SALINITY OF IRRIGATION WATER — SCOFIELD 281 



It is characteristic of these water solutions of electrolytes that the 

 total number of ions or combining units of each group must be the 

 same, that the sum of the cations must equal the sum of the anions. 

 This equality of numbers may be achieved by dissociation of the 

 water molecule into its potential constituents, hydrogen (H) and 

 hydroxy 1 (OH). The extent of this dissociation and the nature of 

 the resulting equilibrium may be measured by the so-called " hydro- 

 gen ion concentration (pH)." Thus acid waters are those in which 

 hydrogen ions must be included with the cations to achieve equality 

 of number, whereas in alkaline waters the requisite number is con- 

 tributed by hydroxyl ions. 



Two methods are used to indicate the quantities of dissolved salts 

 in a solution. The several ionic constituents of the dissolved salts 

 have different gravimetric values. For example, if we assume that 

 a combining unit of hydrogen has a gravimetric value of 1, then an 

 equivalent combining unit of chlorine has a gravimetric value of 

 35.5, while that of the sulphate ion (SO4) is 48, and that of the 

 nitrate ion (NO3) is 62. This relationship between numbers and 

 weight is involved in the two methods that are in current use for 

 reporting the concentration of the ionic constituents of the salinity 

 of irrigation water. By one of these methods the gravimetric values 

 are reported, usually as milligrams per liter, which is equivalent 

 to parts per million. By the other method the values reported as 

 milligram equivalents per liter imply the relative numbers of each 

 ionic constituent. The value reported by either method may be 

 readily converted into the other scale by the use of appropriate 

 factors for each ion. 



THE EFFECTS OF SALINITY 



The dissolved salt constituents that occur in the soil solution as 

 the result of the use of saline irrigation water produce effects that 

 fall into two categories. One includes the effects on crop plants and 

 the other includes effects on the physical condition of the soil. In 

 both cases the reactions appear to be related to the concentrations of 

 the individual salt constituents, rather than to the concentration of 

 the combined salts. The effects of these salinity constituents on the 

 physiological process of plant growth do not operate independently 

 of each other; but for the most part the interrelationships appear 

 to exist between the constituents of a group rather than between the 

 two constituents of any given salt. For example, the effect of mag- 

 nesium may be influenced by calcium, that of calcium by sodium, or 

 the injury caused by boron may be influenced by the concentration 

 of nitrate or the injury of selenium by the concentration of sulphate. 

 Thus the influence of one cation may be modified by the concentration 



