Chapter 4 Soils of the Tropics Fig. 4.1 Soil order map of the tropics. Adapted from NRCS (2005) This chapter describes the main soils of the tropics, grouped technical. Natural systems classify soils as natural bodies in natural soil classification systems, those that describe and in their own right, while technical systems classify soils classify soils in their natural state. Classification is an according to their suitability for a specificuse,suchas important element of science, as knowledge is organized agriculture, engineering or waste disposal (Chapter 5). into categories and classes that are useful, or necessary for Naturalsoilclassification systems are divided into those communications among scientists (Cline 1949). Categories based on soil genesis (what the soil should be according to are hierarchical levels (for example states and municipalities the factors of soil formation) and those based on soil in a federal government system). Classes are the individual morphology, on actual soil properties (what the soils units that fall in a category (for example the individual are). The factors of soil formation are climate, organisms, states in a particular country and the individual municipal- parent material, topography and time (Jenny 1941). ities in a particular state). Initially, soil classification systems were all genetic; now- Many soil classification systems have been used in the adays, they are morphological but underpinned by tropics. They fall into two broad groups, natural and concepts of soil genesis. 4.1 SOIL TAXONOMY 83 The strong genetic orientation of early Russian and 4.1 Soil Taxonomy American pedologists1 led to the zonality concept. Zonal soils were those that have properties according to what Themountingevidenceagainsttheusefulnessofthedirect genesis theory indicates in terms of climate, vegetation, application of genetic theories in practical classification topography, parent material and age. The simplistic concept gradually eroded the confidence of American scientists in of a hot, humid climate, lush vegetation, old parent material their 1938 United States Department of Agriculture and old landscapes resulted in zonal tropical soils, called (USDA) system and its modifications. Soils considered lateritic soils, as discussed in Chapter 3. zonal for the United States, such as Podzols and The numerous soil classification systems are extensively Chernozems, were found in the tropics, where genetic described by Krasilnikov et al. (2009). There are five major theories dictated they should not exist. Likewise, lateritic systems that are important in the tropics, now all based on soils were found in the United States. This led to the quantitative morphological properties: the Soil Taxonomy development of a completely new soil classification system (Soil Survey Staff 1999) – see Figure 4.1; the World system, based on morphological and chemical properties Reference Base for Soil Resources (WRB) (Bridges et al. 1998, that can be quantified by specifictechniques.TheSoil Deckers et al. 1998, IUSS Working Group 2014); the Austra- Taxonomy system ended the grouping of soils according lian system (Isbell 2002); the Brazilian system (Embrapa to what they should be according to genesis theory and con- 1999); the Chinese system (Gong 1994, Li 2001 and others). centrated on what they actually are, and the system is, as the The Cuban system (Instituto de Suelos 1999, Hernández et al. subtitle states: “a basic system of soil classification 2006) is an example of a national system that was originally for making and interpreting soil surveys” (Soil Survey based on soil genesis and is now quantitative. Staff 1975). The difference between the genetic and the quantitative After about 10 years of development, the first version of morphological approaches was vividly illustrated to me Soil Taxonomy, called the Seventh Approximation (Soil during a study tour of African savanna soils in 1975. While Survey Staff 1960), was presented at the 7th International examining a soil pit in Ghana, a leading scientist of the Soil Science Congress in Madison, Wisconsin. Since then it genetic school, Professor Georges Aubert, looked up and said: has been updated by two major revisions (Soil Survey Staff “The climate is so, the vegetation is that, the topography is 1975, 1999). Keys to the Soil Taxonomy system, similar to such, the parent material is this and the age is that; therefore, those used in plant taxonomy, have been updated periodic- voilà, the soil is classified as that.” He barely looked at the soil ally (Soil Survey Staff 1998, 2014) and made available in the profile or at data sheets with physical and chemical analysis. USDA Natural Resource Conservation Service (NRCS) website A soil scientist from the morphological school, Professor (www.nrcs.usda.gov). Stanley Buol, carefully examined the profile, studied the Many of the revisions of Soil Taxonomy have resulted data, went through the keys to Soil Taxonomy and ended up from the active involvement of tropical soil scientists who with a classification according to quantitative properties. participated and often led international committees deal- The two most extensively used natural classification ing with ten soil orders (for example Beinroth and Osman systems, the Soil Taxonomy and the WRB systems, are 1981, Moormann 1985, Buol and Eswaran 1988). The imple- described in the following sections. The WRB, being a United mentation of these changes scaled-up Soil Taxonomy from Nations sponsored system, has superseded national systems a classification system for the United States into a of European countries that have been commonly used in the global one. tropics, such as the French and Belgian systems. The Cuban Soil Taxonomy is a hierarchical system, with six categor- system is presented as an example of a national classifica- ies: order, suborder, great group, subgroup and family (Soil tion system. Survey Staff 1999). It is unique in introducing quantitative This book deals with the properties and management of definitions of soil temperature and soil moisture regimes, soils as they presently exist. The genesis of soils in the based on normal climatic conditions where the soil is tropics (how soils are formed) is extremely interesting located, and on soil water availability. A normal year is a but will not be discussed in detail in this volume. This topic year that has plus or minus one standard deviation from the is worthy of an entire book of its own. Readers interested in long-term (30-year) mean (Soil Survey Staff 1999). Families tropical soil genesis and its development through time are are further subdivided into two additional categories, series referred to books by Mohr and van Baren (1954), Mohr et al. and phase, in local soil surveys. (1972), van Wambeke (1992) and Buol et al. (2011). Of specificsignificance to soils in tropical latitudes is the definition of “iso” soil-temperature-regime families where the 1 Pedologists are soil scientists who are primarily interested in the study mean soil temperature in June, July and August differs by less of soils as natural bodies. They should be distinguished from than 6 C from the mean soil temperature in December, Janu- edaphologists or agronomists, soil scientists who study soils as a fi medium for plant growth, and from soil ecologists, who study soils ary and February (Chapter 1). This criterion closely identi es all for the ecosystems services they provide. soils in tropical latitudes where the timing of planting crops 84 SOILS OF THE TROPICS depends on local rainfall patterns rather than seasonal • Poorly drained soils: “typic” subgroups of “Aqu” temperatures. suborders. Soil moisture formative elements (syllables) are utilized The Soil Taxonomy system is based on quantitatively in the order, suborder, great group and subgroup categor- defined diagnostic horizons and attributes that are known ies. The depth at which aquic conditions occur is typically to exist in nature. Both are either easily observable and expressed in a spatial landscape arrangement of soils quantified in a soil profile or measured by standard known as a drainage catena, and is subdivided into well- laboratory techniques that have high reproducibility drained, moderately well-drained, somewhat poorly (USDA–NRCS 1996). Those observations that cannot be drained and poorly drained soils. Aquic conditions are quantified are not included (Buol et al. 2011). expressed by the following sequence of formative-element Table 4.1 provides a simplified definition of the orders, usage: suborders and main great groups found in the tropics. This • Well-drained soils: no aquic element present. table is an interpretation deemed sufficient for the purpose • Moderately well-drained soils: “aquic” used as subgroup of this book and does not include all the classification cri- identification. teria. The complete definitions are given in Soil Survey Staff • Somewhat poorly drained soils: “aeric” (aerated) sub- (1998, 1999) and more recent versions. groups of “Aqu” suborders. Table 4.1 Simplified definition of orders, suborders and great groups of the Soil Taxonomy system, based on the Keys to Soil Taxonomy, 8th edition (Soil Survey Staff 1998), Soil Taxonomy, 2nd edition (Soil Survey Staff 1999) and Stanley Buol (personal communication, 2016). Soil orders are arranged in declining areal extent in the tropics; same for suborders within orders. For horizon designations, see Soil Survey Staff (2014). Order Suborder Great group (number of subgroups) Oxisols: Oxisols are deep, of sandy loam or finer texture (> 15% clay,