of Soil Properties Variability of soil properties depends on the property of interest and on
D . D . RICHTER AND L . I . B A B B A R
areal and vertical scale factors (Wilding and Drees, 1983). Especially on ancient land surfaces or in old sedimentary materials, minerals may have been intensively weathered, and most nutrients long since released for plant use or to leaching. Extreme weathering tends to diminish inherited differences in soil materials in oxic horizons. However, the notion that “tropical soil” has properties that are somehow homogeneous, is closely associated with soil genetic ideas that leaching and weathering have been somehow complete in soil materials found in the humid tropics. Certainly the idea that tropical soil is homogeneous over wide areas did not achieve its prominance from soil variability data such as those collected by Lopes and Cox (1977) or Wilding and Drees (1983). Considering that the tropics cover 40% of the earth’s terrestrial surface, much detailed spatial variability data have yet to be collected. A lack of soil horizonation is also supposed to be characteristic of “tropical soil”. Certain types of Oxisols lack distinct soil horizons, and they appear to have relatively simple soil profiles: remarkably uniform with soil depth. Presumably such soils have lost their horizons during intensive leaching of past and present weathering environments. Generalization about an absence of distinct horizonation in Oxisols can be easily exaggerated, especially since many Oxisols actually have prominant, deep horizonation. Some Oxisols have large accumulations of organic matter in surface horizons (Sanchez et a l . , 1982), sometimes to a depth of a metre o r more, due to the organic compounds being stabilized by clays. Moreover, if soils include the entire soil-weathering profile, some Oxisols with well developed horizonation may total tens of metres in depth (Eswaran and Wong, 1978). Apparent similarities among Oxisols in the tropics are often in fact misleading. Physical and chemical properties of Oxisols are often inherited from parent materials, despite long-term intense weathering (Greenland, 1981). Oxisol soils vary considerably, as is described in Table 2 which includes the numerous and varied subtaxa that the Oxisol order now contains (Buol and Eswaran, 1988). The Oxisol order now has five suborders that are based on moisture regime (Table 2), from soils with high water tables and excess water (Aquox) to those that are Table 2 Description of new soil suborders and great groups of Oxisols that demonstrate the diversity of soils within the Oxisol order (Buol and Eswaran, 1988)
Suborders or great group
A. Aquox: B. Perox: C. Udox:
Brief description Water saturated within 30 cm > 30 daysfyear Perudic soil moisture regime (moist all year, but no lengthy saturation) Udic soil moisture regime (< 90 daysfyear too dry to plant) Continued
SOIL DIVERSITY IN THE TROPICS
Table 2 Continued Suborders or great group
D. Ustox: E. Torrox:
Brief description Ustic soil moisture regime (moist > 90 days, < 270 days/year) Aridic soil moisture regime (moist < 90 days/year)
A. Aquox: Most are small in area. Acraquox: Plinthaquox: Eutraquox: Haplaquox:
> 5 in ECEC < 1.5 mEq/100 g clay within 200 cm; PHKC~ oxic horizon Continuous plinthite within 125 cm > 35% BS @ pH 7 within 135 cm Other Aquox soils
B. Perox: Precipitation year. Sombriperox: Acroperox: Eutroperox: Kandiperox: Haploperox:
Potential evapotranspiration in all months of the
High SOM in sombric horizon < 150 cm ECEC < 1.5 mEq/100 g clay; PHKCL> 5 in oxic horizon > 35% BS @ pH 7 within 125 cm > 40% clay in surface 18 cm layer with kandic horizon < 150 cm of surface Other Perox soils
C. Udox: previously called Orthox, “true” Oxisols, in 1975 Soil Taxonomy. Sombriudox: Acrudox: Eutrudox: Kandiudox: Hapludox:
High SOM in sombric horizon < 150 em ECEC < 1.5 mEq/100 g clay; PHKCL> 5 in oxic horizon < 150cm > 35% BS @ pH 7 within 125 cm > 40% clay in surface 18 cm with kandic < 150 cm of surface Other Udox soils
D. Ustox: probably the most extensive Oxisol suborder. Sombriustox: Acrustox: Eutrustox: Kandiustox: Haplustox: E.
High SOM sombric horizon < 150 cm ECEC < 1.5 mEq/100 g clay; PHKCL> 5 in oxic horizon > 35% BS @ pH 7 within 125 cm > 40% clay in surface 18 cm with kandic < 150 cm of surface Other Ustox soils
Torrox: high B.S.; often excellent crop soils when irrigated.
Acrotorrox: Eutrotorrox: Haplotorrox:
ECEC < 1.5 mEq/100 g clay; PHKCL> 5 in oxic horizon > 35% BS @ pH 7 within 125 cm Other Torrox soils
ECEC is effective cation exchange capacity; pHKa is soil pH in 1 M KCI; BS is base saturation at soil pH of 7; and SOM is soil organic matter.
D . D. RICHTER AND L. I. E A B E A R
generally dry (Torrox). Chemically and physically, Oxisols also vary widely (Table 2). Texture ranges from low to very high clay contents, and from relatively nutrient-rich (Eutro- great groups) and to those that have extremely low CEC (Acr- great groups). Conceptual problems often arise because the six characteristics discussed above are used to generalize about soils on regional scales in the tropics, far beyond any bases in observed fact. Popular misconceptions about soils in the tropics have persisted for at least four reason: (1) it has been an exceedingly complex task to characterize and map the soils on the 5-billion hectare tropical landscape; (2) scientists of different scientific disciplines have not communicated effectively; (3) no common soil taxonomy has ever existed that could classify soils and provide a common language for discussing soil problems throughout the tropical world; and (4) perhaps most importantly, nearly all soil taxonomists in the nineteenth through the mid-twentieth centuries have emphasized the factors of soil formation rather than the quantitative properties of soils themselves.
A. The Enormous Challenge of Mapping Soils on the 5 Billion Hectare Tropical Landscape Obtaining a representative view of the soils on 5 billion hectares of tropical lands has been an extremely difficult task. Because the origins of soil science were in the temperate zone, relatively few soil scientists continuously studied soils in the tropics. Not only is the area large, but much of the tropics is difficult to traverse, especially those humid areas covered by forests, and reports were often confined to small areas and short-term studies. To this day, the regions of the tropics that have the least reliable soil data, e.g. the Amazon and Zaire River basin, are precisely those regions which historically have been mapped with the largest area of Oxisols (Fig. 4). There have been few historical incentives for surveying soils in these areas supposed to be dominated by Oxisols. As a result, soils in the rainforests of Amazonia and in the Zaire River basin were mapped using general ideas about climate, vegetation, and physiography, i.e. by factors of soil formation, rather than by directly observed soil data. A major exception to this pattern has resulted from the Brazilian government’s decision in about 1970 to stimulate development of the Amazon basin. Systematic soil surveys (in addition to mapping and classification of vegetation, geomorphology, and land suitability) were initiated by the Brazilian government and took about a decade to complete. The results will be improved in the years to come, but in the short-term the work is nothing less than a major advance for soil science
SOIL DIVERSITY IN THE TROPICS
and ecology. What were large, highly uncertain mapping units of Oxisol covering most of Amazonia, have given way to a variety of soil mapping units that are based on directly observed soil profiles. The Amazonian soils of terra firme, the upland soils between the rivers, have finally been examined with some degree of comprehensiveness. These data are evaluated in detail later in this paper, because the results have much to say about soil diversity in the tropics, especially that within the humid tropics. Similar comprehensive soil surveys have yet to be conducted in the Zaire River basin of Africa.
B. Interdisciplinary Miscommunication about Soils in the Tropics At least some of the problems in developing realistic ideas about soils in the tropics are associated with poor communication among scientists of different disciplines that have a purview over natural resources of the tropics. Historically, interdisciplinary communication has been notably deficient between soil fertility specialists and soil taxonomists, and even worse between general soil scientists and ecologists. This latter deficiency is especially significant considering that soil science is fundamentally a biological and ecological science. Lack of communication between soil scientists and ecologists has allowed overgeneralizations about soils of the tropics to persist within scientific communities that have had little contact with the latest developments in the soil sciences. Large numbers, but certainly not all, of soil scientists and ecologists appear to be generally aware of the remarkable diversity of soils in the tropics. The same may not yet be the case for social scientists who work with the economics and policies that affect natural resource management in the tropics. Nevertheless, many ecologists (including soil scientists) give credence to the popular notions of “tropical soil”, although most of the same scientists would no doubt discount the meaning of a so-called “temperate soil”. Given the many years over which both natural and social sciences have largely ignored the diversity of soil resources in the tropics, an interdisciplinary effort is needed to describe and evaluate tropical soil resources more realistically. More realistic ideas about soils in the tropics are particularly appropriate for new editions of soils and terrestrial ecology textbooks.
C. The Tower of Babel Effect of Too Many Soil Taxonomies and Nomenclatures General understanding of the soils in the tropics has been stimied by the large number of languages, classification systems, and nomenclatures
FAO/UNESCO Soil Map Reliability class1
ClaSsIII (van Warnbeke et al..
Fig. 4 The reliability of 3il mapping units ,aried considerably for the FAO/UNESCO (1971) map of South America, but I marked correlation existed between Ferralsols (Oxisols) and the regions of least map reliability (Amazonia). Soil mapping units with reliability class I were based on soil surveys, in which map-unit composition and boundaries were based on actual field observations of soils. Mapping regions with reliability class I1 were based on soil reconnaissance, in which boundaries were based to a large extent on topography, geology, vegetation, and climatic data; the composition of map units were based on field observations. Reliability class 111, indicated that only general information was used to construct both boundaries and composition of each soil map unit.
Fig. 5 The FAONNESCO map of Africa relied on soil data of vastly different qualities, from detailed soil surveys such as that of Burkina Faso (reliability class I) to general information such as that of Mali (reliability class 111). See caption for Fig. 4 for description of reliability classes. As a consequence, detail of mapping unit borders and density of mapping units were much greater in Burkina Faso than in Mali.
D. D. RICHTER AND L. I . BABBAR
that have been used by soil scientists and ecologists working in the tropics. Even by the mid-l950s, there was serious uncertainty about whether soil science was adequately developed to devise a taxonomy that was applicable not only to known types of soils in the tropics, but that was also flexible enough to accommodate new concepts and types of soils yet to be discovered (Mohr and Van Baren, 1954). In 1948, for example, an International Conference on Tropical and Subtropical Soils was held at Rothamsted Experiment Station in England, to promote mutual understanding of soils in the tropics. According to Mohr and Van Baren (1954), however, the gathering had significant communication problems caused by the numerous taxonomic and soil classification systems and technical vocabularies used by participants. Since even well informed delegates had difficulty understanding soil systems, methodologies, and terminologies that were highly parochial, the conference strongly recommended the need for a unified system of soil classification and nomenclature applicable throughout the tropics. Sanchez (1976) used the analogy of the Tower of Babel to describe the problems faced by scientists in the tropics working with soils that were described by an extremely wide variety of soil taxonomies and nomenclatures. Although there are still a large number of classification systems still in use, Soil Taxonomy and the FAO/UNESCO classification are two systems that are increasingly used, and correlation of soil types among classification systems has been an important activity for soil scientists. Table 3 contains approximate correlates of four major systems currently in use that are suited to characterize soils in the tropics: the FAO/UNESCO system, Soil Taxonomy, the Brazilian system, and the French ORSTOM classification. Technically, the FAO/UNESCO system is not a detailed taxonomic system, but rather a mapping system with a limited number of map units. Other major taxonomic systems that are used in various parts of the tropics include the Australian systems (Stevens, 1962; Northcote, 1960; Moore et al., 1983); the Belgian system of INEAC (Sys, 1960); the Soviet system (Gerisimov et al., 1974); and the 1938 USDA system (Baldwin et al., 1938; Thorp and Smith, 1949; Kellogg, 1950). Discussions about correlating soil taxa are found in Sanchez (1976), Buol et al. (1989), and Duchaufour (1982). In recent years, two systems, Soil Taxonomy and the FAO/UNESCO system have increasingly been used as the common international systems of soil classification.
D. Emphasis on Factors Rather Than on Effects of Soil Formation: The 1938 Soil Classification Despite the challenge of mapping 5 billion hectares of soils, a lack of communication among scientific disciplines, and the confusion caused by
Table 3 Taxonomic correlations among four major classification systems for soils in the tropics based on 1974 FAO/UNESCO Soil Map of the World (Sanchez, 1976; Aubert and Tarvenier, 1972; Duchaufour, 1982; Buol et al., 1989; Beinroth, 1974; Moss, 1968; Carmargo et al., 1986, Soil Survey Staff, 1987b) ~~~
1974 F A O ~ E S C OSystem
1975 (1987) Soil Taxonomy
French (ORSTOM) System
Latossolos with latosolic B < 6.5 mEq/100 g clay CECe
Sols ferralitiques fortement desatures Sffd, typiques ou humiferes
Orthic F. or Acric F.
