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Show PART II PRESENT STATUS OF IRRIGATION (4) Soil types, climatic conditions, and irrigation practices may all influence the reactions of the crop to the soil constituents. (5) Interrelationships "between and among constituents may be highly significant; the effect of one ion may be modified by the presence of another• Irrigation water applied to cultivated land is only partly used by crops. The part not consumed is a combination of surface runoff and per- colation through the soil beyond the root zone. Water or soil containing soluble salts require sufficient water supply in excess of consumptive use by the crop to prevent salt accumulation in toxic amounts. Generally, in the few areas where salinity is a problem, there is sufficient water applied to provide leaching. Although absolute limits cannot be set for salinity and sodium con- tent of irrigation water, the U.S. Department of Agriculture's Salinity Laboratory'57 has established some general classifications which are used as a guide where there are no particular soil problems. These are shown in Table 11. Table 11 - Classification of irrigation waters Salinity hazard^/ Sodium hazard Classification Electrical conductivity (EC) (micromhos/cm.) Total dissolved solids (TDS) (mg./l. °r p.p.m.) Sodium adsorption . ratio SAE=' Low Medium High Very high 100-250 250-750 750-2,250 2,250 60-160 160-480 480-1,440 i,44o 0-10 10-18 18-26 26 1/ For conductivities of less than 5>000 micromhos/cm., the total dissolved solids = conductivity x 0.64. The factor 0.64 is an average. 2/ These SAR limits are for water having low salinity content. When salinity hazard is high the SAR limits are from 0.4 to 0.5 of these values. Toxic constituent effects on irrigation must also be considered in evaluating water quality. Boron is the most common element considered and is usually found in varying amounts in natural waters. A concentra- tion of O.5 milligram per liter is considered critical for most sensitive crops, but it can be higher for more tolerant crops generally grown in the Upper Colorado Region. The quality of soil, drainage, and climatic and otheir environmental factors, such as the amount of rainfall and total amount of irrigation water applied, can modify the safe concentration limits. A nonmineral criterion^".) to be considered is the sediment content. Colloidal, material deposited on soil surfaces can produce crusts which 32 |