Amelioration strategies for salinity-induced land degradation.
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AbstractSalinization of soils is a major impediment to their optimal utilization in many arid and semi-arid regions throughout the world. In several large irrigation schemes, salinity-induced land degradation has increased steadily over the last few decades with concurrent reductions in agricultural productivity and sustainability. Currently, saline soils occur within at least 100 countries. These soils need explicit approaches in their amelioration since soil salinization cannot be reduced by routine irrigation and crop management practices. The approaches used to ameliorate saline soils can be broadly divided into four major categories: (1) leaching of bare saline soils to move excess soluble salts from upper to lower soil depths or out of soil profile in the presence of a natural or artificial drainage system; (2) leaching of cropped saline soils with certain plant species, which can withstand ambient salinity levels; (3) surface flushing or mechanical removal of salts from soils that contain salt crusts at the surface or where drainage is inadequate and leaching is restricted by the presence of a shallow water table or highly impermeable profile; and (4) reduction of salts from saline soils through biological means - the harvest and removal of high-salt accumulating aerial plant parts - in areas with negligible irrigation water or rainfall for leaching. Research and practice have shown that in the presence of a drainage system, saline soils can be brought back to a highly productive state by leaching the excess soluble salts from the effective rooting depth. Among the amelioration methods, cropping in conjunction with leaching has an advantage; cropping during or between leaching events increases salt removal efficiency because of a decrease in soil-water content under unsaturated water flow conditions with a concurrent decrease in large pore bypass and drainage volume. However, anaerobic conditions in soil may occur during leaching of cropped soils, thereby affecting growth of those plant species that are sensitive to such conditions. Thus, in addition to existing salt-resistant plant species, research is needed to explore or develop genotypes that can withstand the twin stresses of salinity and hypoxia. Although significant efforts have been made to develop models for simulating movement and reactions of salts in saline soils during the amelioration process, these models need evaluation and verification under actual field conditions.