Latossolos Vermelho Escuro
Sffd, typiques ou humiferes
Orthic F. or Acric F.
Latossolos Vermelho Amarelo
Sffd, typiques ou humiferes
Sffd, typiques ou humiferes
Eutrustox or Eutrudox
Latossolos Roxo o Terra Roxa Legitima
Sffd, typiques ou humiferes derives de basalte
Podzolicos Vermelho Amarelo Distroficos
Sols ferralitiques fortement et moyennement desatures; eluvies
Various lithic subgroups
Lithosols et sols lithiques
Areis Quartzosas Regossolos Plintos- Sols ferralitiques moyennement on solos fortement desatures de texture sableuse
Podzolico Vermelho Amarelo Equivalente Eutrofico y Terra Roza Estruturada
Sols ferrugineux tropicaux lessives
Table 3 Continued 1974 FAO/UNESCO System
1975 (1987) Soil Taxonomy
French (ORSTOM) System
Sols mineraux brut et Sols peu evolue d’apport eolien
Cambissolos (incipient B horizons)
Sols ferralitiques faiblement desatures, rajeunis et Sols ferrugineux tropicaux
Sols ferralitiques fortemente et moyennemnet desatures, humiferes, rajeunis
Sols ferralitiques fortement et moyennement desatures, rajeunis (p.p.)
Podzolicos Vermelho Amarelo
Sols ferralitiques fortemente et moyennement desatures; eluvies
Podzolico Vermelho Amarelo equivalente y Terra Roza Estruturada
Sols ferrugineux tropicaux lessives
Various aquic suborders
Sols mineraux brut et Sols peu evolue d’apport alluvial et colluvial
Paleudalfs Paleustalfs Aqualfs Aquults Argids Argialbolls
Planossolos (hardpan below A horizon)
Sols ferrugineux tropicaux lessives (P.P.)
Solos Salinos (natric B horizon)
Sols calcimagnes imorphiques
Natargids Natrustalfs Natralbolls
Solonetz Solodizados (natric B horizons)
D. D. RICHTER A N D L. I . B A B B A R
too many soil taxonomies, perhaps the single greatest impediment to the development of realistic ideas about soils in the tropics was the general direction in which soil science itself developed as a science. In the mid and late nineteenth century, Dokuchaev in Russia and Hilgard in the USA originated the seminal ideas about how the complex of environmental factors controls soil development. Both, however, actually stressed climatic factors in soil formation, an emphasis that diminished the importance of factors such as geology, topography, and soil age (Hilgard, 1892; Sibirtzev, 1914). The ideas of Dokuchaev and Hilgard dominated the developing science of soils and they helped create the momentum to organize early soil taxonomies according to environmental factors of soil formation (Basinski, 1959). This approach proved more a taxonomy of the factors that formed soils, than a taxonomy of soils themselves. Fundamental classification problems were created by basing soil taxonomies on soil-forming factors rather than on measurable soil properties. Worst of all, few if any quantitative criteria were set for soil taxa, and even experts had difficulties in classifying soils consistently. By 1900, Sibirtzev (1914) enlarged the climatic emphasis of Dokuchaev to emphasize effects of both biology and climate on soil formation. As a part of this perspective, Sibirtzev (1914) promoted an idea of “zonal soils”, a concept that came to dominate soil and ecological science for much of the late-nineteenth and twentieth centuries (Marbut, 1935; Whittaker, 1975). “Zonal soils” were actually an extension of Dokuchaev’s classification of “normal soils”: well drained, upland soils with well-developed horizons, that were supposed to reflect regional climatic regimes. These ideas are illustrated in Fig. 1. Major objections were raised by Milne (1935a,b) to the zonal and normal soil concepts. Milne (1935a,b) described repeating soil catenas in west Africa, and questioned whether true “zonal” soils were those on the uplands or those in the swales, because soils of both landscape positions were in a dynamic balance with regional and local climates. The concepts of zonal and normal soils demonstrated that bioclimatic factors were clearly overemphasized at the expense of the entire complex of interactive environmental factors that control soil formation. Perhaps it is instructive that Marbut’s (1935) map of “non-normal” soils of the USA (not including Alaska) included about half of the USA. Included as “non-normal” soils were poorly drained soils; soils with imperfectly developed profiles; unstable soils in mountainous landscapes; soils with fragipans, claypans, or any indurated hardpan horizons; and soils with high limestone contents. Effects of geologic materials, landforms, and hydrology were especially diminished in importance by the zonal concept. The priority that the zonal concept gave to upland soils may also have contributed to
SOIL DIVERSITY IN THE TROPICS
other soil conceptual problems, such as those mentioned by Daniels and Nelson (1987): that soil variability influenced by stratigraphy, geomorphology, and hydrology has yet to be adequately appreciated, even by most soil scientists in the late twentieth century. Some soils and ecological scientists still employ zonal concepts (e.g. Whittaker, 1975; Burnham, 1984), but from most modern perspectives of soils, concepts of zonal and normal soils are too arbitrary to be useful. On balance it even might be argued that the zonal and normal soil concepts are intellectual constructs that have obscured as much as they have enlightened. The ideas of Sibirtzev (1914) and Marbut (1927, 1935) about zonal and normal soils had a great influence on the 1938 USDA soil classification, the taxonomic system that has at least indirectly promoted the popular notion of “tropical soil”. The authors of the 1938 USDA system, led by C. E. Kellogg, were given only a short time by the US Secretary of Agriculture, Henry Wallace, to prepare a comprehensive classification of soils to be published in the 1938 Agricultural Yearbook, Soils and Men (Baldwin et a l . , 1938; Forbes, 1986). Kellogg’s group relied heavily on the ideas of Marbut (1927, 1935), and given time constraints of publication, the group developed few new concepts of soil classification. Although the 1938 USDA system was later refined (Thorp and Smith, 1949; Rieken and Smith, 1949; Kellogg, 1950), the system in many ways solidified past ideas about soils. The 1938 system and its revisions were entirely within the old paradigm of soil taxonomy in which soils were classified based on inferred factors of soil formation. The 1938 system had three major categories that were operationally useful: soil orders, soil great groups, and soil series. There were three soil orders: zonal, azonal, and intrazonal soils, categories that were taken directly from Sirbitzev (1914) and can be traced back to ideas of Dokuchaev. There were up to 36 great soil groups, up to 20 of which were zonal soils, some of which are illustrated in Fig. 1. Each great group contained soils with broadly similar soil profiles that reflected generally similar formation processes. The number of soil series are of course indefinite. With use, many shortcomings were found with the 1938 system. A lack of specific and quantitative limits for each great group allowed different experts to classify the same soil series in different great groups. Moreover, the system was not able to accommodate changes in conceptual ideas very easily. Despite important revisions (Thorp and Smith, 1949; Rieken and Smith, 1949; Kellogg, 1950), the system simply had difficulty in being applied or in being updated effectively (Beinroth, 1974). The 1938 system was nonetheless very widely publicized in the USA
D. D. RICHTER A N D L. I . BABBAR
and internationally, and was promulgated in major textbooks of soil science, ecology, agronomy, biology, forestry, as well as other disciplines. As a result, the 1938 system has promoted, especially among non-soil scientists, a relatively monolithic concept of zonal tropical soils, the red laterite of Fig. 1. For example, Fig. 1 is from the classic ecology textbook,The Study of Plant Communities by Oosting (1956), which like other texts emphasized the effects of regional climates on vegetation types and on zonal soils. Similar diagrams were rationalized by Lutz and Chandler (1946) in Forest Soils, by Whittaker (1975) in a contemporary plant ecology text, Communities and Ecosystems, and in Odum (1971) in Fundamentals of Ecology. Several generations of students have thus been educated entirely within the 1938 system. The system has had a profound effect on the way natural and social scientists think about soils, and specifically about soils in the tropics. In fact, many concepts and great soil groups of the 1938 system, such as the Latosol, continue to be used with little modification. Whittaker (1975) justified the use of the 1938 system in his classic text Communities and Ecosystems because as he argued, useful ideas about “typical” soil and ecosystem conditions could be communicated easily by using great soil groups that were associated with “typical” ecosystems. Whittaker (1975) described the tropical rainforest type as a forest supported by Latosol, a forest with a “rich nutrient economy perched (italics added) on a nutrient-poor substrate.” While important rainforest ecosystems are supported by nutrient-poor Latosols (i.e. Oxisols), Whittaker’s (1975) typification approach can easily diminish the importance of the diversity of soil taxa and edaphic conditions that support tropical rainforests, and the enormous areas of tropical rainforest supported by non-Latosol (non-Oxisols) soils (Whitmore, 1984; Cochran et al., 1985). Indirectly, the typification approach has narrowed our perspective of soil and forest vegetation in the tropics, a result that may have major practical consequences. Moran (1981), for example, argued that a narrow perspective of tropical rainforest soils has caused the potential for agricultural and forestry development to be greatly underestimated.
IV. ADVANCES IN SOIL TAXONOMY AND THE CREATION OF THE WORLD SOIL MAP A. Soil Taxonomy: A new Scientific Paradigm To understand modern perspectives of soil diversity, one needs at least some understanding of the structure and development of modern soil taxonomy and classification. By the 1950s, an international group of soil
SOIL DIVERSITY IN THE TROPICS
taxonomists initiated a new soil classification system that radically altered the taxonomy of soil. Development of the new system continued through the 1960s and 1970s. In 1975, the definitive proposal for the new system was published as Soil Taxonomy (Soil Survey Staff, 1975). Major amendments to Soil Taxonomy have been published since 1975 (Soil Survey Staff, 1987a), amendments which have demonstrated the vitality and evolution of this new approach and system. Although some soil concepts in the Soil Taxonomy system are inherited from the 1938 USDA system (Smith, 1983), the new system breaks from the past because it is quantitatively based. Prior to Soil Taxonomy, soil forming factors were emphasized in taxonomic systems; however, following the development of Soil Taxonomy, the effects of soil forming factors, i.e. quantifiable soil properties, are emphasized by most soil taxonomies. Currently, the FAO/UNESCO, Brazilian, and Australian systems are all quantitatively based, in contrast to the Soviet and French (ORSTOM) soil classification systems which are still heavily influenced by soil formation ideas (Buol et al., 1989). The Soil Taxonomy system is a flexible, multicategorical system, that is based on key soil horizons, i.e. diagnostic soil horizons, characterized by their physical, morphological, and chemical properties. Diagnostic soil horizons include mollic, histic, oxic, argillic, spodic and kandic horizons each of which is briefly explained in Table 4. In Soil Taxonomy, diagnostic soil horizons help distinguish many soils at the highest taxonomic level, i.e., the order level, as is illustrated in Table 4. A number of secondary diagnostic horizons of surface and subsoils are used to differentiate groups of soils below the order level. Twenty-one of 22 diagnostic soil horizons listed in Soil Taxonomy (Soil Survey Staff, 1975) are found in soils of the tropics, an indication of the diversity of soil taxa found in the tropics. One of the outstanding features of the Soil Taxonomy system is its conscious openendedness; a system that intentionally accommodates change and improvement without drastic revision of the structure of nomenclature. In contrast, the structure of the 1938 USDA system was typical of many older soil taxonomy systems, in that it could not survive pressures created by the expanding scientific knowledge about soils. A seminal and continuing idea of the Soil Taxonomy system was to devise a structure that would not self-destruct as new information was assimilated. Although Soil Taxonomy is not complete, it is not meant to be. A major intention of its publication was to have it used and criticized by as many people as possible (Smith, 1983). The system evolves to meet the wide range of soil conditions found throughout the world. The Soil Taxonomy system is highly organized. Its nomenclature appears complex initially, but once the system is even partly understood,
Table 4 Primary diagnostic soil horizons in Soil Taxonomy used to classify soils in both Soil Taxonomy (Soil Survey Staff, 1986, 1987a) and the FAO/UNESCO (1974) system.
Soil Taxonomy Diagnostic Horizon
Location within Profile
Soil Taxonomy Order
1974 FAO/UNESCO Primary Map Unit
Kastanozem Phaeozem Rendzina
Dark, deep (> 25 cm), friable, organic rich, with relatively high pH, and nutrient content.
Deep, accumulation of organic matter. Peat or muck.
Low cation exchange capacity with Fe and Al-oxide rich, variable charge, low activity clays; highly structured friable.
