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1051 ,ca++ Na+ + Mg++ 2 SAR ; where concentrations are expressed as epm (equivalents per million). Bicarbonate (HC03-) concentration is important because it can cause a water to lose calcium and magnesium, thus increasing the sodium hazard. Calcium will react with bicarbonate to form calcium carbonate (CaC03)' Magnesium can be lost by an indirect reaction. It enters the exchange complex of the soil, re- placing calcium which reacts with bicarbonate and precipitates as CaC03' This hazard can be evaluated in terms of the residual sodium carbonate(RSC) which is defined as: RSC ; (C03 + HC03 ) ++ ++ (Ca + Mg ) where the concentrations are expressed as epm. Studies referred to in the above publication indicate that waters greater than 2.5 epm RSC are probably not suit- able for irrigation purposes. Water containing 1.25 to 2.50 epm are marginal, and those containing less than 1.25 epm RSC are probably safe. Good management practices may make use of marginal waters possible. The occurrence of boron in toxic concentrations in certain natural waters requires that it be considered when assessing the quality of an irriga- tion water. In addition, the type of crop must be considered, as different species have different tolerances. Some crops such as sugar beets, alfalfa and onions can tolerate up to about 4.0 ppm. Semi tolerant crops including corn, tomatoes and potatoes are unaffected by concentrations up to 2.0 ppm. Sensi- tive crops primarily fruits can tolerate up to about 1.0 ppm. The U. S. Salinity Laboratory has devised a procedure for classifi- cation of irrigation waters into degrees of salinity hazard and sodium hazard. A graph has been developed which has SAR for the ordinate and specific electri- cal conductivity as the abscissa. The graph has been divided into separate areas representing degrees of salinity hazard and sodium hazard. The procedure is to plot a point using the SAR and specific electrical conductivity of the water sample being classified, and the salinity and sodium hazards are deter- mined by the area on the graph containing the plotted point. The significance and interpretation of these hazard ratings are summarized below. Salinity Classification Cl - LOW SALINITY WATER can be used for irrigation with most crops On most soils, with little likelihood that a problem will develop. C2 - MEDIUM SALINITY WATER can be used if a moderate amount of leaching OCcurs. Plants with moderate salt tolerance can usually be grown without special practices. C3 - HIGH SALINITY WATER cannot be used on soil with restricted drainage. With adequate drainage, special management may be required and crops with good salt tolerance should be selected. - 11 -