Owing to the existence of several thousands of types of soils throughout the world, it is necessary to classify them into groups that have similar observed properties. This practice helps us to understand and remember the traits of a particular soil and be able to develop a good land policy for efficient soil management.
This article discuses about the practice of soil classification which is normally derived from soil survey reports. Soil maps are produced to define areas with similar soils and the location of other land resources.
Concept and Definition of Soil Classification
According to Esu (1999), soil classification is defined as the systematic arrangement of soils into groups or categories on the basis of their observed properties. Observed properties are those that can be seen in the field or measured in the laboratory.
Classification systems based on observed properties are usually called natural classification or soil taxonomy.
When soils are classified based on inferences from the observed properties, the classification system is called technical classification. For example, it could be inferred that crops grown on dark loam soils would have high yields. Therefore, soils classified on the basis of potential yield are a technical system.
The current system worldwide has six levels of classification. The soil order is the highest level and broadest group which are 10 in number. The soil order is divided into suborder which is further divided into Great Group, Subgroup, Family, and Series which is the lowest soil grouping similar to species in animal and plant grouping.
The order and suborder levels of soil classification are usually used at state and regional levels and by soil scientists. Soil series group is more relevant to farmers, builders, extension workers and those that require information on soils at the local level.
Purpose of Soil Classification
Soil classification is useful for the following specific purposes:
– To organize information and knowledge on the soil that are understandable and useful.
– Soils are classified into groups to help us remember their names and important properties.
– Organizing soils into groups with similar properties minimizes the problem of locating information about any one soil.
– It makes us to easily understand the relationships among individual soils being classified.
– Soil classification enables us to predict soil behavior and even estimate their productivity.
– By means of soil map, extrapolation of knowledge and information of one soil to others in other places by different soil scientists, becomes possible.
– Soil classification helps us to identify the best use of a particular soil.
Process of Soil Classification
The following steps as articulated by Esu (1999) constitute the general process of carrying out a natural soil classification, especially those meant for agricultural purposes.
A comprehensive study of the physical environment in which the soils to be classified are located. The parameters so studied include the geology, geomorphology, vegetation, land use, drainage, and climate of the area.
A field mapping, soil characterization, and field sampling of soils is then carried out.
More adequate knowledge of the soil characteristics is then sor through morphological and micro-morphological study of the soils; laboratory characterization of the fine earth fraction of each horizon within pedons; elucidation of genetic factors of soil formation and the relation between the soils and the environment in which they are located.
Armed with an adequate knowledge of the characteristics of the soils, the soil classifier (pedologist) then formulates differentiate at the lowest category.
The next step involves the clustering of soil individuals at lower categories into higher level taxa or classes on the basis of similar characteristics (Hierachial, or Multiple Category system).
This is followed by the regrouping of higher classes into a yet higher classes.
More extensive soil mapping is then carried out in areas of similar environmental settings to test the classification carried out at a different location.
The classification is then tested and adapted several times so as to discover and learn new relationships. At this point, the classification is sent out to colleagues for necessary inputs and valid criticisms.
The new relationships learned, the criticisms, and other ideas are finally incorporated into the classification and a Final Classification is thus obtained.