Luvisol Acrisol Nitosol
Illuvial clay, with high or low acidity (Ultisol or Alfisol, respectively). Nitosols have high argillic clays in subsoils.
Acidic coarse textured horizon, high in organic matter and amorphous Fe and Al oxides.
Mutually exclusive from oxic but not of argillic horizons. Sharp increase in low activity clay with depth; no argillans evident.
SOIL DIVERSITY IN THE TROPICS
much of the nomenclature can be translated, if the person has some experience with soils. It begins with eleven upper-level classes, called orders (Table l ) , and the remaining levels of the hierarchy are suborders, great groups, subgroups, families, and series. Specifying a soil at any level above the family automatically specifies the higher hierarchial classes to which that soil belongs. This nomenclature has the appearance of complexity, but actually contributes greatly to communication. All subdivisions of a soil order end in a characteristic syllable: for Oxisols this is -ox, for Ultisols this is -ult. Buol et al. (1989) give introductions to the different levels of the system, and more complete descriptions of the system are found in Soil Taxonomy and its amendments (Soil Survey Staff, 1975, 1986, 1987a,b). In the years since the publication of Soil Taxonomy, the system has received much use throughout the tropical world. The amendments published since 1975 are most significant because many refine concepts of soils found in the humid subtropics and tropics. Many of the amendments have originated from scientists based in the tropical world (Moormann, 1985; Forbes, 1986). A new diagnostic horizon, the kandic horizon, has been recently accepted for Ultisol, Alfisol, and Oxisol orders (Soil Survey Staff, 1986, 1987a). Major revisions have been made in the Oxisol order (Buol and Eswaran, 1988), and a new soil order, the Andisols, has been adopted to include many soils of volcanic ash origin (Leamy, 1988). Each of these three changes (the kandic horizon, Oxisol revisions, and the new Andisol order) have made the Soil Taxonomy system significantly more applicable to soils in the tropics and each are briefly described here. The kandic subsoil horizon is a new diagnostic horizon, recently introduced to ease problems in differentiating Oxisols, Ultisols, and Alfisols that have subsoils with low activity kaolinitic clay. The kandic horizon shares properties with both argillic and the oxic horizons; it is mutually exclusive with oxic horizons, but not with argillic horizons. Kandic horizons have sharp increases in clays between surface and subsoils (similar to argillic horizons required for Alfisols and Ultisols), and need not have clay skins (peds that are coated with clays) that indicate active clay movement from surface to subsoils. Kandic horizons must have clays of very low buffering capacities, i.e., C 160 mmol,/kg of clay of CEC determined at pH 7. The 1987 revisions of Soil Taxonomy make the definition of Oxisol much more detailed and have slightly broadened the Oxisol concept compared with that of 1975 (Soil Survey Staff, 1975, 1987b; Buol and Eswaran, 1988). Changes in concepts have been based on the wealth of data and experience with soil in the tropics that have gained in the last decades. Such revisions have caused some confusion; e.g. Van Wam-
D. D. RICHTER AND L. I . EAEEAR
beke (1989) questioned why some high clay soils (> 40% clay) with clayey accumulations in subsoils are now included as Oxisols. For the most part, however, the 1987 Oxisol criteria (Soil Survey Staff, 1987b) combined with the new kandic horizon should make classification of low activity clay soils more straightforward. A recent significant change in the Oxisol order was the replacement of Udox for the older Orthox (“orth” meaning the “true” or the “common” Oxisol). Buol and Eswaran (1988) describe how this change responded to the concern that more Oxisols may actually be located in drier climate regimes than previously appreciated: the so-called ortho-Oxisol may well be the Ust-ox (an Oxisol soil moist enough to grow crops for 90-270 days/year) rather than the Ud-ox (an Oxisol soil moist enough to grow crops for > 270 days/year). The new 1987 revisions of Oxisols have also added considerable detail to the order. In 1975 there were 30 subgroup taxa (the fourth level of the Soil Taxonomy system hierarchy), whereas according to Buol and Eswaran (1988), the 1987 Oxisol order has 212 subgroups. Evidence of the flexibility of the Soil Taxonomy system is the recent inclusion of an 11th soil order, the Andisols or volcanic-ash soils. This is the first additional soils order added to the system since publication of Soil Taxonomy (Soil Survey Staff, 1975). Soil and ecological scientists throughout the world, many from the tropics, have argued that chemical and physical properties of such volcanic soils (formerly classified as Andepts, a suborder of the Inceptisol order), warranted an upgrade to order status to be adequately classified (Forbes, 1986; Smith, 1968).
B. FAO/UNESCO Soil Map of the World Because of the practical and theoretical importance of soil taxonomy, the classification and mapping of the world’s soils have a long-term activity of the International Society of Soil Science (ISSS). Marbut (1927) presented an initial scheme for world soil classification at the 1st International Congress of ISSS. Mapping of the world’s soils emerged as a top priority of the 6th International Congress held in Paris in 1956. At the 7th International Congress held in Madison, Wisconsin in 1960, soil maps of each continents were presented at scales of 15 million to 1:lO million. At the Madison meeting it was recognized that the complexity of the task had been previously underestimated. Nomenclature, survey methods, legends, and systems of classifications varied so widely that comparisons between and within continents were very limited in their usefulness. In response to recommendations of the 7th ISSS Congress, F A 0 and
SOIL DIVERSITY IN THE TROPICS
UNESCO agreed to join ISSS in preparing a comprehensive world soil map to be based on the most complete soil survey data and field correlation available. The FAO/UNESCO world maps were eventually based on over 600 individual soil maps and many different mapping systems. Considerable attention had to be given to development of the map legend, mapping units, and nomenclature (FAO/UNESCO, 1974). It is no coincidence that like the Soil Taxonomy system, FAO/UNESCO categories were based not on speculative concepts of soil information, but on quantitative criteria of diagnostic soil horizons (FAO/UNESCO, 1974). This was clear evidence for the success of the new quantitative soil taxonomic paradigm. To quantify soil diversity in the tropics we will evaluate mapping data of the FAO/UNESCO map of the world. Although the FAO/UNESCO map unquestionably contains the most comprehensive soil data of the tropics, the classification system has three characteristics that are important to interpreting the data of the FAO/UNESCO world soil map: (1) the system is not a complete classification system; it has at most three hierarchical levels; (2) the system is not very flexible, although the original legend, but not the map, has been revised once (FAO/ UNESCO, 1988); and (3) the system is based on national and regional soil surveys that have a wide range of quality.
1. A Limited Soil Classification The FAO/UNESCO system is not a complete soil classification; it has only two and at most three levels of organization. Primary FAO/ UNESCO map units like the soil orders of the Soil Taxonomy system, are grouped by diagnostic soil horizons. For example, the primary map unit of Ferralsol includes soils with a diagnostic oxic-like ferralic horizon, whereas the primary map unit of Acrisols and Luvisols (mainly Ultisols and Alfisols, respectively) includes soils with diagnostic clay-enriched (argillic) subsoils that are acid or non-acid, respectively. Secondary map units of the FAO/UNESCO system are used to account for variation within primary units. Humic Ferralsols o r Humic Acrisols are Ferralsols or Acrisols with deep surface soils (> 25 cm) and large carbon accumulation throughout the surface soil (> 0.6% carbon throughout the surface horizon). Soil texture and slope data are also included in mapping units. The FAO/UNESCO system, however, as a two and at most three-tiered system, contains no application to local soil series. At 1:5 million or 1:l million scale, it has no ability to serve local land-use purposes. The system should not be criticized for objectives it does not claim to have, for it supplies considerable information for a two- or three-level classification system used to map soils.
D . D . RICHTER A N D L. I . BABBAR
2. System Flexibility The FAO/UNESCO system lacks much of the flexibility possessed by the Soil Taxonomy system. The FAO/UNESCO system has been updated once since its publication in 1974 (FAO/UNESCO, 1988), although the maps remain as originally published. The next soil map of the world may not be completed until the twenty-first century, due mainly to the magnitude and expense of a world soil mapping project. Nonetheless, despite being relatively inflexible, the FAO/UNESCO world map contains the best current summary of global-scale soil taxonomic data. It is, however, important to appreciate its limitations, most important of which is that the quality of the mapping varies greatly between regions, especially in the tropics.
3. Quality of Data Base A major limitation of the FAO/UNESCO map is that the map is based on soil data of a wide range of primary sources, ranging from systematic soil surveys to general surveys which were based on practically no soil data at all. Most of the mapping units of the soils map of the world were entirely dependent on soil mapping that was accomplished prior to about 1975. Figure 5 is taken from the FAO/UNESCO map of west Africa, a region that has soils mapped with relatively detailed resolution in Burkina Faso and with much less resolution in Mali. The figure illustrates how differences in map resolution of the original soils surveys used in the FAO/UNESCO project had a direct effect on the size and detail of mapping units used in this soil map of Africa. Primary soil surveys of Burkina Faso were relatively detailed compared with those available for Mali. Apparent soil taxonomic diversity shown on the soil map of the world can be due to political boundaries between nations rather than real soil differences (Fig. 5 ) . The recent revisions of the FAO/UNESCO system (FAODNESCO, 1988), are important for understanding soils in the tropics and for anticipating the future direction of soil taxonomic concepts. The 1974 FAO/UNESCO system was similar to the 1975 Soil Taxonomy in many ways and some of the recent revisions made in Soil Taxonomy (Soil Survey Staff, 1986, 1987a, 1987b) were similar to those recent revisions made in the FAO/UNESCO (1988) system. Significantly, recent changes in the FAO/UNESCO and Soil Taxonomy system also suggest that they are moving away from each other, since similar classification problems were solved in markedly different ways. Two diagnostic horizons that have often been difficult to distinguish in the field, the oxic and the argillic, were revised and even renamed by the revised FAO/UNESCO
SOIL DIVERSITY IN THE TROPICS
(1988) system, the ferralic and the argic, respectively. The new ferralic B horizon now contains all soils with very low activity clays, regardless of textural differences between surface and subsoil. The ferralic B also contains very low concentration of weatherable minerals, low water-dispersible clays, low silt-clay ratios, and < 5% by volume that shows any rock structure. The argic B is now subordinate to the ferralic, and represents most fine-textured subsoils that underlie relatively coarse-textured surface soils. In contrast, the Soil Taxonomy system has maintained both oxic and argillic concepts, but added the kandic concept in an effort to distinguish between low activity argillic from the oxic horizon (Table 4). A second revision that moves the FAO/UNESCO system further away from the Soil Taxonomy system, is the elimination of nearly all criteria based on transient soil properties that respond to climate. Although the 1974 FAO/UNESCO system contained little reference to soil moisture or temperature, such climatic regimes are central to the Soil Taxonomy system’s practical objectives of soil and ecosystem management. At a very high level (the suborder), the two systems have no direct correlates. The French and Brazilian systems of classification also include criteria that only involve relatively permanent soil morphologic, chemical, or physical properties, i.e., no soil moisture or temperature-regime data are included. Taxonomically, such an approach by the FAO/UNESCO system is extreme, in that it seems to overestimate the permanence of soil properties as well as the difficulty of distinguishing between soil moisture and temperature regimes. Van Wambeke (1989) severely criticizes this movement away from using soil dynamic properties (such as soil moisture regime) to classify soils, due to the ecological importance of such soil properties. In summary, the advancement of soil taxonomy has been a notably difficult task, especially in developing a framework for soils of the tropics. The progress in soil taxonomy is, however, very impressive, given that soils are difficult to observe and highly variable and local in the expression of their properties. The development of the Soil Taxonomy system and the completion of the FAO/UNESCO Soil Map of the World have both contributed to our understanding of soil diversity in the tropics. Although the systems are not suited for all soil survey applications (nor should they be expected to), both systems are recognized increasingly as systems that can classify soils repeatably throughout the world and in ways that have meaning to land management and ecology (Isbell, 1984). It appears, however, strongly desirable that future revisions of Soil Taxonomy and the FAO/UNESCO classifications be much more co-ordinated, especially concerning issues that bear on the soils of the tropics.
