There is no unique parameter relating crop yield to the average soil salinity, since crop growth changes with soil water deficit and other soil features. There are mathematical models for estimation of crop yield under saline conditions dealing with the physics of water movement through soil, plant and atmosphere (Grant et, al., 1993). The model named ecosys was expanded to include an ion transfer-equilibrium-exchange model used to calculated electrical conductivity and soil osmotic potential, and it was proposed for general use in assessing salinity effects on crop growth and water use on different soils and environmental conditions (Grant, 1995). Some other models propose the application of static and dynamic salinity stress indices (SSI) in order to determine root zone salinity and ion flux to the shoot, respectively (Dalton et al., 1997), and the separation of physical and biochemical processes governing plant salt tolerance (Dalton et al., 2000).
The conceptual models, which do not neglect solute reactions in the root zone, the surface evaporation and influence of immobile wetted pore space can accurately predict the leaching requirements (Lr) for crops from salinity of irrigation water and the crop salt tolerance threshold (Alsaeedi and Elprince, 1999). The application of a sprinkler (such is triple line source sprinkler system) and a drip irrigation system was successfully used in screening for salinity tolerance in barley (Isla et al., 1997). Several other models dealing with estimation of specific parameters of soil salinity, two- and three-dimensional equilibrium solute transport, field-scale spatial salinity patterns, etc., have been proposed during the last twenty years by USDA (USDA Salinity Laboratory, http://www.ussl.ars.usda.gov/modelsmenu.htm). Among them, the newest model ("WATSUIT"), developed in 2001, predicts the salinity, sodicity and the toxic solute concentrations of the soil water within a simulated crop root zone. This model allows an evaluation of crop salinity threshold, slope and yield at given salinity level. Nevertheless, in case of increasing and variable salinity within farmer fields, an estimated yield for the most of the cultivated species is far from acceptable. Hence, it seems that the only reasonable option for utilization and management of highly salinized soils should be domestication and use of halophytes.
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