TR-22 : Leaching requirement of agricultural land and study of movement of salts

Abstract

One of the major problems associated with irrigated agriculture land is accumulation of salt in the soil which results in low productivity if proper water management practices are not adopted. Irrigation water contains salts from 0. 1 to 4 metric tons/1000m3 and are generally applied to soils at annual applications rates of 10,000 to

15,000 m /ha. The concentration of soluble salts in soils increases as the applied water is removed by evaporation and transpiration. Evapotranspiration creates a suction force that produces an upward flow of water and salt into the root zone from lower soil depth. To prevent harmful accumulation of salts in soils, an additional amount of water, over and above that required to meet crop evapotranspiration needs, must be passed through the root zone for leaching of salts.

A model has been formulated using Green and Ampt equation for estimation of infiltration volume from different soil layers. The soil system was divided into 4 layers of thickness 50 cm each having some initial salt concentration and soil moisture. When irrigation water having low salt concentration is applied to soil system for leaching, the solute movement from 1st layer to 2nd layer, 2nd layer to 3drd layer, likewise have been determined using salt balance approach. The time required to fill the different layers have been estimated from the initation of the infiltration. The variation of salt concentration with time and depth have been estimated and presented.

Break-through curves have been presented for different layers. It is found that when soil is near saturation and when one pore volume of water passes through the top soil of 50 cm thickness, the solute concentration predicted by the present model is 0. 54. For an ideal break-through curve, 50 per cent displacement of salts take place at one pore volume. Thus the salt prediction by the present model is satisfactory. It has been said that 1 .2 to 2. 0 times the pore volume replacement of the soil solution should remove about 80 per cent of the original salt content. From this study, it is found that wren water twice the pore volume is passed through the 50 cm of the top soil, 76 percent of salt is removed.

For a silty loam soil, it is found that for wheat crop when 50 cm of irrigation water having salt concentration 1 .00 mm ho/cm passes through the soil having initial solute concentration 13. 00 mm ho/cm, the salt upto 106 cm depth has been leached out.