D. D . RICHTER AND L. I. EABBAR
V. DIVERSITY OF SOIL TAXA IN THE TROPICS At a global scale, soil taxa in the tropics are diverse. Even on soil maps of the tropics with 1 5 0 million scale, all ten of the original ten soil orders of the Soil Taxonomy system are mappable (Aubert and Tavernier, 1972). At this scale, each of eight soil orders exceed 50 million hectares of the tropics (Aubert and Tavernier (1972); Drosdoff as cited by Sanchez (1976)). According to such maps, common soils in the tropics (Table 5) include soils that are iron and aluminium-rich, friable, and well drained (Oxisols); soils that are sandy, droughty, and occasionally salt-rich (Aridisols) ; soils that have clay-enriched subsoils that are either nutrient-poor or nutrient-rich (Ultisols or Alfisols, respectively); soils with high-water tables (Aquepts, Aquults, and Aquents); rocky soils on unstable slopes (Inceptisols or Entisols); sandy soils deposited by rivers or on coastal plains (Psamments); clayey soils that are nutrient-rich and have clays that shrink and swell upon drying and wetting (Vertisols); deep, organic-rich soils that are often highly fertile (Mollisols); and soils of deep accumulations of peat (Histisols). To quantify the diversity of soil types found on a regional scale throughout tropical Africa, America and Asia, we summarized areal estimates of all soil map units found in each of the five volumes of the world soil map containing tropical areas, Volumes 111, IV, VI, VII, and IX (FAO/UNESCO, 1971, 1975, 1977a,b, 1979). Our approach was to collate soil-area data that are found in each of the five volumes of the 1974 FAO/UNESCO system, based on nations that are mainly of lower latitudes than the Tropics of Cancer or Capricorn. Tropical America included all nations in Central and South America including Mexico, but excluding Argentina, Chile, and Uruguay. Tropical Africa included all nations except South Africa, Morocco, Algeria, Libya, and Egypt. Tropical Asia included India and all nations of the Southeast Asia peninsula and islands including Indonesia, the Philippines, and New Guinea. Our rationale was to quantify the very wide range of soil types found throughout tropical Africa, America and Asia, and our results are presented in Table 6 and 7 for primary and secondary categorical levels, respectively. All descriptions and analyses in this section of the paper (Part V) use the 1974 FAO/UNESCO legend except where stated. We use the world soil map data ( 1 5 million scale) to demonstrate diversity of soil taxa in the tropics as a whole. The FAO/UNESCO map is not adequate in much of the humid tropics, and recent Brazilian soil survey data of the Amazon River basin (at 1 5 miilion scale) are analysed later in this paper to demonstrate that soils in the tropics are
SOIL DIVERSITY IN THE TROPICS
Table 5 Areas of major soils in the tropics as estimated at 150 million scale by Drosdoff, Aubert and Tavernier (after Sanchez, 1976).
Soil Order or Suborder Aridisols Ustalfs Orthoxs Mountain soil complexes Udults Psamments Ustoxs Aquepts Tropepts Ustults Usterts Mollisols Aquults Udalfs Aquents TOTAL
Description Desert soil, dry > 50% of most years Clayey, nutrient-rich, nonacid subsoil; dry > 90 days, < 180 dayslyear Extr. weathered; no clayey B hor; moist > 270 days/year Various; rocky, shallow, steep, volcanic ash. Clayey, nutrient-poor, acid sub-soil > 270 dayslyear Deep sand, well drained, weakly developed horizons Extr. weathered; no clayey B hor; dry > 90 days, < 180 dayslyear Weakly developed, young soil; water table at surface each year Weakly developed, young soil; isotropical temperature regime Clayey, nutrient-poor, acid subsoil; dry > 90 days, < 180 dayslyear Shrink-swell clays, neutral pH; dry > 90 days, < 180 days /year Deep organic-rich soils of grasslands Clayey, nutrient-poor, acid subsoil; water table at surface each year Clayey, nutrient-rich, nonacid subsoil; moist > 270 dayslyear Weakly developed, young soil; water table at surface each year
Area millions of ha
Percent of Tropics
Table 6 Areas of major soil mapping units of the l:5-million sacle FAO/UNESCO Soil Map of the World for nations mainly within the Tropics of Capricorn and Cancer.
Primary Map Unit
Percent of total
loooS of hectares
Sesquioxide-rich clay (Oxisols)
Acid, argillic (Ultisols)
Rocky, shallow (Lithic subgroups)
Non-acid, argdlic (Alfisols)
Thin soil over unconsolidated matter (Orthents)
Desert soils (Aridisols)
Incipient change in structure, consistence (Inceptisol)
Low CEC in argilhc (Alfisol, Ultisol)
Shrink-swell, clayey (Vertisol)
Reduced horizons due to wetness (Aquepts, Aquents)
Dry soils of semi-arid regions (Aridisols) Alluvial soils (Aquents)
Poorly drained with abrupt A-B boundary (Alfisols, Ultisols)
Organic-rich with low acidity (Mollisols) 36 OOO
Volcanic-ash, high organic, amorphous, 19 530 (Andisols)
Organic soils (Histosols)
High soluble salt concentrations (Aridisols)
Shallow soil over limestone (Udolls and 13 550
Thick, base-rich organic horizon (Mollisol)
High sodium salt concentrations (Aridisols)
Light E horizon, subsoil accumulation of Al, Fe, and organic matter (Spodosols)
2 258 920
4 935 270
Table 7 Areas of secondary soil mapping units in 1000s of hectares of the 1:s million scale FAO/UNESCO soil map of the world for nations mainly between the Tropics of Capricorn and Cancer
Primary Mapping Unit
Acric Humic Orthic Plinthic Rhodic Xanthic Ferric Gleyic Humic Orthic Plinthic
Luvic Albic Cambic Ferralic
Central America 499 152
523 342 8269 10 937 1236 24 660
Chromic Ferric Gleyic Calcic Orthic Plinthic Vertic
16 927 3340 1441
Calcaric Dystric Eutric
3743 545 9266 20 344 2407 1885
Haplic Calcic Gypsic Chromic Dystric Eutric Ferralic Gleyic Humic Calcic Vertic D ystric Eutric Humic Chromic Pellic
13 182 575
8 160 6786 8881
4647 2336 4610 6181 58 16 253
3278 236 096 35 123 33 338 109 804 58 929 20 062 5966 234 075 81 643 5163 190 347 94 185 88 1 18.517 162643 17 758 4325 207 14533 1929 35 921 20 150 101466 136 676 40 771 15531 24 083 31 378 6288 23 584 17 076 105 7591 9479 5265 63 370 40 438 11581 1706 52 339 35 715
68433 15647 229617
221 5852 4416
3464 1167 32 188 16317 14 944 180 772 6366 83003
69 153 24 777 470 28 1 35 123 60 059 388 536 91 640 17 113 23 213 334 006 80 484 487 255 81 643 9679 223 416 165 598 88 1 130 895 253 102 20 542 4409 40 878 20 103 1863 1929 41 061 27 596 265 295 157 404 62 353 28 584 24 083 41 063 38 139 73 185 22 609 3521 30 811 17 883 16 179 108 480 71 327 11943 1706 105 988 72 9.50
454 122 235 66916 145517 1422 70973 41922 65479 5249 5409
2792 152196 384 2038
11320 1383 9120 2369 12868 7343 2934 4749
3094 33 069 440 53529 21640 1343 84 22240 161 1288 1397 4109 236717137 11168 1525 13745 39337 5533 3416 14100 1388 8578 27632 17365 362 50607 16233
Primary Mapping Unit
Rendzina Phaeozem Solonetz Podzol Total
Calcaric Dystric Eutric Humic Mollic Plinthic
339 330 2019 2852 807
Haplic Calcic Luvic Gypsic
750 6840 7385
Calcaric D ystric Eutric Thionic
354 69 1 2092
Dystric Eutric Humic Calcic Mollic Solodic
349 628 218 330
Haplic Calcic Luvic Humic Mollic Ochric Vitric
12 075 5320 18 610 1624 1203 898 15 807
Gleyic Orthic Takyric Luvic Haplic Mollic Orthic Gleyic Humic
242 13 550 460 626
8204 2917 63 048 55 666 742 10 615 2453 1970 54 162 46 026 8841 24083 4036 22 226 22 945 72 705 11681 82 1 1421 36 232 7205 4446 8236 3518 22 22 271 5375 26 170 9212 6687 6035 22 610 2421 20518 8287 1708 11191 9668 212 17 583 11472 2744 820 11515 1400 4423
2 258 920
1538 650 860 630
4 935 270
8204 10713 23 738 3302 1646 1970 42 980 35 732 999 24 083 4036 11059 2624 34 864 5099 82 1 12 457 1
295 1 22
158 2824 1695 144 508 1237 1708 740 8285 212 589 343 9363
47 763 4476 742 4461
2578 4242 25 433
10 432 3454 457
4538 19 768 183 1051 31 033 8236 237 10 196 55 7560 4364 1762 5189 2421
6288 408 11012
10813 15 092 15 981 6399
3066 898 3442 1470 17518 7050 4163 733 3444 1775
D . D . RICHTER AND L. I . BABBAR
even more diverse than that indicated by the 1974 FAO/UNESCO world map. For these latter analyses, a geographic information system (GIS) was used to estimate areal extents of various mapping units in the 500 million hectare Brazilian Amazon by comparing the older FAO/ UNESCO soil map with that of the recent Brazilian efforts (EMBRAPA, 1981; Carmargo et al., 1986).
A. General Soil Taxonomic Variation in Tropical Africa, America, and Asia Soil taxa found in tropical America, Asia, and Africa indicate wide variation of soil conditions among continents (Table 6). Tropical Asia is mapped by the FAO/UNESCO (1977b, 1979) mainly as acidic Acrisols; nutrient-rich Luvisols; weakly developed Camhisols; rocky shallow Lithosols; and shrink-swell prone Vertisols (Table 6). Most of tropical Africa is mapped by the FAO/UNESCO (1977a) as extremely weathered Ferralsols; sandy, droughty Arenosols; Lithosols; weakly developed desert Yermosols; and Luvisols (Table 6). Most of tropical South America is mapped by FAO/UNESCO (1971) as extremely weathered Ferralsols; acidic Acrisols; weakly developed and sedimentary Regosols; and rocky Lithosols (Table 6). Central America is mapped mainly as organic-rich Kastanoszems; nutrient-rich Luvisols; acidic Acrisols; weakly developed Cambisols; rocky, shallow Lithosols; and volcanic-ash Andosols. The Soil Taxonomy system correlates of these FAO/UNESCO mapping units are for tropical Asia mainly Ultisols, Alfisols, Inceptisols, lithic soils, and Vertisols; for tropical Africa mainly Oxisols, Entisols, Aridisols, and Alfisols; for tropical South America mainly Oxisols, Ultisols, Entisols; and lithic soils; and for Central America mainly Mollisols, Alfisols, Ultisols, Inceptisols, lithic soils, and Andisols. The FAO/UNESCO maps of soils in the tropics include a total of 22 primary mapping units and 97 secondary mapping units. In tropical Africa, South America, and Asia, and Central America, the total number of primary FAO/UNESCO mapping units total 22, 20, 19, and 19, respectively. Secondary mapping units total 81, 60, 69, and 57, respectively. Although large numbers of taxonomic classes are not necessarily equivalent to wide soil diversity, the specific collection of soil taxa demonstrate a marked diversity of soil between and within continental areas, considering the marked differences among many of these soils and the 1:5 million scale of these soil maps. Many of these mapping units also contain substantial within taxa variation in soil properties. The following discussions are summaries of major soil mapping units
SOIL DIVERSITY IN THE TROPICS
of tropics according to our GIS summary and analysis of FAO/ UNESCO (1971, 1975, 1977a,b, 1979) primary and secondary map-unit data. The summaries include definitions of each primary mapping unit, a description of the variation in soil properties within each primary mapping unit, and an estimate of the areal and geographic distribution of these soils. Tables 6 and 7 include areal estimates of primary and secondary units arranged from largest ot smallest in areal extent according to the FAO/UNESCO world map.
