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 (United Nations Environment Programme, 1993).
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.
Waste lands are those which for one or the other reason have poor life sustaining property. Out of 100 per cent potentially active lands only 44 per cent are available for cultivation and 56 per cent of land are non-available for cultivation.
The wasteland can be made useful by increasing productivity of land by using some useful methods as afforestation or by using bio-fertilizers. Soil degradation is a complex phenomenon derived by interaction between natural and socio economic factors.
The degradation or deterioration of soil may be caused by the following factors:
1. Physical factors, e.g. loss of fertile top soil due to water or wind erosion.
2. Chemical factors e.g. depletion of nutrients or the toxicity due to acidity or alkalinity (salinization) or water logging.
3. Biological factors which affect the micro-flora and reduce the microbial activity of the soil. These factors reduce the yield.
Some other factors as deforestation, extensive cultivation on marginal land, improper cultivation practices like mono-cropping, poor manuring, misuse of fertilizers or excess use of fertilizers, excessive irrigation, over-grazing, fragility of soil, adverse weather and mining may accelerate the process of soil degradation.
During last decade the nutrients deficiency has been considered as the main cause of poor productivity and crop failure. A study of the current trends in agronomic practices has suggested that the nutrients deficiency is further aggravated by continued use of high yielding crop varieties, intensive cropping pattern and relatively poor fertilizers.
Among the major causes of degradation, water erosion is considered to be the most severe one which covers almost 87% of the affected area. The main cause of water erosion is removal of vegetation, over exploitation of vegetation, over grazing and improper agricultural practices. The latest data revealed that erosion has rendered 200 million hectares or 36% of the total area of the country barren (Table 27.1).
Soil degradation is a global phenomenon. Of the world’s total land area of 13.5 billion hectares, only 3.03 billion hectares (22 per cent) is actually cultivable and about 2 billion hectares is degraded. The annual loss of land is expected to go up to 10 million hectares by 2000 A.D.(Yadava 1996). In India alone, about 188 million hectares or almost 57% of total land area is degraded (Sehgel and Abrol, 1994).
Causes of Soil Degradation:
The main reasons for unproductiveness or degradation of soils are as follows:
1. Nutrient disorder
5. Biological degradation
6. Other causes
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.
According to an estimate of 1992, every year 20.2 million tonnes of NPK is removed by growing crops. The data published by National Bureau of Soil Survey and Land Use Planning (Sehgal and Abrol, 1994) show that about 3.7 million ha land suffers from nutrient loss or depletion of organic matter.
The problem is more severe in the cultivated areas of the subtropical belt. Out of 20.2 million tonnes NPK removed by the plants, only 2.66 million tonnes comes from fertilizers and 3 million tonnes from organic sources. If the loss of nutrients due to soil erosion is included, the loss of nutrients from top soil is 43 million tonnes.
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.
In highly productive areas, canal irrigation is responsible for a rapid rise in water table. Expansion of canal irrigation is also directly concerned with widespread water-logging and salinity problems in arid and semiarid areas. Disturbances in the hydrologic cycle due to inefficient use of surface irrigation water, poor land development, seepage and poor drainage have resulted in higher water tables.
Most of the canal areas in arid and semiarid regions are rich in soluble salts. In irrigation these salts are dissolved in soil water and rise to the surface through capillary action. When the water dries up, the salts are left on the upper surface as a crust or layer. According to National Commission on Agriculture (1976), about 6 million ha area is under water-logged condition. Data of World Bank Survey (1995) reveal that India loses 1.2 to 6 million tonnes of food grains production every year due to water-logging of soil. The water-logging and salinity cause a loss of Rs. 12 billion to 27 billion annually.
Salinity (Saline and alkali soils):
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.
The saline degradation is due to natural causes and poor irrigation practices which disturb the water cycle in areas. Most of the crops in India are affected due to salinity. Productivity loss of some crops is given in Table 27.5.
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.
According to Global Assessment of Soil Degradation (GLASOD), deforestation is the main cause of soil erosion by wind in about 98% of the area. Overgrazing, cutting of timber trees, collection of fuel wood, shifting cultivation and encroachment of forest areas are some of the important factors responsible for the loss of vegetation cover on the soil which ultimately causes soil erosion. The latest data provided by Sehgal and Abrol (1994) show that the total degraded land in India is 187.8 million ha, of which 162.4 million ha is degraded due to soil erosion alone (Table 27.6).
Table 27.6 presents the area under different types of soil degradation in different years:
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.
Applications of some pesticide chemicals (e.g., amitrole, atrazine, bromacil, picloram, etc.) inhibit nitrification. The nodulation and growth of some leguminous crops and nitrogen fixation are inhibited by different pesticides. Disposal of oil shales, heavy metal contamination of soil and spillage of crude oils adversely affect soil micro flora which ultimately affect soil productivity and cause soil degradation.
Other Causes of Soil Degradation:
Extension of cultivation to marginal land:
Due to tremendous population increase the use of land is increasing day by day. Marginal lands though sustainable for farming are less fertile and more prone to degradation. Examples of marginal lands are steep sloppy lands, shallow or sandy soils and the lands in dry and semi-dry areas.
Improper crop rotation:
Due to shortage of land, increase of population and economic pressure, the farmers have adopted intensive cropping patterns of commercial crops in place of more balanced cereal-legume rotations. During last two decades the area under food crops decreased and that under non-food crops increased. Intensive cultivation leads to removal of large quantities of nutrients from the soil which results to in loss of soil fertility.
Soil fertility is reduced due to prolonged intensive cultivation. The farmers maintain productivity of soil by applying chemical fertilizers but make less use of organic manures. Although the yield can be maintained by using fertilizers that provide deficient minerals yet their use often results in deficiencies of other nutrients.
In India pasture land area is decreasing day by day due to expansion of agricultural land. Recent satellite data show that the area under pasture land is severely degraded. This poor condition of pasture lands is due to excessive grazing. The unchecked and indiscriminate grazing on forest land also leads to degradation of forest soils. Overgrazing directly leads to disappearance of vegetation which is one of the important causes of wind and water erosion in dry lands.
Mining disturbs the physical, chemical and biological features of the soil. The impact of mining on soil depends on the physical, chemical properties of the waste generated. The soil profile is changed; the top soil is turned deep inside the dumps. The erodible material is almost devoid of organic matter and lacks in mineral plant nutrients. According to an estimate, about 0.8 million ha soil is degraded due to mining activity.
Impact of Soil Degradation:
The following are the impacts of soil degradation:
1. Degradation leads to reduction in crop yield in the affected lands and a possible decline in cropping intensity.
2. In extreme cases, soil becomes unfit for cultivation.
3. Silting of drainage, canals, rivers and reservoirs results in increased floods and droughts.
4. In some cases farmers use more fertilizer inputs to compensate reduced soil productivity while in other cases, they use excess fertilizers.
5. The rate of siltation in many water reservoirs are significantly high. According to Central Water Commission (1991), nearly 11 per cent of the total capacity of water reservoirs has been silted.
6. Soil degradation has several adverse impacts on the environment. It affects global climate through alterations in water cycle and energy balances and disruptions of carbon, nitrogen and sulphur cycles.
The estimated annual loss of different crops due to soil degradation rangs from Rs. 89 billion to 232 billion which represent a loss of 11 to 26 per cent yield (Table 27.6).