Soil is not only a mixture of rocks; it contains living organic substances also. These organic substances continue to operate in plant and animal organisms of the rock mixture.

This mixture, through chemical, physical and biological actions convert carbohydrates, proteins, etc. into many types of substances which supply the vegetation with food. When the vegetation dries, it (i) becomes the food for others, (ii) changes the form of surface, (iii) causes weathering of mineral substances and (iv) develops the soil formation.

The biological portion of the soil consists of a mixture of leaves, fruits, branches, the residue as well as parts of animals, etc. in various stages of decay.

The micro­organisms which are usually present are fungi, bacteria, protozoa, insects, etc. The number of these micro-organisms in one gm of soil exceeds one million.

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It is now clear that soil is not a lifeless mixture of rocks but it is in fact a mixture of living and non-living things.

It is a medium in the form of layer on land surface, of rocky substances and living organisms in which climate, organisms, etc. create such a mixture through physical, chemical and biological activities that it actively contributes to the growth of vegetation.

Long well and Flint consider it as a link between living and non-living things. With time the structure and outer form of soil continue to change.

“Soil is, therefore, not a static thing but is a dynamic and developing part of the surface of the earth, which adjusts itself according to water, air and structuring of the surfaces.” Science related to the study of soils is called Pedology.

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The unconsolidated and loose material of rock lying on the surface of the earth which is fit for the growth of plants or soil is that layer of the earth’s surface which is composed of minerals and organic matter and is capable of sustaining plant life.

Soils are formed due to weathering of rocks and minerals. This process of disintegration had been there since ages. Heat, cold, frost, running water, rain, tides and wind are the major agencies of weathering.

The sudden heating and cooling of the rocks breaks them into fragments which are further weathered through chemical reactions where moisture is there. Plants and animals are also responsible for weathering.

Thus soil is continuously being formed out of rock and in different layers.

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India is a large country with various types of land forms, different rock cover, different climates and numerous vegetation zones having a wide variety of natural vegetation. All these factors effect soil and produce a number of soil types.

The study of soil is done with respect to soil characters like texture, structure, colour and drainage conditions. Based on this study soil classification is made and proper land use planning is done accordingly.

The soil productivity depends upon physical and chemical properties of the soil. The physical properties include soil colour, texture and structure.

Soil Colour.

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Colour is the result of organic matter and iron compounds present in the soil. When the iron compounds are less as in the case of poorly drained soils, the soil colour is normally grey.

The soils arc red in colour when the iron compounds are in an oxidized form. The soils are yellow in colour when the iron compounds present in the soil are in a hydrated state.

Soil Texture.

Soil texture is an index of size and distribution of soil particles. These particles are classified on the basis of their size as clay, silt, sand etc. The standards fixed for different fractions arc as under:

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(i) Soil is heavy when proportion of clay in it is high.

(ii) Soil is medium when proportion of clay in it is small and sand and silt are in nearly equal proportions.

Soil is light when there is a large proportion of sand and gravel but very little of clay and silt in it.

Clayey soils are prone to water erosion. Sandy soils are prone to wind erosion.

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Soil Structure.

It refers to the arrangement or grouping of soil particles. It is also known as soil aggregation. It is caused by sand, silt and clay particles adhering together to form larger soil mass.

On the structural pattern of the soil depends the moisture holding capacity of the soil, its porosity and resistance of the soil to erosion or leaching