B. Ferralsols: Modern Inheritors of the Latosols Concept Ferralsols of the FAO/UNESCO system are generally the most weathered of all soils; they are similar to Oxisols in the Soil Taxonomy system. Ferralsol profiles are often exceptionally deep, as was well documented by Eswaran and Wong (1978) who separated the top 20 m of a weathered profile into four layers above bedrock granite in a Malaysian Udox (an Oxisol with a humid climate regime). Ferralsols are often well aggregated, porous, and friable. The diagnostic ferralic horizon, which results from very strong weathering, has low CEC that is often an important consideration for management. These horizons are dominated by sesquioxides of aluminium and iron, and often accumulate other sparingly soluable metal oxides, for example, of titanium, and more occasionally chromium and nickel. Ferralsols typically have resistent, low acticity clays, i.e., kaolins and gibbsite. Ferralsols may contain a wide range of clay contents, however, and in contrast to Ultisols and Alfisols, they have relatively constant clay contents throughout their profiles. Often Ferralsols have clays that form very durable aggregates, a structure that can give them water-holding characteristics that are similar to coarse-textured soils. As a result, even Ferralsols with high clay contents are often drought prone, due to low water holding capacities. Low water holding capacity on a per unit weight or volume basis may be compensated by the fact that many Ferralsols can be deeply rooted by certain plant species. Some Ferralsols are derived from mafic or basic rocks such as basalt with readily weatherable iron-containing minerals that allow such Ferralsols to form in place. More commonly, Ferralsols are formed in previously weathered, transported materials, rather than formed in place from primary materials that are located on stable landscape positions (Lepsch et al, 1977; Macedo and Bryant, 1987; Buol et al., 1989; Buol and Eswaran, 1988). As a consequence, Ferralsols often obtain their pre-weathered mineral characteristics from processes not related to their present locations. The regional distribution of Ferralsols is markedly independent of current regional precipitation patterns, and many Ferral-
D . D. RICHTER AND L. I . BABBAR
sols support tropical savanna, such as the Cerrado in Brazil (Macedo and Bryant, 1987; Buol et al., 1989; Buol and Eswaran, 1988). A wide range of natural vegetation and ecosystems is supported by Ferralsols, including tropical rainforest, shrub and thorn forests, semi-deciduous forests, savannas, and grasslands. Many of these Ferralsols in semi-arid climates are thus relics, formed mainly in ancient humid environments. Ferralsols may be acidic to neutral in pH, but many have low CEC with a large fraction of variable charged exchange sites (Uehara and Gillman, 1981). Although variable charge may dominate Ferralsols, under field conditions, relatively few appear to have large net positive charge (Cochrane et a1 ., 1985). Nevertheless, relatively low contents for plant-available P and exchangeable Ca, Mg, and K may readily create nutritional problems for plants. On the other hand, physical properties of many Ferralsols make these soils well suited for agronomic use, although many Ferralsols have not been intensively managed over the long-term. Ferralsols are the most prominant of the world’s potentially arable but currently unexploited soils of the world (Brady, 1984). Recent estimates of the Cerrado of Brazil, for example, indicate that about 100 million hectares of Ferralsols could be potentially cultivated and another 100 million hectares converted to pasture (J. Macedo, personal communication). Ferralsols are not uniform in their properties (Tables 2 and 7). In the 1974 FAO/UNESCO system, six secondary mapping units of Ferralsols were recognized (Table 7). These secondary mapping units of the 1974 FAO/UNESCO system represent Ferralsols that have high concentrations of organic matter (Humic Ferralsols), or have different degrees of weathering, mineralogies, and plinthic materials. The secondary classification of the 1974 FAO/UNESCO system differ greatly from the suborder classes of the 1988 Oxisols in the Soil Taxonomy system (Table 2), or from the major subtaxa of Latossolos in recent descriptions of the Brazilian classification system (Carmargo et al., 1986). The major subcategories of Latossolos in the Brazilian system of classification depend on CEC, colour, minerology, drainage, and iron chemistry. Although much soil survey and conceptual work has been devoted to Ferralsols in recent years (Buol and Eswaran, 1988; Duchaufour, 1982; Carmargo et al., 1986), these soil taxa need additional mapping and continued conceptual development (Buol and Eswaran, 1988; Van Wambeke, 1989). According to the FAO/UNESCO data, Ferralsols occupy about 21% of the total tropical land mass, or about 1 billion hectares (Tables 6). Ferralsols occupy markedly different proportions of the FAO/UNESCO maps of tropical South America, Africa, Asia, and Central America; about 40.0, 18.5, 1.8, and 0.2%, respectively. According to the FAO/
SOIL DIVERSITY IN THE TROPICS
UNESCO map, the majority of Ferralsols cover the Amazon and Zaire River basins. In the FAO/UNESCO map of Africa, enormous mapping units of Ferralsol cover the central African basement, the Zaire River basin, the Zambian highlands, the Mozambique belt, and the eastern half of Madagascar. In the FAO/UNESCO map of South America, enormous mapping units of Ferralsols cover the Guyana Shield, the Cerrado uplands, the Brazilian Shield, the Amazon planalto, and Pleistocene terraces. As we will describe later in this paper, recent systematic soil surveys demonstrate that many of these areas are actually not covered by Ferralsols. To a significant extent, the large proportion of the tropics (21%) that is mapped as Ferralsols by the FAO/UNESCO map is the direct result of archaic ideas about the domination of Latosols in the humid tropics. Sanchez (1976) and Buol et af. (1989) suggest that the FAO/UNESCO map of Ferralsols in South America greatly overestimates their occurance in that continent. As more of the soils of the tropics are surveyed, Ferralsols will be recognized to occupy much less area than was indicated on the FAO/UNESCO maps in which many Ferralsols were mapped mainly by regional climate and vegetation types rather than by soil properties. Based on an analysis of the recent Brazilian soil surveys, later in this paper we hypothesize that as little as 580 million hectares (about 12% of the total tropics) will eventually be mapped as Ferralsols, assuming that the diagnostic criteria of this taxa are not radically modified.
C. Acrisols: the Underestimated Soil Order Acrisols are mainly Ultisols in the Soil Taxonomy system. Acrisols are strongly weathered, acid soils with clay-enriched, illuvial subsoils. Like Ferralsols, Acrisols vary greatly between continents in the proportional area they cover, but are most concentrated in humid regions, especially on slightly younger materials compared to Ferralsols. The 1974 FAO/UNESCO system had five secondary mapping units for Acrisols (Table 7), which reflect differences in weathering and mineralogy, moisture conditions, organic matter content, and presence of plinthic material in subsoils. Major management problems with Acrisols are associated with their acidity. Like Ferralsols, Acrisols have few primary minerals remaining, although secondary clay minerals may be prominant, especially in subsoils. Acrisols are mapped by the FAO/UNESCO as covering 29% of tropical Asia, but only about 12, 8 and 4% of South America, Central America, and tropical Africa, respectively. Major Acrisols mapping units in the FAO/UNESCO map are in southeastern China, the Shan
D. D. RICHTER AND L. I . BABBAR
Plateau of Burma, the Indochinese and Malaysian peninsulas, the Philippines, and the non-volcanic parts of the Sumatran and Kalimantan Islands of Indonesia. Acrisols are mainly mapped in South America by FAO/UNESCO in the upper Rio Madeira and Araguaia watersheds; in Africa, they are in the west African basement complex and the Inter-Rift Valley in Tanzania. Estimated coverage of Acrisols of tropical regions according to the FAO/UNESCO mapping is about 500 million hectares. The actual extent of Acrisols in the tropics is probably substantially higher than the 500 million hectares estimated by the FAO/UNESCO maps. Because the pedogenic time required for Ferralsol formation has been underestimated in the past, many areas of the humid tropics that have been mapped as Ferralsols are actually Acrisols. Our analyses of the Brazilian Amazon detailed later in the paper, indicate that Acrisols in this region were underestimated by FAO/UNESCO mapping by nearly 100%. The new soil surveys of the Brazilian Amazon include about 133 million hectares of Acrisols, compared with the FAO/UNESCO estimate of about 74 million hectares.
D, Lithosols, Arenosols, and Luvisols: From Extremely Fertile to Infertile, 500 Million Hectares Each Three primary mapping units are mapped by the FAO/UNESCO map as each occupying nearly 500 million hectares, or each about 10% of the total area of the tropics (Table 6). These primary mapping units include Lithosols, Arenosols, and Luvisols. Some of the world's most fertile and infertile soils are included in these soils. Lithosols have no equivalent order in the Soil Taxonomy system, but are grouped as a variety of lithic subgroups of several soil orders. Lithosols are typically shallow 'with frequent rock exposure (< 10-cm depth over solid rock), and are most frequently found in mountainous terrains. Many Lithosols found on hill slopes are highly unstable, a situation that greatly limits development of soil horizons and profiles. No secondary Lithosol mapping units are represented in the FAO/UNESCO system (Table 7), which should not suggest uniformity of soil properties. Chemical and physical properties of Lithosols are strongly influenced by characteristics of parent materials which vary widely on regional and local scales. In contrast to Ferralsols and Acrisols, Lithosols occupy relatively similar proportions of tropical Africa, Central and South America, and Asia, about 10% of each (Table 6). In Africa, major Lithosol mapping units are located in Taoudenni basin in Mauritania, in the Guinean highlands, and throughout the Sahara Desert; in South America, Lithosols are found throughout the Andes mountains. Due to the large area
SOIL DIVERSITY IN THE TROPICS
of tropical Africa, about half of the tropical world’s Lithosols are mapped in Africa, about 230 million hectares. Arenosols are similar to Psamments in the Soil Taxonomy system, soils that form from coarse, sandy materials. Arenosols may have a variety of subsoils, but all such subsoils have < 15% clay. Arenosols are potentially droughty and have only very weakly developed horizons. These sandy soils are divided into four secondary mapping units to reflect differences in colour, structure, accumulation of illuvial clays, accumulation of sesquioxides, and incipient eluvial E horizons. Most of the tropical Arenosols are found in Africa, where they occupy about 371 million ha or 77% of the total Arenosols mapped by FAO/UNESCO in the tropics. Large mapping units are found throughout the sub-Sahara from Chad to Senegal, and in the Kalahari Desert in southwestern Africa. In contrast, Arenosols are not mapped in Central America, and only 4-5% of tropical Asia and South America is estimated to contain Arenosols. Luvisols are similar to Alfisols in the Soil Taxonomy system, soils with clay-enriched, illuvial subsoils that are generally nutrient-rich and low to moderate in acidity. This major soil taxon is perhaps the most well studied of any major group of soils in the world (Rust, 1983). Luvisols are extremely variable as a primary FAO/UNESCO mapping unit. Eight secondary mapping units are recognized, which account for difference in moisture regimes, calcium carbonate content, minerology, shrink-swell capacity, and sesquioxide concentrations. Many Luvisols are highly fertile agricultural soil in the tropics (Sanchez and Buol, 1975). Although seasonality of rainfall often limits annual crop productivity, due t o high native soil fertility Luvisols often are associated with high human population densities. Luvisols occupy major areas scattered throughout the tropics, between about 8 and 13% of each of the four tropical areas detailed in Table 6. Major areas mapped as Luvisols are in the west African basement complex from Nigeria to Mali; in southeastern Africa from Zimbabwe to Kenya; in eastern and southern regions of the Indian peninsula, much of Sri Lanka; and in scattered areas of southern Mexico, Nigaragua, northeastern Brazil, and Andean valleys of Colombia and Bolivia.
E. Regosols, Yermosols and Cambisols: Weakly Developed Soils Three primary mapping units of the 1974 FAO/UNESCO system have relatively weakly developed soil horizons. The three taxa total about 850 million hectares, with each representing between about 4-7% of the tropical total. Regosols are classified as Orthents and Psamments in the Soil
D . D . RICHTER A N D L. I . BARBAR
Taxonomy system, two suborders of the Entisol order. Regosols are thin soils derived from unconsolidated materials, that lack diagnostic soil horizons. Regosols differ widely in fertility, calcium carbonate content, and temperature regime (Table 7). Regosols are mapped to occupy about 336 million hectares in the tropics, about 10% of tropical South America, about 7% of tropical Africa, but only about 1% of tropical Asia. Major mapping units of Regosols are on the Horn of Africa in Ethiopia and Somalia, and across the Saharan Desert generally north of the Arenosols and Luvisols in the region. Yermosols are similar to Aridisols in Soil Taxonomy. Yermosols are very low organic matter soils of deserts which incipient A horizons (surface soils). Yermosols have one of several kinds of B horizons which are usually composed of non-sodium salt, such as calcium sulphate, or alkaline acumulations such as calcium carbonate. The FAO/UNESCO maps them to cover about 270 million hectares of the tropics. About 80% of the Yermosols in the tropics are found in Africa, mainly across the Saharan Desert, whereas only 0.1% are mapped in South America. Cambisols are classified as Inceptisols in the Soil Taxonomy system. They are highly diverse soils which all have weakly developed B horizons, i .e. cambic horizons, that are distinguished by color, structure, or consistence. Eight secondary mapping units are used in the 1974 FAO/UNESCO system to classify a wide variety of soil conditions that differ in acidity, nutrient content, drainage, sesquioxide content, mineralogy, and organic matter content (Table 7). Cambisols are mapped by the FAO/UNESCO to cover about 243 million hectares of the tropics, about 10% of tropical Asia, but only about 1.5% of tropical South America. Major Cambisol mapping units are in Ethiopia, scattered throughout India, and in western Burma. In sum, according to the FAO/UNESCO data, about 80% of the tropical soil surface is covered by eight primary mapping units: Ferralsols, Acrisols, Lithosols, Arenosols, Luvisols, Regosols, Yermosols, and Cambisols. Each primary unit has considerable variation in chemical and physical properties, and in their potential to support various land uses. This diversity in soil properties is demonstrated by the fact that this 80% of the tropics (4 billion hectares) is mapped by 1974 FAO/UNESCO system with 40 secondary mapping units, even at the 1 5 million scale (Table 7).
