Soil is the topmost layer of the earth's surface. It consists of a mixture of minute particles of disintegrated rocks, minerals, organic matter and bacteria. Soil is formed when forces of nature such as temperature, rain, wind, waves, animals and plants act on rocks and break them into tiny pieces over a long period of time. The depth of soil is not the same in all parts of the country. Soil may be only a few centimeters deep in some places while in others it may extend to as much as 30 meters.
Soil consists of four layers. The first or topmost layer of soil is made up of minute soil particles and decayed plant and animal matter. This layer is vital for the cultivation of crops. The second layer is made up of fine particles like clay; the third layer is a combination of weathered basic rock materials and soil while the fourth layer consists of un-weathered hard rocks.
India has various types of soil ranging from the fertile alluvial of the Indo-Gangetic plains to the black and red soils of the Deccan Plateau. For example, if one is travellers through the State of Tamil Nadu, one may observe that the ploughed fields in the districts of Salem and Periyar are red while those in Coimbatore and Ramanathapuram are black.
Each type of soil benefits different types of crops through their unique physical, chemical and biological properties. Alluvial soil is a fertile soil rich in potassium. It is highly suitable for agriculture, especially for crops such as paddy, sugarcane and plantain. Red soil has high iron content and is fit for crops like red gram, Bengal gram, green gram, groundnut and castor seed. Black soil is rich in calcium, potassium and magnesium but has poor nitrogen content. Crops like cotton, tobacco, chilly, oil seeds, jowar, ragi and maize grow well in it. Sandy soil is low in nutrient content but is useful for growing trees such as coconut, cashew and casuarinas in areas with high rainfall. Soil may sometimes get eroded through factors such as wind, running water, overgrazing of animals and human activities such as construction. In addition, soil may also be depleted of its fertility if a particular crop is cultivated repeatedly in the area.
This is when soil testing is vital. The quality of soil available in an agricultural area may be tested at soil testing laboratories. Here, the sample of soil is analysed and recommendations are made about what elements are needed to optimize it. Quality soil is one of the most important farming inputs. High yields and good produce can be achieved only when the right type of soil is used for a certain crop. For areas in which suitable soil is not available, one may add nutrients in the form of fertilizers to enrich it.
Keeping this in mind, the government has set up a large number of soil testing labs all over the country