Soil Microbiology Introduction:

Soil is the surface layer of earth on which the human civilization depends for its existence. Actually soil represents the loose upper crust of the earth surface distinctly different from the underlying bed rock. Its depth, colour, composition vary from place to place, but all soils are common in consisting of inorganic (mineral) and organic matter, water, and gaseous phases. Every soil is made up of a succession of layers, collectively known as soil-profile, reaching down to the parent material.

The soil-profile consists of two or more horizontal layers, called horizons. The soil horizon may vary in thickness, mineral composition, and structure; they are indicated by the letter A1, A2, A3, B1, B2, B3, C1, etc. A1 horizon is the uppermost or surface layer of the soil and its fertility level is very important from viewpoint of an agriculturist. Soil fertility depends not only on the presence of inorganic and organic substances, but also on the presence of various species of microorganism which influence the qualitative composition of the soil.

To a farmer, it is a medium to grow his crops in and from which the plants obtain their mechanical support and many of their nutrients. Chemically, the soil is endowed with a magnitude of organic and inorganic substances not found in the underlying starta; indeed it functions as natures chemical laboratory in which various dissolutions and synthetic processes go on continuously in a hidden manner.

A lay man, however, is always of the opinion that soil is dust, essentially a dead material, sustaining nothing like life within it. With regard to origin and evolution of life, it can be considered that soil is the depository of all lives within which are carried out most of the transformations that enable life to continue. Ecologically, soil is the most dynamic component (lithosphere) of the global environment encompassing distinct communities of organisms in its realm. For a builiding engineer, the soil is a substratum on which structures can be built. But nothing could be farther from the truth, a microbiologist would say. For him, soil appears to be a dynamic body on the surface of the earth.