F. The Other 1 Billion Hectares: Extreme Variation The other 20% of the tropical land suface, about 1 billion hectares, is mapped by FAO/UNESCO with a total of 14 primary mapping units,
SOIL DIVERSITY IN THE TROPICS
and a total of 52 secondary mapping units. These soils range widely in their properties, from clayey, base-rich Vertisols with extreme shrinkswell capacities to sandy, nutrient-poor and acidic Podzols. The 14 primary map units range in area from about 200 to 6 million hectares each. Nitosofs are similar to certain high clay Alfisols and Ultisols in the Soil Taxonomy system. They are noted for their well structured clayey subsoils that have good water-holding properties and relatively high phosphorus availability. They may be acidic or relatively neutral. Nitosols are mapped to total about 192 million hectares of the tropics. According to the FAO/UNESCO map, Nitosols are mainly found in the African tropics (Table 6), in Ethiopia, Cameroon, Nigeria, and eastern Zaire. Vertisofs of the FAO/UNESCO system are comparable to Vertisols of the Soil Taxonomy system, soils that are marked by very high activity clay that swell and shrink in response to wetting and drying, respectively. Often wetting and drying cause prolific and deep cracking, as well as major volume changes. Although Vertisols are often potentially fertile, they require specialized management techniques to accomodate their unique physical properties. Nonetheless, Vertisols have become soils with major agricultural producing capability. Vertisols occupy about 180 million hectares of the tropics, and according to the FAO/UNESCO map, Vertisols are most common in Asia and Central America, where they cover 7.8 and 5.9% of the tropical land surface, respectively. Major Vertisol mapping units are in west-central India, in Sudan from Kartoum south along the Nile River, scattered throughout southern Chad, and in Mexico. Gfeysofs are not directly comparable to any order of the Soil Taxonomy system, but are classified as aquic suborders in several different orders. Gleysols are poorly drained, often with seasonally pronounced reducing conditions, due to high water tables. Many have high potential productivity for specific agricultural use. Gleysols are a highly diverse soil taxa, and have seven secondary mapping units in the 1974 FAO/UNESCO system (Table 7). Gleysols may vary in acidity, calcium carbonate contents, organic matter accumulation, rooting depth, and presence of plinthic material. Gleysols average about 3% of the total tropical land surface, and are distributed relatively evenly on each of the four continental areas (Table 6 ) . Major mapping units of Gleysols include floodplains of the Amazon and Orinoco Rivers, along the Niger River on the large saltflats southwest of Timbuktu in Mali, in the Niger River delta in Nigeria, surrounding the confluence of the Zaire and Oubangui Rivers in northeastern Zaire and northern Congo, in the headwaters of the Zambezi River in eastern Angola and western Zambia, at the Mouths of the Ganges in Bangladesh, in the lower
D . D . RICHTER AND L. I . BABBAR
floodplains of the Irrawaddy River in Burma, and in the lower Mekong River floodplains of Cambodia and Vietnem. Xerosols are most closely similar to Mollic Aridisols, in the Soil Taxonomy system, weakly developed dry soils, with incipient A horizons that contain slightly higher concentrations of organic matter than Yermosols. Xerosols are mapped by the FAO/UNESCO map mainly in tropical semi-arid areas of Africa, mainly in Namibia in southwest Africa and in central Sudan, where they occupy an estimated 106 million hectares. Because of their location in semi-arid regions, Xerosols are not well leached, and calcium sulphate and calcium carbonate often dominate soil horizons in the profile. Fluvisols are not correlated with an order in the Soil Taxonomy system, but are most similar to the suborder Fluvents, alluvial Entisols that range widely in soil properties. Of the 134 million hectares mapped throughout the tropics, most are found in Asia and Africa. Fully 5.6% of tropical Asia is mapped as Fluvisols, and much of this land is extremely fertile, such as the many Eutric and Calcaric Fluvents that are under intensive paddy rice management. Most of the major rivers in tropical Asia, Africa, and America have Fluvents as important mapping units along their floodplain terraces. About 35 million hectares of Eutric Fluvisols (nutrient-rich alluvial soils) are mapped in tropical Africa and South America, and have been identified as areas with a large potential for rice production (Greenland, 1981). Fluvisols are, however, diverse with four secondary map units in the 1974 FAO/UNESCO system. Some Fluvents are extremely nutrient poor and acidic, and others are calcarious or sulphidic (Table 7). Planosols are soils that are widely scattered in several orders of the Soil Taxonomy system, namely Alfisols, Ultisols, Aridisols, and Mollisols. Planosols are soils on nearly level landforms with poor drainage, and distinct and abrupt textural boundaries between A and B horizons, a distinction that has earned them the popular name of duplex soils. Planosols have a wide range in organic matter contents, and they are occasionally acidic, whereas others are affected by salt accumulations in climates with relatively high evaporati0n:precipitation ratios. Soil properties range widely, as indicated by the seven secondary mapping units in the FAO/UNESCO system that are found in the tropics (Table 7). The FAO/UNESCO maps Planosols on about 62 million hectares in the tropics (Table 6). About half the tropical world’s Planosols are mapped in South America as nutrient-rich Eutric Planosols, with the largest mapping units located in central Brazil and eastern Bolivia. Kustunozems are most closely similar to Ustolls in the Soil Taxonomy system, organic-rich soils of grasslands, with low acidity and high base saturation. Some Kastanozems have high fertility. The FAO/UNESCO
SOIL DIVERSITY IN THE TROPICS
map indicates that the taxon covers about 54 million hectares of tropical lands, about 35 million hectares of which are found in Central America, mainly in central Mexico. Andosols are nearly equivalent to the new Andisol order in the Soil Taxonomy system, soils that are derived from volcanic-ash deposits. Although all Andosols are derived from volcanic materials, mainly from deposited ash layers, they range widely in their physical and chemical properties (Leamy, 1988). Due to high allophane contents, Andosols are typically very low in bulk density and high in organic matter often to great soil depths. Andosols are noted for their pronounced ability to stabilize organic compounds. but also for their potential anion adsorption capacity, especially when acidified. They often have high native fertility and excellent physical properties that make them especially suitable for agriculatural even on very steep slopes. Andosols are especially important in Central America where they occupy 7.1% of the land surface. The FAO/UNESCO map indicates that Andosols cover about 20 million hectares in Central America mainly in southern Mexico, Guatemala, El Salvador, Nicaragua, and Costa Rica. These areas represent nearly half the total 44 million hectares of Andosols mapped throughout the tropics. Histosols are nearly equivalent to Histosols in the Soil Taxonomy system. Histosols are formed from accumulations of organic matter, due to high water tables, restricted drainage, or climatic conditions with extremely high precipitation and low evapotranspiration. Histosols can be high or low in acidity. About 66% of the 31 million hectares of Histosols mapped in the tropics have high acidity. About 25 of the 31 million hectares of tropical Histosols are in Asia, most of which are located in coastal lowlands of the Indonesian islands, especially in the eastern coast of Sumatra and other coastal plains of the Indonesian Islands and along the coastline of the Malaysian peninsula (Whitmore, 1984; Burnham, 1984). Solonchaks are similar to some Aridisols in the Soil Taxonomy system, and are those soils dominated by non-sodium salts. They range widely in their concentration and composition of salts, and in their amount of organic matter and moisture regime. About half of the tropical Solonchaks have excess water problems (high water tables), in addition to high salt concentraions. In central Mauritania and Djibouti in Africa, about 210 thousand hectares of Solonchaks are so high in salts and so impenetrable and toxic to plant roots that their secondary mapping modifier is “takyric”, meaning barren of vegetation. Rendzinas are similar to Rendolls in the Soil Taxonomy system, soils that are relatively shallow, organic-rich, and that directly overlie weathering calcium carbonate. The FAO/UNESCO maps nearly 80% of the
D . D . RICHTER AND L . I . BABBAR
tropical Rendzinas in Central America. They are mapped on about 18 million hectares, and are mainly found on the Yucatan Peninsula of Mexico. Phaeotems are similar to Udolls and Aquolls, two suborders in the Soil Taxonomy system. Phaeozems are moist to wet soils with deep, extremely organic-rich surface horizons with relatively neutral pH and high base saturation. Some are fertile, and according to the map of the world, they are mainly found in southern Brazil. Nearly 80% of Phaeozems in the tropics are mapped in South America. Solonetzes are sodium-dominated soils that are scattered throughout the Soil Taxonomy system as Aridisols, Alfisols, and Mollisols. Solonetzes may have high or low organic matter. The FAO/UNESCO map includes about 12 million hectares in the tropics, most of which are mapped in tropical Africa, mainly in Chad and Somalia. Podzols are closely similar to the Spodosol order in the Soil Taxonomy system. Podzols are acid, coarse-textured sandy soils with accumulations in the subsoil (spodic horizon) of organic matter, and amorphous iron and aluminium oxides. In order of aerial extent, Podzols are last on this list of primary FAO/UNESCO soil mapping units in the tropics, and are estimated to cover about 6 million hectares according to the FAO/ UNESCO map. The world soil map includes Podzols only on maps of Asia and Africa, with none mapped in tropical South America. Podzols were mapped on Sumatra and Kalimantan in Asia and in Zambia and Angola in Africa. In the 1970s, sandy Podzols in Amazonia received considerable attention (Klinge, 1975; Stark, 1978), attention that appears to be out of proportion to their relatively small areal extent in the Amazon and in the tropics as a whole. The new Brazilian soil survey, however, includes about 13 million hectares of Podsols, approximately 2.8% of the 500 million hectare Brazilian Amazon. In sum, the 1974 FAO/UNESCO classification and map demonstrate that the soil taxa in the tropics have a very wide ranging diversity, and that “tropical soil” is a meaningless concept in describing soils in this region. The 1974 FAO/UNESCO map presents Ferralsols to cover about a billion hectares of tropical land, about 20% of tropics, whereas highly weathered Acrisols are estimated to cover an additional 10% of the tropics. It is critical to appreciate that these estimates of Ferralsols and Acrisols are overestimates and underestimates, respectively, probably on the order of hundreds of millions of hectares each. These major mapping errors largely resulted from reliance on outdated concepts of soil taxonomy. Estimates of the extent of soil taxa throughout the tropics can not be quantified in accurate detail until the soils in the tropics are mapped much more systematically than they are at present. It is clear that
SOIL DIVERSITY IN THE TROPICS
although the FAO/UNESCO map illustrates the diversity of soils in the tropics as a whole, it contains little evidence about the diversity of soils in the humid tropics. We use recent Brazilian soil survey data and a GIS system to analyse soil taxa and mapping units in the Brazilain Amazon, specifically to evaluate the diversity of soils in the humid tropics.
VI. HOW MUCH AREA IN THE TROPICS IS COVERED BY OXISOLS? RESULTS OF THE FIRST SOIL SURVEYS OF THE AMAZON BASIN Misconceptions about soils in the tropics are probably most firmly held about soils in the humid tropics. Such regions include vast areas of the Amazon River basin, the Zaire River basin, throughout the lowlands of southern and southeastern Asia, much of coastal west Africa, and the coastal plains of Central America. In the last two decades, soil surveys, mapping, and agricultural experiments have proceeded in many of these humid regions. Soil taxonomic concepts have been tested in these regions as well. The most impressive advances have arguably come in Brazil, especially in the Brazilian Amazon basin, an enormous 500 million hectare area that represents about 10% of the tropics as a whole. Of the large areas of South America and Africa where so little is known about the details of soils geography (Figs. 4 and 5), recent advances made by the Brazilian soil surveys are of major significance. We use a geographic information system to analyse results of these new Brazilian soil surveys and maps (EMBRAPA, 1981; Carmargo et a l . , 1986) not only to indicate how the results pertain to soils in Brazil per se, but for what the results suggest about soil diversity throughout the humid tropics.
