A number of classification have been made on different basis by the agricultural scientists, geographers and geologists. The objects of classifying the soils differ in different classifications.
The geographic and geologic classification of soil are only important in1 the present context of our study.
Geographic Classification
Geographers have classified soil in terms of their areal distribution over the earth’s land surfaces which are linked with the climates, parent materials etc. As we know, under similar climatic conditions similar soil types develop.
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The U.S. Department of agriculture proposed classification system in 1938, which is much simpler and recognizes the existence of three orders of soils
1. Zonal Soils
2. Intrazonal Soils
3. Azonal soils
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Zonal soils are by far the most important and widespread soils. These are exhibiting a mature and well-developed soil profile indicating the fullest play of various soil-forming factors.
They develop on well- drained areas, on parent material which has ramained in the original place for a sufficiently long time to have been affected by various soil- forming processes.
Intrazonal soils are developed under conditions of poor drainage, on regolith where soluble salt contents are high. Soils of bog areas and alkali flats are examples of intrazonal soil. All of them have distinct profile characteristics.
Azonal soil lack well-developed soil profiles which may be due to non-availability of sufficient time for them to develop fully or due to the location on very steep slopes which prohibits profile development. Alluvial soils, dune sands, lithosols (i.e. mountain soils on steep slopes) and organic soils, which develop on peat bogs are examples of azonal soils.