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Soil degradation processes

Soil degradation has become a serious problem in both rainfed and irrigated areas of India.

3.1 Salinization of soil

The sources of salts in soil are from the soil itself or also from ground water, irrigation water, canal water and tide water. The primary source of salts in soil is from rock weathering. During the process of rock weathering and soil formation, soluble salts are formed. Solute movement in the water is the determining factor in the soil salinization process. In the humid and sub humid regions with adequate rainfall, most of the soluble salts are leached down either at some depth below the surface or into the ground water. If the rainfall is not adequate and the evapotranspiration exceeds rainfall, leaching is not adequate to remove the soluble salts. Soil salinization is quite common in the arid and semi arid regions having an annual rainfall of less than 55 cm. Further, incrustation of salts on the land surface also occurs.

Fluctuating depth of ground water or the water table leads to salinity in soil. Soluble salts move upward along with rise in the water table. The salts are left behind when the water table recedes and accumulate at varying depth below the soil surface. Upward movement of soluble salts that are already accumulated at some depth of soil also takes place. The process of salinization is accelerated by rapid evaporation from the surface. The higher the depth of the water table, the higher is the rate of evaporation. The critical depth of the water table is defined as the depth of the water table above which soil solution can move upward by capillar}' action of the surface to cause soil salinization. This critical depth depends on the texture, water retention and transmission characteristics of soil in the profile. Soil salinization in the coastal area is due to the accumulation of salts from inundated sea water. Irrigation water containing high concentration of soluble salts, particularly sodium, leads to soil salinity, if proper drainage is not provided to leach the salts beyond the root zone. The soils which have salt accumulation at certain depths below the surface may also lead to salinity if the depth of irrigation water is inadequate to leach down the salts. Seepage from canal or irrigation channel may cause salinity. Soluble salts move along with seepage water and are accumulated at the water front.

3.2 Alkalization of soil

Formation of carbonates of Na and alkalization in the soil take place as a result of carbonation of alumino-silicate minerals in the presence of water. Sodium carbonate is highly soluble and its hydrolysis results in high alkalinity (pH up to 12).

In the presence of C02, the pH is lowered because of the formation of bicarbonates of Na, according to the reaction

The release of C02 with decomposition of organic matter in soil accentuates the process of NaHCOj formation in soil. In arid region, these reactions go on indefinitely, resulting in excessive accumulation of Na2C03 and NaHCOj in the soil. With excessive evaporation and extreme arid conditions, carbonates and bicarbonates of sodium may accumulate in the soil as double salt crystals (Na2C03. NaHCOj. 2H20) or pure NaHC03.

With increase in the concentration of soil solution due to evapotranspiration, the solubility limits of calcium sulphate, calcium carbonate and magnesium carbonate are exceeded and hence they get precipitated. Therefore, the relative proportion of Na+ ions with respect to Ca2+ and Mg2+ ions becomes high in soil solution, resulting in the increase of Na+ ions on the exchange complex as per equilibrium and ESP increases to produce alkali soils. Most alkali soils, particularly in arid and semi-arid regions, contain CaC03 in the profile in some form, and constant hydrolysis of CaC03 sustains the release of OH' ions in soil solution.

The OH" ions so released results in the maintenance of higher pH in calcareous alkali soils than that in non calcareous alkali soils. The increase in ESP can markedly affect the physicochemical properties of the soil. High sodium saturation in the soil results in the dispersion of clay particles, i.e., they tend to repel each other and remain independent of others. Highly hydrated Na" ions increase the zeta potential of the exchange sites, resulting in the repulsion of clay particles from each other. These soils therefore have very poor structure. The dispersed clay particles move downwards through soil pores and produce a dense or compact layer of very low permeability at some depth. Due to the decrease in hydraulic conductivity of alkali soils with increase in ESP, irrigation water stagnates at the surface of the field. Clogging of pores in the surface soils, followed by drying, results in the development of crust, which hinders seedling emergence and deteriorates air-water relations in the root zone. Alkali soils become very hard when dxy and sticky when wet.

 
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