A. Background The Amazon basin is the world’s largest watershed. It covers nearly 500 million hectares in Brazil alone, provided that the Tocantins drainage of the northern Cerrado is included in eastern Amazonia, an inclusion also made by Bates (1864) in his map of the watershed boundaries of Amazonia. The Amazon is covered by a rich variety of vegetation communities, including wet evergreen forests, seasonal forests from almost evergreen to mainly deciduous, freshwater and brackish swampforests which vary depending on duration and depth of inundation, savannas (campo cerrado) on uplands and on poorly drained lowlands, and tropical Andean montane forests and paramo vegetation above treeline. Its rivers dissect the eastern Andean Mountain chain, cut
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through uplands on old continental platforms, and drain immense flat to undulating alluvium, to flat and frequently flooded lower terraces. Seventeen of its tributaries are over 1500 km in length (Shoumatoff, 1986). A wide variety of rock types which form parent materials for soils are represented within its boundaries. Soil and natural resource information has been very late in coming from the Amazon, not without reason since economic development of the region has been slow. There has until recent years been little economic incentive for soil surveys or evaluations of this enormous area. Following native Indian settlement which was extensive throughout the basin, the quest for gold, religious converts, and political subjects stimulated some agriculatural production and established communites over several centuries. Mining and rubber exploitation brought extensive settlement toward the end of the nineteenth century. By the 1940s, serious efforts began to be made to encourage colonization and agricultural development as population pressures along coastal Brazil and in the Andean highlands became more intense (Cochrane et a l . , 1985). Since the 1940s, settlements and agricultural exploitation in Amazonia have had variable success, a trend that suggests a limited knowledge of the various soil resources and their potentials. In fact, the varying success of settlements emphasizes the importance of understanding the diversity of soil resources in the Amazon and elsewhere in the lowland humid tropics (Moran, 1981). Prior to the 1970s, a major soil study of the Amazon was an exploratory investigation conducted by the Division for Soil Survey and Soil Fertility of the Ministry of Agdculature in Rio de Janeiro with collaboration from US-AID and F A 0 (Beek and Bennema, 1966). Although these soil maps provided a wealth of soil data for soils in otherwise unknown regions, the reliability of these maps was relatively poor compared to what is known today. Even during the FAO/UNESCO mapping project, most of the basin was assigned to a soil reliability class I11 (Fig. 4) indicating that as of the early 1980s, few soils maps of Amazonia were based on any soil observations at all. In retrospect, the FAO/UNESCO map of South America might better have included large mapping units of “little known soil complexes” in the Amazonian basin rather than the enormous and deceptively uniform map units of Ferralsols. The misclassification of more than 100 million hectares as Ferralsol west of the confluence of the Rio Madiera, the Amazon, and Rio Negro will be described subsequently, an error that supports the idea that much of the Amazon River basin might have been better mapped by the FAO/UNESCO with a mapping unit of “little known soil complexes” rather than as Ferralsol. The Brazilian government’s decision in the 1970s to stimulate development of the Amazon led to high quality and systematic natural resouce
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inventories which have supplied a wealth of new data about soils, ecology, vegetation, and geologic resources. For the first time a systematic soil survey has been conducted of the Brazilian Amazon. Compared to maps of the past including the FAO/UNESCO, the new Brazilian surveys and maps present a markedly more diverse perspective of soil taxa in Amazonia.
B. Some Common Properties of Amazonian Soils irior to describing some of the diversity of soils within the Amazon basin, it is worthwhile to briefly describe several properties shared by most or all soil in the Amazon. Two characteristics common to most or all soils of the Amazon (and to most soils of the lowland, humid tropics) are the generally seasonal cycle of soil moisture, and the relatively constant soil temperature regime (Sanchez, 1976; Van Wambeke, 1978). Relatively few places in the tropics have rainfall in excess of evapotranspiration (Et) every month of the year. With Et relatively high, plants may exhaust soil water from great depths during dry seasons, which in some parts of eastern Amazonia may be up to about 5-6 months per annum. A second characteristic shared by all soils in the lowland Amazon is that soil temperature is relatively high, with little seasonal variation. Other soil properties are shared by many but not necessarily all soils of Amazonia. Soils have a range of acidities, but most are acidic and at least somewhat low in nutrient status, in that most soluble nutrients that are products of weathering have been removed from soils by leaching or taken up by plants. During the process of soil formation, losses of nutrients to leaching tend to be balanced by inputs from the decomposition of weatherable minerals (if they exist in the parent materials), a source which can also provide nutrients for plant uptake. However, if soil processes have proceeded to form acidic soils by intense leaching and weathering, or if acid soils are formed from geologic materials containing only inert minerals, soil nutrients can be a finite resource. There are many acid soils in Amazonia, and nutrients under these conditions are not readily resupplied by mineral weathering. As is the case with most if not all acid soils throughout the world, long-term cropping systems that remove significant amounts of nutrients in harvests require inputs of nutrients from organic matter and inorganic fertilizers to prevent decreases in soil nutrient availability. Another characterisitc shared by many upland soils is their relatively limited ability to supply water to plants. Plant-available water storage capacity in upland Oxisols may be less than 10% by volume, and plants without deep roots are susceptible to periodic drought stress. Cochrane et al. (1985) mapped about half of Amazonian soils to have water
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holding capacity <7.5% in the surface metre of soil, and many others to have < 15%. A final characteristic of most Amazonian soils is their relatively great susceptibility to water erosion following disturbance of native vegetation. Rainfall is often intense and erosion can be rapidly accelerated from bare ground following forest cutting for agriculture, pastures, roads, towns, or mines. Management of vegetative cover and establishing control over run-off water is very important to minimize such on-site and off-site problems with water resources.
C. The New Soils Map of the Brazilian Amazon Results of our GIS analyses of soil mapping in the Brazilian Amazon are summarized in Figs. 6, 7, and 8, and Tables 8 and 9. The new systematic soil surveys of Amazonia conducted by EMBRAPA indicate that Oxisols were overestimated considerably in the past, and that Ultisols and 'other soil taxa were underestimated. (In the FAO/UNESCO classification, Ferralsols have been overestimated and Acrisols underestimated; in the Brazilian classification, Latossolos and Podzolicos Vermelho-Amarelo Distroficos have been over- and underestimated, respectively). The new maps reduce the coverage of Oxisols in the Brazilian Amazon from about 67.4% to 39.1% (Tables 8 and 9, Fig. 7). These first surveys and maps based on actual soil data indicate convincingly that soil forming processes have not weathered Amazonian soils as intensively as previously thought. The Podzolicos Vermelho-Amarelo Distroficos (closely similar to the Ultisol soil order) is the soil taxonomic unit with the largest increase in area in the recent Brazilian soil surveys (Tables 8 and 9) Ultisols according to the older FAO/UNESCO maps covered approximately 15% of the Brazilian Amazon (mapped as Acrisols), which increased to nearly 30% in the recent Brazilian soil maps (EMBRAPA, 1981). The largest increases of Ultisol mapping units are concentrated in the upland areas west of the confluence of the Rio Negro, the Amazon, and Rio Madeira (Figs. 7, 8). The enormous region of Ultisols, on the order of 100 million hectares, also extends well into Amazonian Peru and Colombia, and was entirely misclassified as Oxisols by the FAO/ UNESCO map (Beek and Bennema, 1966; FAO/UNESCO, 1971). The sedimentary materials that have produced these Ultisols date from the Tertiary in this part of the western Amazon (RADAM, 1977), and are materials that are apparently not sufficiently weathered to have formed Oxisols. This misclassification probably represents one of the largest soil
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Fig.6 New soil maps of the Brazilian Amazon basin (EMBRAPA, 1981; Carmargo el al., 1986) present a clearer perspective of soil diversity in the humid tropics (Table 9 summarizes individual data). Major changes compared to all previous soil maps include far fewer Oxisols (Latossolos in the Brazillian classification) and many additional soil taxa, especially Ultisols (Podzolicos Vermelho Amarelo Distroficos).
mapping errors made in modern times. The recent mapping and surveys also indicate a modestly greater area of potentially fertile soil taxa. Alfisols (both Luvisols and Eutric Nitosols in the FAO/UNESCO classification) were mapped as only about 1.1% of the Brazilian Amazon by the FAO/UNESCO (1971), whereas the new map includes about a four-fold increase in the areal extent of soils with high base saturation and low acidity (Podzolicos VermelhoAmarelo Eutroficos and Terra Roxa Estruturada in the Brazilian classification). Approximately 18.5 million hectares of such soils are mapped by the recent Brazilian survey (Table 9). Important areal changes have also been estimated in soils mapped with plinthite. Estimated areas of Ultisols with plinthite, Plinthic Acrisol in the FAO/UNESCO map of the world, plus Podzolicos Plinticos in the
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Table 8 Soil mapping units of the Brazilian Amazon River basin based on GIS-interpreted 1:s million scale FAO/UNESCO Soil Map of the World (FAO/UNESCO, 1971)
Primary Map Unit
Area 100 km?
Percent of Basin
Secondary Map Unit
Area 1000 km2
Acric Orthic Xanthic
34 1 1092 1787
7.14 22.87 37.43
Percent of Basin
Brazilian map, are mapped as 7.8 to 4.6% of the Brazilian Amazon, respectively, on the two maps. Areas of Psamments (Arenosols and Solos Arenoquartoso, on the two maps, respectively) have changed slightly from 5.9 to 4.4%. Spodosols (Podzols in both maps) were not mapped in the FAO/UNESCO map of the world, but occupy about 2.8% of the new Brazilian map, or about 13 million hectares. Fig.7 (Facing page) New soil maps of the Brazilian Amazon highlight how the FAO/UNESCO map greatly overestimated the distribution of Oxisols (Ferralsols or Latossolos). Ferralsols are blackened on the FAO/UNESCO map (Fig. 7A), and Latossolos blackened on the EMBRAPA map (Fig. 7B). Most of the areas mapped as Ferralsol by the FAO/UNESCO (1971) were not based on actual soil data (Fig. 4), whereas the EMBRAPA map is based on systematic soil surveys. Note how the region west of the confluences of the Amazon, Rio Negro, and Rio Madeira was entirely misclassified as Ferralsol by the FAO/UNESCO. Elsewhere in the basin, monolithic mapping units of Ferralsol have given way to more detailed and more diverse mapping units. Tables 8 and 9 quantify these data.
Table 9 Soil mapping units of the Brazilian Amazon River basin based on GIs-interpreted 1 5 million scale Map0 de Solos do Brasil (EMBRAPA, 1981; Carmargo et af., 1986) Primary Mapping Unit
Area loo0 km2
Percent of total
Secondary Mapping Unit
Amarelo Distrofico Vermelho-Amarelo Distrofico Roxo Distrofico e Eutrofico Vermelho-Escuro Distrofico Vermelho-EscuroDistrofico e Eutrofico
Vermelho-Amarelo Distrofico Vermelho-Amarelo Eutrofico Plintico Distrofico
Percent of Total
787.3 923.4 1.0 87.6 6.6
17.06 20.01 0.02 1.10 0.14
1118.4 160.8 210.4
24.24 3.48 4.56
Distrofico (Tb) Distrofica e Eutrofica (Ta) Indiscriminadas
273.6 3.0 45.7
5.93 0.06 0.99
Distroficos Distroficos e Eutroficos
Distroficos Distrofico e Eutroficos Humicos Distroficos
148.6 56.9 2.4
3.22 1.23 0.05
Distroficas Hidromoficas Distroficas
Distrofico (Tb) Eutroficos (Tb, Ta) Humico Distrofico (Tb)
19.0 42.1 0.9
0.41 0.91 0.02
Grupamento Indiviso de Solos Terras Roxas Estruturadas
Solos Salinos Planossolos Total Primary Units
Distroficos e Eutroficos Eutroficos
32.9 8.5 2.4 14.1
0.71 0.18 0.05 0.30
Solochak Indiscriminad Costeiros
Eutroficas Similar Distrofica e Eutrofica Similar Eutrofica
Total Secondary Units
SOIL DIVERSITY fN THE TROPICS
D. How Much Area in the Tropics is Covered by Oxisols? Based on the results of the recent soil surveys of the Brazilian Amazon, we can approach the question of how much of the tropics is covered by Oxisols. We start by assuming that the estimates of 1.0 to 1.1 billion hectares of Ferralsols in the tropics are too high (Tables 5 and 6). This seems a safe assumption given the FAO/UNESCO method of mapping Ferralsols by typically using climatic and vegetation data rather than soils data, and the outcome of the Brazilian soil survey in relation to the FAO/UNESCO map of the Brazilian Amazon (Tables 8 and 9). A first-order approach is to assume that Oxisols in the remainder of South America (mainly in Bolivia, Colombia, Venezuela, and southern Brazil) and in Africa (mainly in the central African basement and the Zaire River basin) have been overestimated by the FAO/UNESCO project to a similar extent as they have been in the 500-million hectare Brazilian Amazon. This apparently bold assumption may not be unreasonable since boundaries of nearly all Ferralsols in South America and in Africa were drawn using much the same procedures (FAO/UNESCO, 1971, 1977a), relying on very few soil observations and almost entirely on a very general knowledge of climate and vegetation (Figs. 4 and 5 ) . According to the new EMBRAPA (1981) soil map, newly mapped Latossolos actually occupy only about 56% of the total area covered by FAO/UNESCO Ferralsols in the Brazilian Amazon (Tables 8 and 9). Given that the FAO/UNESCO maps of the tropics include about 1.05 billion hectares of Ferralsols, 56% of this total amounts to about 580 million hectares of Ferralsols, or about 12% of the tropics as a whole. If a lower-limit estimates of Oxisols in the tropics is about 580 million hectares, an upper limit might simply be estimated by reducing the FAO/UNESCO total for Ferralsols (1.05 billion hectares, Table 6) by the overestimates apparent solely in the Brazilian Amazon that are based on the new EMBRAPA data. This upper-limit estimate of global Oxisols is about 900 million hectares. Given the manner in which Ferralsols were mapped in the FAO/UNESCO project, however, this is almost certainly an overestimate. Considering the enormous FAO/UNESCO mapping units for Ferralsols in tropical South America and Africa, and the way climate and vegetation information rather than Fig.8 (Facing page) Soil maps of the Brazilian Amazon highlight how the FAO/UNESCO (1971) map greatly underestimated Ultisols (Acrisol or Podzolicos Vermelho Amarelo Distroficos). Blackened areas on the FAO/UNESCO map are Acrisols (Fig. 8A), and on the EMBRAPA map. Podzolicos Vermelho Amarelo Distroficos are blackened (Fig. 8B). Tables 8 and 9 quantify these
D. D . RICHTER AND L. I . BABBAR
actual soil data were used in mapping most of these areas (Figs. 4 and 5), the so-called lower-limit estimate of global Oxisol coverage, 580 million hectares, is probably our best estimate of Oxisol coverage at this time. Many additional field surveys combined with continued conceptual progress will be needed to estimate the areal extent of Oxisols with more accuracy.
