Soil is a natural resource of immense value to man. Its nature and fertility determines the crop- productivity and agricultural production through which man draws his food and essential raw materials. An area rich in fertile soils yields good agricultural harvests and supports large human population. On the other hand, if the soil cover is shallow and lacks in fertility, the agricultural economy tends to be depressed, and the population density as well as the levels of living tend to be low.
‘Soil may be defined as finely divided rock material which contains mineral matter in soluble form that will support the growth of plants’ (P.G. Worcester). It is either the altered residue of the underlying rocks, after the soluble constituents have been removed, mingled with some proportion of decomposed organic matter (residual soil); or the soil-cap may be due to the deposition of alluvial debris brought down by the rivers from the higher grounds (drift soil) (Wadia, 1975, p. 478).
The process of soil formation (pedogenesis) is very complex and time consuming. It largely depends upon the nature of parent rock, surface features of relief, climatic conditions, natural vegetation and activities of man. A number of animals, insects and bacteria are also associated with this process.
The material for soil formation, termed by the soil scientists as the parent material is derived from the rock cover. It largely determines the chemical composition of the soil. The relief and slope characteristics-along with the various agents of weathering-determine conditions for the disintegration of the rock cover. Climate, through its elements like temperature and rainfall, controls the type and effectiveness of weathering of the parent material, the quantity of water seeping through the soil and the type of microorganisms present therein. Similarly the natural vegetation enriches the soil by providing it with the much needed content of humus.
The process of soil-formation passes through different stages. The lowest stage is termed as bed rock or the parent rock which is overlain by various layers called horizons. The D horizon immediately over the bed rock accumulates the fresh parent material. This is succeeded by C horizon as a zone of weathered parent martial, B horizon as zone of alleviation or accumulation, and lastly A horizon as a zone of eluviations or leaching.
The A horizon near the surface consisting of fine-grained material is rich in organic matter and supports plant life. The vertical section of the soil from A horizon to the bed rock is called soil profile. While in residual soil a horizon may just overlie its bed rock same may not be true in case of drift (alluvial) soil.
Soils have different particles of varying size. The finest soil is known as ‘colloidal clay’ whose particles cannot be identified through naked eyes. Others are called clay, silt, sand and gravel depending upon the size of their particles.
Their mixing leads to the formation of various types of loans. If clay is more and silt is less, the loam is called salty clay. If opposite is found it is called clayey silt. If three or four types are mixed together the loam becomes very complex. Most of the Indian soils are found in the form of loams which do not exhibit donation as in case of temperate or Alpine soils.