While the nature takes from 100 to 400 years to build one centimetre of top soil, man can and often does destroy it almost overnight by haphazard land use and improvident husbandry. Irrational methods of cultivation, deforestation, destruction of natural vegetation due to overgrazing by pasturing animals etc. , accelerate denudation.
Besides, failure of rains, floods, depopulation and loss of cattle caused by famine and pestilence, disturbance caused by war and interference with or change in the natural drainage system have had their deleterious effect on the soil at some time or the other. a) Irrational Methods of Cultivation (i) Faulty method of cultivation Particularly on the steeper slopes when the virgin land is ploughed and naked soil is exposed to the rain, the loss of fertile soil is enormous. The potato cultivation in the Himalayas and the Nilgiris, where the rows run straight up and down hill, causes an abnormal rapid loss of soil. (ii) Shifting cultivation It is a primitive form of soil utilization. In shifting cultivation, framers grow food only for themselves and their families.
In this system of farming a patch of forest is selected. Its tree and bushes are than cut and burnt down on the ground in order to clear room for a field. The ground is then lightly ploughed and seed is sown broadcast and racked into the soil at the first fall of rain. The soil gives rise to a better yield as it is immensely fertile owing to the wood ashes and accumulated humus. After two or three years’ crop, when the fertility of the soil is seriously reduced, the people again change their land of cultivation.
Thus the essential feature of shifting cultivation is the rotation of fields rather than crops. As a result more and more land are exposed to erosion. (iii) Nature of crop grown In India, as it has been noticed the dry crop producing regions (such as millet, maize, potato, tobacco, cotton and even wheat growing region) of high temperature, low humidity and scanty rainfall are attended by heavy loss in soil specially at the time of a heavy shower. In contrast to these, rice, jute and sugar cane account for very little loss of soil.
b) Deforestation The cover of the vegetation not only reduces the velocity of surface runoff, but also binds the soil particles through the roots and increases its strength. Thus vegetation cover protects the soil from the attack of erosional processes. Deforestation leads to increased runoff of rain water and its diminished seepage and storage in the soil. The structure of the soil suffers, the runoff increases which loosens the soil and carry it. Once the channel is filled up with the load, it causes devastating floods at times.
In a natural forest, the force of rain is checked by the leaves of trees and thick carpet of vegetation, while surface covering of soil and the humus soak up the rain water like sponge and let it sink into the ground to emerge afterwards as springs and streams. When the rain falls gently, the whole is absorbed and violent floods in the stream is lessened but deforestation makes the problem more severe.
(c) Over-grazing Over-grazing by pasturing animals like cattle, goats and sheep makes the grass cover on the soil worn and thin. As a result, rain drops begin to fall directly on the soil and clogging up the pores with mud and form an impervious crust which reduces infiltration into the soil and increases surface runoff. This leads to an increase in the area of bare ground. As a consequence, they fall easy victims to various erosional processes. Apart from the above causes, the systems of farming, size of the farm tenancy, tenant-landlord relationship etc. also constitute the socio-economic factors of soil erosion.
January 9, 2018
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