E. A Realistic Concept of Soil Diversity in the Humid Tropics The new soil surveys of the Brazilian Amazon document important improvements in areal estimates of different soil taxa but they also signal major conceptual changes about soils in the humid tropics. This conceptual change departs from that of a “tropical soil”, the relatively monolithic concept that has been critiqued throughout this paper. A more realistic perspective of the diversity of soils in the tropics accomodates a much wider range of soil conditions and soil taxa found in the intertropical regions as a whole, and in the humid tropics specifically. The FAO/UNESCO map of the world demonstrates that soil taxa in the tropics are numerous and varied (Tables 6 and 7), even when considering the small 1 5 million scale of this map. The FAO/UNESCO map is not, however, able to demonstrate soil diversity within the humid tropics, such as the Amazon or Zaire River basins (Figs 4, 5, 6, 7, 8). The major contribution of the new soil surveys of the Brazilian Amazon is that they demonstrate that even in the humid tropics, soils are not as pedogenically aged or as uniform as previously suggested (Fig. 6). Compared to ideas in the past, soils found in the humid tropics represent a wider range of soils taxonomically, a range that spans substantially more pedogenic time and intensity of weathering. Figure 9 (after Smeck et a l . , 1983) helps describe this change in concepts. The figure illustrates free energy of soil profile horizons as a function of pedogenic time, using a simplified evolution of soils from Entisols. to Inceptisols, Alfisols, Ultisols, and finally to Oxisols (Table 1). In fact, soils of a number of tropical landscapes include Entisols, Inceptisols, Alfisols, Mollisols, Ultisols and Oxisols. The more realistic concept of soils in the tropics must consider that pedogenic time can be easily underestimated. Soil formation diagrams such as that in Figure 9 must also be conceived as being multidimensional, since the composition of geologic materials, landforms, local hydrology, biota and various disturbances all influence soil properties and processes and help insure considerable heterogeneity of soils between and within soil orders.
SOIL DIVERSITY IN THE TROPICS
Pedogenic time + Fig. 9 Since the pedogenic time to form Oxisols has often been greatly underestimated in the past (Van Wambeke, 1989), there are many other soil taxa in the humid tropics than previously suspected. This simplified diagram (after Wilding and Drees, 1983) illustrates free energy content of soil profiles of several soil orders as a function of pedogenic time (the duration over which bioclimatic factors operate on parent materials in local landscape positions to form soils). Recent soil surveys (e.g. EMBRAPA, 1981) demonstrate clearly that the pedogenic time required for Oxisol formation must be conceived to range far more widely than previously suspected.
VII. MESO- AND LOCAL-SCALE SOIL VARIATION IN THE TROPICS This paper has emphasized that soil diversity in the tropics is relatively easy to demonstrate using maps with scales of 1 5 million. A very large number of soil taxa are represented in the tropics, soils that contrast greatly in their physical, chemical, and biological properties. Local variations in soil properties are also significant in the tropics and at least a comment is worth making about meso- and micro-scale diversity of soil taxa. Local variation in soil properties is at least as great in the tropics as elsewhere in the world. Soil genetic factors that cause soil meso- and micro-scale variability (geology, geomorphology, biota, hydrology, climate and age) are not qualitatively different between tropical and temperate regions, such that spatial variation might be expected to be lower in the tropics (Lepsch et al., 1977; Moormann and Kang, 1978).
I). D . RIC'HTFR A N D L . I . B A H B A K
There are as yet? notably few analyses of local spatial variability of soils in the tropics, such as that evaluated by Wilding and Drees (1983). Mausbach et al. (1980), in a study of contrasting pedons, hypothesized that for many soil chemical properties, spatial variation of Vertisols< Mollisols = Alfisols < Entisols = Inceptisols = Ultisols < Spodisols. No Oxisols were included in the study. Anecdotal examples illustrate extreme microvariation of soils in the tropics. Moormann and Kang (1978) and Trangmar et al. (1987) described how growth of upland crops in the tropics was sometimes characterized by substantial unevenness over remarkably short distances, variations attributed to edaphic effects and to edaphic by weather interactions. This local variation in productivity may be very great on recently cleared fields with low fertilizer inputs and with large quantities of forest organic matter that ranges widely in chemical composition (e.g. from burned ashes to decomposing wood and foliage) and in nutrient immobilization and mineralization potentials (Trangmar et al., 1987). In tropical forests or savannas, trees may alter soil properties, especially in surface horizons and in the immediate soil environment adjacent to trunks and roots. Forest-soil, tree-soil, and root-soil studies in temperate forests indicate large inter-specific variation in the effects of trees on soil properties (Gersper and Holowaychuk, 1971); Jenny, 1980; Binkley, 1985; Richter, 1986). Factors such as volume and chemistry of stemflow, nutritional requirements of trees, organic chemistry of litter, and N-fixing abilities affect how different tree species alter soils on micro- and meso-scales. Animals also have profound local effects on variations in soil properties. Large areas of many upland soils in humid and subhumid tropics are mixed by termites, sometimes to many meters in depth (Goudie, 1988). In addition to termites, ants, worms, and other soil fauna can also mix great volumes of soil material in many different soils. Human activities also affect soil heterogeneity on a local scale; old village sites often have accumulations of soil organic matter, phosphorus, nitrogen, and other nutrients associated with human and animal wastes, gardening, and kitchen middens. Old anthropic soils along the Amazon River and its tributaries attest to large native populations who formerly inhabited terraces along the river. Humans are also responsible for fires in many locations in the tropics. Fires can have extremely variable effects on properties of surface soils; effects that are both physical and chemical, and are affected on a wide range of spatial scales. Finally, two impressive examples of microscale soil diversity are described in tropical Asia and Africa. At the International Rice Institute in the Philippines, a 50-m trench through an agricultural field contained soil profiles that were common to 6 of the 10 original orders of Soil Taxonomy (Van Wambeke and Dudal, 1978). In west Africa, agronomic
SOIL DIVERSITY IN T H E TROPICS
field studies have presented plant breeders with complex analytical problems due to extreme within-field variation in edaphic conditions (Moormann and Kang, 1978). Conventional experimental designs proved inadequate for plant-genetics trials in these fields with soils having extreme local variation in fertility.
VIII. CONCLUSIONS The recent paradigm-change in soil taxonomy has encouraged the collection of soil data, and contributed directly to an increased understanding of soil diversity in the tropics. Soil data have accumulated that make it impossible to ignore the diversity of soils found in the tropics, and in the humid tropics specifically. Even small-scale maps (e.g. the FAO/UNESCO soil map of the world) demonstrate elaborately the diversity of soils of the tropics (Tables 6, 7). Such maps document that tropical Africa, America, and Asia have greatly different distributions of primary and secondary map units. Even at the 1:5 million scale, the FAO/UNESCO soil map of the world includes in the tropics 22 different primary mapping units and 97 different secondary mapping units. Many of these mapping units include soils with a wide range of properties. The idea of “tropical soil” is still invoked frequently, despite good evidence and authoritative statements that attest to the complexity of soil taxonomic distributions in the tropics. Perhaps part of the problem is that few ecological scientists know much about soil taxonomy, or have much appreciation for the scientific difficulties created by the absence of an accepted, global system of soil taxonomy. Isbell (1984) contends that even many pedologists do not understand the fundamental principles and limitations of soil classification. It seems extremely important that at the present time the two most widely used soil classification systems (Soil Taxonomy and FAO/UNESCO) appear to be moving apart in their approach and in how each classifies soil (Van Wambeke, 1989). Both systems actually share many principles and approaches, yet neither will remain static in the years ahead. Whether or not these systems continue to diverge, soil taxonomy should be an important part of broadening the perspectives about soils in the tropics. Why misconceptions about “tropical soil” have persisted for so long is a question that a philosopher of science might well examine with interesting results. To the critical natural scientist, misconceptions have managed to persist due to the enormous challenge of surveying soils on the 5 billion-hectare tropical landscape; an absence of interdisciplinary communication about soils in the tropics; problems associated with too many taxonomies and nomenclatures used to describe soils in the tropics; and the course of the development of soil science itself which
D. D. RICHTER A N D L. 1. RABBAR
too often has emphasized the factors of soil formation rather than the quantitative properties of soils themselves. Emphasis of many soil and ecological scientists has been on climatic factors of soil formations, which has diminished the importance of complex interactive effects imposed by parent materials, landforms, biota, and soil age. The FAO/UNESCO map of the world contains little indication about soil diversity in the humid tropics, because soil maps of the lowland humid tropics are among the world’s least reliable. Recent soil surveys of Amazonia (EMBRAPA, 1981) and recent conceptual taxonomic developments (Buol and Eswaran, 1988) have made it relatively easy to criticize the notion of “tropical soil” as it applies to soils in the humid tropics. A fundamental soil mapping principle predicts that as larger and larger scale maps are constructed, increasing heterogeneity of soil map units will be recognized. Since mapping units attempt to represent natural occurrences of soils on the landscape, mapping units always contain some associated soils with properties that are outside the range permitted by the specific unit. As soil maps become more detailed (due to more information and larger map scales), discrepancies between soil cartographic and soil taxonomic units will decrease, and increasing numbers of soil taxa will be represented on soil maps. As soil maps of > 1:50,000 scale continue to be constructed for local human settlements and land use planning, the diversity of soils in the tropics will be increasingly obvious and better understood. In sum, the tropics are covered by a variety of soils which in aggregate are much less homogenous than has often been suggested. In place of the archaic notion of the highly weathered “tropical soil”, soils in the tropics, and specifically soils in the humid tropics, range widely in their properties and in their intensities of weathering. There are far fewer Oxisols and many more Ultisols and other soil taxa than estimated in the past, even in the humid tropics. This is well documented by the first soil surveys of the Brazilian Amazon that have been recently completed. To speak carelessly about “tropical soil” greatly oversimplifies the complexity and diversity of ecosystems in this 5-billion hectare region. As soil diversity at all spatial scales (from regional to microsite) is better documented, understanding and use of tropical ecosystems can only improve.
ACKNOWLEDGEMENTS Many thanks to K. Korfmacher for Geographical Information System management; to Drs S. W. Buol and P. A. Sanchez for critical
SOII. I)IVEI' IN T H I . TROPIC'S
discussions a n d t h e use of their library: t o I . F. Lepsch. M . C r a v o . J . Macedo, a n d J . S. Reynolds f o r field trip discussions of soils and landscapes; t o E. Bornemisza, D. Binkley, N . Christensen. R. G . Healy, P. Heine, M. H u s t o n , K. Lal, D. Livingstone, C. H. Periera, W . M. Post. W. Schlesinger, A . Van W a m b e k e , a n d J . Wright for reviews of t h e manuscript: t o M . Doelle a n d N . Stevens for c o m p u t e r work; t o A . M a c F a d y e n f o r constructive editorial c o m m e n t s ; a n d t o P. Wilson and D. Fourqurean for typing.
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