Soil is usually defined as any part of the earth’s crust in which plants root.
Soil is composed of three types of constituents: (i) soil particles (ii) water and (iii) air. The soil particles consist of minerals, inorganic or organic materials.
These substances get dissolved in the soil-water and are thus, made available to the roots of the plants.
The soil particles contain tiny spaces which are filled with water and air. This soil water is called the hygroscopic water and cannot be removed by drainage. There is another form of water, outer to hygroscopic water which is called capillary water. This water circulates in the soil and is utilized by the root hairs of plants.
Kinds of Soil:
On the basis of its texture, soil can be divided into the following five kinds:
1. Sandy Soil:
This soil is composed of coarse particles of silica (0.04 to 1.0 mm diameter). The soil is easy to plough and retains very little amount of water. It is warm and dry, contains little or no food material, and thus is unfit for cultivation.
2. Clay Soil:
It is composed of very fine particles of silica together with oxides of iron, aluminium and other materials. It can hold and retain large amounts of water. These soils lose heat rapidly by evaporation and are thus known as cold soils.
3. Calcareous Soil:
It supplies food to the plants in the form of lime, magnesium, phosphorus. Iron, aluminium and ammonium compounds it supplies alkaline material to the soil and neutralizes the acidity of the humus. In the presence of calcareous matter, nitrifying bacteria work more easily as the process of nitrification takes place in alkaline medium.
The soil which possesses the quality of sandy and clay soils is known as loam. It is best suited for the cultivation of plants.
This is the decaying organic matter of both animals and plants. It has great capacity to hold water and contains 4 to 9% of nitrogen and is very useful to plants.