Soil degradation has been defined as a process that leads to decline in the fertility or future productive capacity of soil as a result of human activity. It occurs whenever the natural balances in the landscape are changed by human activity through misuse or overuse of soil. Degraded soils which result in poor or no production are also called problem soils.
Causes of Soil Degradation:
Nutrient disorder: Most of the Indian soils are deficient in nutrients and organic matter. Organic matter is rapidly decomposed and leached or eroded by heavy rains. In addition to these causes, intensive cultivation using high-yielding short-duration and fertilizer-responsive cultivars of crops has further accelerated the loss of plant nutrients which is much greater than what is supplemented through fertilizers.
Water-logging: Soils become water-logged when the water balance of an area is disturbed because of excess recharge. Important sources of water are heavy rains, overland water flow towards basin, seepage from canals and distribution system and tidal flooding. Natural basins without outlet for water, low permeability of subsurface horizons, internal drainage, low intake rate of surface soils and obstructions to natural flow of rain water etc. are conditions cause water logging.
Salinity: Salinity directly affects the productivity by making the soil unsuitable for crop growth. Indirectly it lowers productivity through its adverse effects on the availability of nutrients. The adverse effect of alkalinity on availability of nutrients is due to deflocculating effect of sodium ions. An area of about 21.7 million hectares of soil is rendered unproductive due to salinity and water-logging.
Erosion: Soil erosion is the major cause of soil degradation. In the soil erosion, uppermost fertile layer of soil which contains essential nutrients is lost. Thus soil becomes deficient in essential minerals and this results in productivity loss. Deforestation or destruction of forests accompanied by reduced frequency of rainfall leads to soil erosion and causes damage to agriculture property. Deforestation causes fast degradation when the soil is steep sloppy or easily erodible. Destruction of natural vegetation cover is a major factor responsible for erosion of soils by water and wind.
Biological degradation: The factors which affect soil micro flora and fauna also reduce the biological or microbial activity of soil adversely. These factors reduce the yield. It is well known that mono cropping (growing the same crop on the same land year after year) often leads to increasing attack of pests and diseases. The fatal nematodes threaten potato cultivation in the Nilgiris and, if not controlled they may pose threat to potato cultivation in that area. Excess use of pesticide reduces microbial activity and biomass.
Other causes: Extension of cultivation to marginal land, Improper crop rotation, Fertilizer misuse, Overgrazing, Mining.