The Soil Taxonomy developed since the early 1950’s is the most comprehensive soil classification system in the world, developed with international cooperation it is sometimes described as the best system so far. However, for use with the soils of the tropics, the system would need continuous improvement.
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Just as plants, animals, and other living things are named and classified using taxonomic systems, soils are also classified. Many nations, as well as the Fisheries and Agricultural Organization of the United Nations, have their own system to classify their own soils. The one used in the United States (as well as several other countries) is called Soil Taxonomy (Soil Survey Staff 1975). Almost every soil on earth can be classified according to this system, and it has become the standard soil classification system in scientific journals throughout the world. Soil Taxonomy is periodically updated to encompass the results of new research and studies, with the latest complete key published in 1998.
Categories in the Soil Taxonomy
There are six levels in the hierarchy of categories: Orders (the highest category), suborders, great groups, subgroups, families and series (the lowest category) (USDA, 1978).
There are ten orders, differentiated on gross morphological features by the presence or absence of diagnostic horizons or features which show the dominant set of soil-forming processes that have taken place. The ten orders and their major characteristics are shown in Table 1. The occurrence of the major soils in the humid and subhumid tropics.
It is the next level of generalization. It permits more statements to be made about a given soil. In addition to morphological characteristics other soil properties are used to classify the soil. The suborder focusses on genetic homogeneity like wetness or other climatic factors. There are 47 suborders within the 10 orders. The names of the suborders consist of two syllables. The first connotes the diagnostics properties; the second is the formative element from the soil order name. For example, an Ustalf is an alfisol with an ustic moisture regime (associated with subhumid climates).
The great group permits more specific statements about a given soil as it notes the arrangement of the soil horizons. A total of 230 great groups (140 of which occur in the tropics) have been defined for the 47 suborders. The name of a great group consists of the name of the suborder and a prefix suggesting diagnostic properties. For example, a Plinthustalf is an ustalf that has developed plinthite in the profile. Plinthite development is selected as the important property and so forms the prefix for the great group name.
Table 1. Brief descriptions of the ten soil orders according to Soil Taxonomy.
|ALFISOLS||– Soils with a clayey B horizon and exchangeable cation (Ca + Mg + K + Na) saturation greater than 50% calculated from NH4OAc-CEC at pH7.|
|ULTISOLS||– Soils with a clayey B horizon and base saturation less than 50%. They are acidic, leached soils from humid areas of the tropics and subtropics.|
|OXISOLS||– Oxisols are strongly weathered soils but have very little variation in texture with depth. Some strongly weathered, red, deep, porous oxisols contain large amounts of clay-sized Fe and Al oxides.|
|VERTISOLS||– Dark clay soils containing large amounts of swelling clay minerals (smectite). The soils crack widely during the dry season and become very sticky in the wet season.|
|MOLLISOLS||– Prairie soils formed from colluvial materials with dark surface horizon and base saturation greater than 50%, dominating in exchangeable Ca.|
|INCEPTISOLS||– Young soils with limited profile development. They are mostly formed from colluvial and alluvial materials. Soils derived from volcanic ash are considered a special group of Inceptisols, presently classified under the Andept suborder (also known as Andosols).|
|ENTISOLS||– Soils with little or no horizon development in the profile. They are mostly derived from alluvial materials.|
|ARIDISOLS||– Soils of arid region, such as desert soils. Some are saline.|
|SPODOSOLS||– Soils with a bleached surface layer (A2 horizon) and an alluvial accumulation of sesquioxides and organic matter in the B horizon. These soils are mostly formed under humid conditions and coniferous forest in the temperate region.|
|HISTOSOLS||– Soils rich in organic matter such as peat and muck.|
Table 2. Occurrence of major soils in the Humid and Subhumid Tropics.
|1. Alfisols||Savanna and drier forest zones|
|2. Hydromorphic Soils||Valley bottom of a rolling topography|
|3. Vertisols||Alluvial plains in savanna|
|4. Ultisols||Rain forest zone and derived savanna|
|5. Oxisols||Rain forest and savanna|
|6. Inceptisols||All regions|
|7. Andepts (suborder of Inceptisols)||Limited and localized distribution relating to present and past volcanic activities|
There are three kinds of subgroups:
1. The typical subgroup which represents the central concept of the great group, for example Typic Paleustalfs.2. Intergrades are transitional forms to other orders, suborders or great groups, for example Aridic Paleustalfs or Oxic Paleustalfs.
3. Extragrades have some properties which are not representative of the great group but do not indicate transitions, for example, Petrocalcic Paleustalf.
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The grouping of soils within families is based on the presence or absence of physical and chemical properties important for plant growth and may not be indicative of any particular process. The properties include particle size distribution and mineralogy beneath the plough layer, temperature regime, and thickness of rooting zone. Typical family names are clayey, kaolinitic, isohyperthermic, etc. There are thousands of families.
The soil series is the lowest category. It is a grouping of soil individuals on the basis of narrowly defined properties, relating to kind and arrangement of horizons; colour, texture, structure, consistence and reaction of horizons; chemical and mineralogical properties of the horizons. The soil series are given local place names following the earlier practice in the old systems in naming soil series. There are tens of thousands of series.
Classified Soils in the Tropics
According to the USDA Soil Taxonomy, Oxisols are the most abundant soils in the humid and perhumid tropics covering about 35 percent of the land area. Ultisols are the second most abundant, covering an estimated 28 percent of the region. About half of the Ultisols and 60 percent of the Oxisols are located in humid and perhumid tropical Africa and Asia. In tropical Africa, they are abundant in the eastern Congo basin bordering the lake region; in the forested zones of Sierra Leone; in Ivory Coast; in parts of Liberia; and in the forested coastal strip from Ivory Coast to Cameroon (Figure 2).
The Alfisols, which have high to moderate fertility, cover a smaller area of the humid tropics. In west Africa they are found in Ivory Coast, Ghana, Togo, Benin, Nigeria and Cameroon. They are, however, the most abundant soils in Africa’s subhumid and semi-arid zones, covering about one third of these regions. The Alfisols are widely distributed in the subhumid and semi-arid tropical regions of Africa, including large areas in western, eastern, central, and southeastern Africa.