In general, two broad kinds of soil classification are recognized; a Natural or Taxonomic Classification also termed scientific classification, and a Technical Classification.
A Natural Classification is one in which the purpose of the classification is, to bring out relationships of the most important properties of the population being classified without reference to any single specified and applied objective.
In a natural classification, all the attributes of a population are considered and those which have the greatest number of covariant or associated characteristics are selected as the ones to define and separate the various classes.
Most soil classification systems try to approach a natural classification system as an ideal, though some more weight tend to be given to properties of higher agricultural relevance.
The most common examples of a natural soil classification system are the USDA Soil Taxonomy System (Soil Survey Staff, 1975) and the FAO – UNESCO Soil Map of the World Legend (FAO – UNESCO, 1988).
These two systems have received worldwide attention and are the most used in Nigeria. The salient features of these systems of soil classification will be briefly discussed later.
A technical Classification, on the other hand, is one which is aimed at a specific, applied, practical purpose. For example, classifying soils for agriculture or engineering purposes or even more specifically classifying soils for maize production or for irrigated agriculture.
The most common examples of a technical classification which are widely used worldwide are the USDA Land Capability Classification System and the FAO Land Suitability Classification for rainfed agriculture.
Read Also: Physical Properties of Soils (Soil Physical Properties)
USDA Soil Taxonomy System
In 1951, the Soil Survey Staff, Soil Conservation Service of the United States Department of Agriculture (USDA) started the development of a new system of soil classification.
The system was developed by a series of approximations, testing each one to discover its defects and thus gradually approaching a workable system.
In 1960, the 7th Approximation was published to secure the widest possible criticism. It was adapted and changed in various supplements and in 1975, the final text was published under the title “Soil Taxonomy: A basic System of Soil Classification for Making and interpreting Soil Surveys”.
In the 1975 edition, ten Soil Orders were recognized (Soil Survey Staff, 1975), but several revisions have since been carried out and in 1994, one additional Soil Order of Andisols was added making it a total of eleven Soil Orders (Soil Survey Staff, 1994).
The System is a multiple category or Hierarchical System and contains six categories. From highest to lowest levels of generalization, they include; the Order, Suborder, Great group, Subgroup, Family, and Series.
FAO-UNESCO Soil Map of the World Legend
In response to a recommendation of the International Society of Soil Science (ISSS) at its 7th congress held at Madison, Wisconsin in 1960, the preparation of the FAO/UNESCO Soil Map of the World project at a Scale of 1:5,000,000 was started in 1961.
Successive drafts of the soil map and of the legend were prepared from a compilation of existing materials combined with systematic field identification and correlation.
The first draft of the Soil Map of the World was presented to the 9th Congress of the ISSS held at Adelaide (Australia) in 1968. Since then, the Legend of the map has undergone many revisions, the latest being in 1988 (FAO-UNESCO, 1988).
The objective for developing a soil map of the world legend was to correlate all units of the various soil maps in the world (and they are numerous) and to obtain a Worldwide inventory of soil resources with a common legend.
The nomenclature has, however, been drawn from a number of national systems in a successful exercise of international diplomacy. It was perceived that this exercise will provide for the ready transfer of land use and management knowledge.
The soil Map of the World Legend thus has a bicategorical system, with the highest or upper class being approximately but not completely equivalent to the Great group level of
Soil Taxonomy (USDA). The lower category is composed of intergrades or soils with special horizons or features of note.
The latest revisions also includes phases to subdivide the secondary classes according to the difference in characteristics or qualities important in use and management of the soils such as texture, stoniness, presence of indurated (pan) layers, presence of hard rock, slope, and presence of salinity.
Today, the Legend consists of 28 major soil groupings, subdivided at the second level into 153 soil units. Additional information is contained in the texts recommended for further reading.
Classification of African Soils
The most prevalent groups of African soils are usually the four groups described below.
Ferruginous Tropical Soils
These are soils which have the A, B, C, and D – horizons designation. Some may have B1, B2, where the B could be a textural one in which clay is accumulating.
Spores which are rich in sesquioxide may be present in the soil; the silt content is usually low. Clay type of this group is mainly kaolinite though pockets of 2:1 lattice clay may occur. This trend makes the cation exchange capacity of these soils to be very low.
The silica- sesquoxide ratio is less than 2 while the color characteristic is 10YR – 7.5YR. The soils of the cocoa belt in West Africa are classified in this group.
Ferrisoils
Soils in this group are characterized by high base content, good structure, and high biological activity. The surface of aggregates may have a gloosy appearance due to the aggregation of sesquoxide clay.
The silica- sesquoxide ratio (i.e Si O2: Al2O3 + Fe2O3) may be less than 2, especially in the clay component which confers low cation exchange capacity on the soils.
Similarly, base saturation is low (less than 50). These soils are further divided according to the type of parent material from which they are formed. Some of these soil types are found in northern Nigeria.
Ferrallitic Soils
These soils reflect the final stage of weathering and leaching whereby only the least mobile and least weather-able constituents remain; even kaolinite may be altered. Soils are very deep with poor horizon differentiation whereby only A2 or textural B (heaviest texture formed due to clay eluviations) may be present.
There are no glossy surfaces as the clay type is mainly kaolinite associated with a large quantity of iron oxide. The silica-sesquoxide ratio is <2, while base saturation is also <40. The soils in this group are poor in weatherable materials.
Most soils formed form sandstone parent material in Nigeria have been classified into this group e.g. soils along Ankpa – Anyigba – Idah axis in the kogi State of Nigeria.
Hydromorphic Soils
These are soils that are under the influence of the water table. They are known by the degree of mottling present. There is usually the rise and fall of water table over the soils resulting in the variegated color of mottling in the soils.
