Soil degradation is defined as the decline in soil fertility, when the nutritional status declines and depth of the soil goes down due to erosion and misuse. Soil degradation is the main factor leading to the depleting soil resource base in India. There are spatial variations in the extent of soil degradation depending on the topography, wind velocity and amount of the rainfall.
Soil erosion is the removal of soil by the forces of nature, particularly wind and water, more rapidly than the various soil forming processes can replace it. Water and wind are the most important factors which are simultaneously engaged in soil formation and soil erosion. Under normal circumstances, there is a balance between solid forming and soil erosion processes. This means that the rate of deposition of particles to soil layer is almost the same as rate of erosion and the soil remains intact. However, sometimes that balance is disturbed partly by the action of natural forces but mainly by human factors and the process of soil erosion starts. Soil erosion mainly takes place due to man’s ill-judged activities such as deforestation, over-grazing and faulty methods of cultivation. Excessive grazing by cattle on the slopes of hills cause rapid soil erosion. With the increase in human population, demand for land is increasing at a rapid pace and forest and other natural vegetation is removed, exposing the soil to the forces of erosion.
Agent of Soil Erosion
Wind and water are the two main agents of soil erosion because of their ability to remove and transport soil. Erosion by water is more significant in areas of heavy rainfall and steep slope. Soil erosion by water takes place in the form of sheet and fully erosion. Sheet erosion takes place on level lands after a heavy shower and the soil removal is not easily noticeable. But it is harmful since it removes the inner and more fertile top soil. Gully erosion is common steep slopes. Gullies deepen with rainfall, cut the agricultural on lands into small fragments and make them for cultivation. The areas of excessive gully erosion are the badlands of Chambal and Yamuna and the dry areas of Rajasthan and Haryana. In Chambal Valley, running water cuts deep ravines in the absence of vegetatiion. Ravines are widespread in the Chambal basin. They cover over 6 lakh hectares of land in Gwalior, Morena and Bhind districts of Madhya Pradesh, and Agra, Etawah and Jalaun districts of Uttar Pradesh. In Tamil Nadu, ravines are common in South Arcot, North Arcot, Kanniyakumari, Tiruchiraplli, Chingleput, Salem and Coimbatore districts. In West Bengal numerous gullies and ravines exist in the upper catchment areas of the Kangsabati river in Purulia district. The country is losing about 8,000 hectares of ravines every year.
It is estimated that over 80,000 hectares of cultivated land of India have already been lost and a much larger area is rendered less productive by soil erosion every year. Soil erosion is a national menace for the Indian agriculture and its bad effects are seen in other spheres also. Eroded materials are carried down to rivers and they lower down their carrying capacity, and cause frequent floods and damage to agricultural lands. The bed of the Kaveri river in Tiruchirapalli and Tanjavur districts of Tamil Nadu, for example, has gradually risen and many of the old irrigation sluices and drainage inlets have got blocked. The shallowing of the Brahmaputra causes flood every year. Silting of tanks is another serious consequence of soil erosion. A large number of tanks in different parts of the country get silted every year.
Large parts of soil is blown away by strong winds in dry sandy areas of Rajasthan and adjoining areas. As a result of this, the top layer of the soil is eroded. This is an example of Sheet erosion. The large scale erosion in the dry areas due to over-grazing and intensive cultivation may lead to a general deterioration of the ecosystem known as desertification. Floods cause extensive damage to our soils.
Another major cause of soil erosion is deforestation. Plants keep soils bound in locks of roots and save soil from erosion. Leaves and twigs shed by the plants get mixed in soil and add humus to the soil. Large scale soil erosion has been noticed in areas where forests have been destroyed. This process is more pronounced in hilly areas of the country. Large structures of fertile arable land in irrigated parts of Punjab and Haryana etc. are becoming saline due to over irrigation. Salt lodged in lower proles of soil comes up to the surface and destroys its fertility. Use of chemical fertilisers is also harmful to the soils. In the absence of adequate humus, chemicals harden the soil and adversely affect is fertility in the long run. This problem is common to almost all the command areas of the river valleys especially the areas which were the first beneficiaries of the Green Revolution. It is estimated that about half of the total land of India is under some degree of degradation.