280 ANI^UAL. REPOET SMITHSONIAN INSTITUTION, 19 3 5 



in the soil solution of an irrigated field is four to eight times that 

 of the water with which the field is irrigated. Such differences 

 do not occur with respect to all the salt constituents. Those con- 

 stituents that form salts of low solubility cannot exceed that limit. 

 Furthermore, there may occur in the soil reactions of base exchange 

 by which the proportions of some of the constituents may be modi- 

 fied. The point here to be emphasized is that in considering the 

 relationship between the salinity of irrigation water and the salinity 

 tolerance of crop plants it should be kept in mind that the plant 

 must obtain its water supply from the soil solution, which may be 

 very much more saline than the irrigation water. Furthermore, any 

 standards to be used for the quality of irrigation water must be in- 

 fluenced by consideration of the regimen of irrigation and the 

 conditions of soil and climate involved. 



THE CONSTITUENTS OF SALINITY 



In order to understand the nature and apparent complexity of the 

 problem of salinity in irrigation water it is necessary to recognize 

 as a fact that this salinity includes not only a number of different 

 salts, but what is even more important, that each of these salts is 

 composed of at least two constituents. Our best information is that 

 the reactions and effects that occur in the soil solution or subse- 

 quently in the plant are produced by the salt constituents or ions 

 rather than by the salts as such. In water solutions of common salts 

 or electrolytes it is believed that the salt constituents exist largely 

 as independent dissociated ions. Furthermore, the analytical meth- 

 ods, by which their concentrations in water solutions are measured, 

 are based on determinations of the individual constituents rather 

 than of the salts in their combined form. 



Natural waters, such as are used for irrigation, contain in solution 

 a very large number of constituents. Rarely if ever does an analyst 

 attempt to determine all of them. Probably most of the analyses 

 of irrigation water are limited to the determination of the anions: 

 Carbonate (CO3), bicarbonate (HCO3), sulphate (SO4), chloride 

 (CI), and nitrate (NO3), and in the cation group the determinations 

 seldom include more than four: Calcium (Ca), magnesium (Mg), 

 sodium (Na), and potassium (K). In addition in some cases it 

 is necessary or desirable to determine the concentration of certain 

 elements such as iron (Fe), aluminum (Al), silicon (Si, or as silica, 

 SiOa), boron (B), fluorine (Fl), and selenium (Se). These last 

 named constituents usually, though not always, occur in small 

 quantities, and although they may exist in the solution in ionic 

 combinations, the nature of these combinations is not well enough 

 known to warrant their identification except as elements. 



