Soils develop as a result of a long process of weathering and disintegration of rocks which change them into sand, silt, clay and gravel.
A few centimeters of soil form in hundreds of years depending on the nature of parent rock material. However, rock fragments alone do not form a soil. A large variety of living organisms occur in a soil which include: algae, bacteria, actinomycetes, fungi, protozoans, helminths, insects, reptiles, rodents, plants and animals. It is these organisms which actually transform rock fragments into a soil.
The inorganic material derived from the parent rocks is gradually enriched with organic matter produced by the living organisms during their lives as well as after their death. This organic matter serves as a food or a source of energy for much of the living communities in the soil. Partially decomposed organic matter, called humus, gives the soil a loose texture, and ensures better aeration and a greater capacity to imbibe water and nutrients.
Had soil been a lifeless material, its store of plant nutrients, howsoever large, might have disappeared at some point of time. Plant nutrients in the soil are regularly regenerated by decay, decomposition and mineralization of organic matter deposited by living organisms which also solubilize nutrients from rock fragments and soil particles. Atmospheric nitrogen is fixed by a number of bacteria and blue green algae.
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A number of microbes are known to produce growth-promoting substances. Insects, helminths, rodents etc. turnover and mix a large quantity of soil. Root decay leaves in the soil a network of channels. Burrowing animals create innumerable passages in the soil which helps in exchange of gases.
Mucilage produced by blue green algae, bacteria and a number of other micro-organisms along with other organic matter binds the soil particles in crumbs or aggregates which prevents excessive leaching and erosion. Fungal hyphae weave a network which helps in keeping these aggregates together.
Soil is, therefore, a living entity. Living communities in the soil are the basis of its fertility and other properties which promote growth of a healthy plant life on it years after years in succession. Anything detrimental to the living community within the soil tends to cause its degeneration, loss of fertility and an increased rate of soil erosion. In absence of living communities soil turns into a useless heap of sand, silt and clay.