Many arid or semiarid soils contain concentrations of soluble salts that have a negative impact on the efficiency of water use. In irrigated agriculture, salinity is probably the second most important yield constraint to irrigation. In addition to a direct osmotic effect and a possible toxicity of specific ions, soil salinity and salts present in the irrigation water may have a deleterious impact on physical properties such as infiltrabil-ity, water holding, and aeration, especially if the soil or the water are rich in exchangeable sodium.
The most desirable characteristics in selecting a crop for irrigation with saline water are high marketability, high economic value, ease of handling, tolerance to salts and specific ions, ability to maintain quality under saline conditions, and compatibility in a crop rotation . However, no crop is outstanding in all of these categories. For example, the economic value per crop area is negatively correlated with crop salt tolerance , and many high-value crops are sensitive to specific ions.
Because saline conditions reduce both plant growth and seasonal ET, it is important to develop information on crop water-production functions under saline conditions.
Van Genuchten  established four crop response models or functions of salinity. SALT software offers several options for each model, whose application depends on information experimentally generated. Most salinity response studies have reported only the economic-yield component. Few studies have reported any differential yield effects between the economic yield and dry-matter yield as a result of salinity, although crop quality may be differentially affected.
A linear response model was developed :
Y = Ym - Yms(EC - ECt) ECt < EC < EC0 (5.185)
where ECt is the salinity threshold (dSm-1) (salinity from Y starts to decrease), s is the sensitivity of the crop to salinity above the threshold level and EC is the electrical conductivity of the soil solution on the basis of saturation extracts (dS m-1). These relationships are applicable where C is the main ion that affects yield.
Even though the model in Eq. (5.184) is the most widespread due to its conceptual comprehension and the relative simplicity of fits, it is evident that the concept of threshold salinity is a simplification of the crop response, which is actually curvilinear in shape.
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