Using water of marginal quality for crop production: major issues

Abstract

A considerable amount of data is available regarding the effect of soil salinity on crop yield. Most of the data was obtained under uniform spatial and temporal distribution of salts, at high levels of fertility and with crop established prior to the introduction of saline conditions. In practice, under realistic field conditions, uniformity is the exception rather than the rule, soil fertility may not be optimal and salinity may be present before the crop is established. In addition, crops may have different sensitivities at different stages of growth. This review attempts to answer the principal questions which are relevant to the use of marginal quality water for irrigation. Duration of exposure and stage of growth: plants are more sensitive during the seedling stage than during later stages of growth. But the preponderant temporal effect of salinity is the duration of exposure. Spatial distribution: the best estimate of the effective salinity when salt is non-uniformly distributed with depth is the mean salinity within the root zone. Under some conditions normalization on the water uptake basis is superior. Soil fertility: the level of soil fertility has no effect on the tolerance of crops to salinity. Varietal differences: differences in salt tolerance among varieties exist mainly in fruit trees, which are specifically sensitive to chloride and sodium salts. Differences among field and garden crops are not common and are usually small. Irrigation requirement: crop water production functions relating yield to evapotranspiration are not influenced by water salinity. It is still controversial whether reduction in water uptake with increasing salinity is the cause or the result of reduction in growth. Leaching requirement: leaching is the key to the successful use of saline water for irrigation. Under normal field conditions with free drainage the leaching provided by the normal inefficiencies in irrigation should be sufficient to control salinity. When leaching is necessary, it should be provided at the time when the soil salinity reaches hazardous levels. Irrigation frequency: the bulk of the evidence shows no advantage to increasing irrigation frequency when saline water is used, except possibly under excessive leaching. Fertilizer application: the response to nitrogen and potassium fertilization under non-saline conditions is the same as or even greater than under saline conditions. Excessive phosphorous application may be toxic at high salinity, especially in hydroponic conditions. Availability of more than one water source: blending of saline with non-saline water is a questionable practice. It is preferable to use the non-saline water source early in the growing season and the source of saline water successively. Irrigation method: drip irrigation, where feasible, gives the greatest advantages when saline water is used. Sprinkler irrigation may cause leaf burn on sensitive crops. The damage may be reduced by night irrigation and by irrigating continually rather than intermittently. Drainage: the critical depth to the water table is determined mainly by the aeration requirement of the crop, as long as a net downward flux of water is maintained by natural or properly designed man made drainage system. The design drainage coefficient is determined by the leaching requirement. Soil hydraulic conductivity ( K) and drainable porosity: important parameters in drainage design, are strongly influenced by the composition and concentration of the irrigation water. The higher the sodium adsorption ratio (SAR), the greater the reduction in K. The detrimental effect of high SAR is mitigated as the total salt concentration increases.