SALINITY OF IRRIGATION WATER — SCOFIELD 279 



whatever quantity of dissolved salt is contained in the irrigation 

 water. Insofar as the water of the soil solution is dissipated by di- 

 rect evaporation from the soil, the remaining solution becomes more 

 concentrated because the dissolved salts do not evaporate. The effect 

 on the soil solution of the absorption of water from it by crop plants 

 is almost the same as the effect of evaporation, because plants absorb 

 a much larger proportion of water than of the dissolved salts when 

 the concentration of the solution is above a very low level. Thus 

 when saline irrigation water is used the trend of events is in the 

 direction of an increasing concentration of salinity in the soil 

 solution. 



This trend toward increasing concentration is limited by two 

 processes. One of these operates through the precipitation from 

 the solution of the salts of low solubility, which occurs with such 

 salts as calcium carbonate and calcium sulphate at concentration 

 levels well below the limits of tolerance for most crop plants. In 

 other words, the addition of these salts to the soil solution might con- 

 tinue indefinitely and at any rate without impairing the productivity 

 of the soil because they would precipitate from the saturated solu- 

 tion and become a part of the solid phase of the soil. The other 

 limiting process operates through the percolation downw^ard of the 

 soil solution with its dissolved salts to levels below the root zone. 

 This process occurs naturally if and when the downward path is 

 open and when the quantity of irrigation water applied is greater 

 than the water-holding capacity of the soil of the root zone. In 

 some situations the downward path is not open because of a sub- 

 surface layer of rock or impermeable clay. In other areas it is im- 

 peded by a water-saturated zone of subsoil known as a " water table." 



In order to prevent the excessive accumulation of soluble salts in 

 the root zone of irrigated soil it is not enough that the way of 

 escape below shall be unimpeded. There must be some displace- 

 ment of the solution, or leaching, through the application, at least 

 occasionally, of moie than enough water to replenish the root zone 

 to its capacity. In many places, particularly along the Pacific coast, 

 the necessary leaching occurs as a result of winter rains that come 

 during the dormant season. When such rains are inadequate to 

 cause some leaching it becomes necessary to do it with irrigation 

 water. 



As a result of the processes of concentration that occur in the soil 

 solution of an irrigated field the soil solution is always more saline 

 than the irrigation water. Where subsoil drainage is free and water is 

 used copiously, the concentration of the soil solution may be only twice 

 that of the irrigation water. If the subsoil drainage is poor or if irri- 

 gation water is used too sparingly, the difference will be much greater. 

 In general, it is found that the concentration of total dissolved salts 



