250 SOILS. 



all such cases, close attention to the measure of water that will 

 wet the soil to a certain depth, and " eternal vigilance " with 

 respect to the accumulation of alkali near the surface, must be 

 the price of immunity from injury. In all cases the farmer 

 should know how much of alkali salts he introduces into his 

 land with the irrigation water, and watch that it does not ap- 

 proach too closely, or exceed, the tolerance of his crops for 

 alkali salts, as given in chapter 26. 



Use of Drainage U'aters for Irrigation. When lands 

 charged with alkali salts are being reclaimed by drainage, the 

 question sometimes arises whether the drainage-water may not 

 be used for irrigation, lower down. This of course depends 

 entirely upon the amount of alkali in the water, the nature of 

 the lands to be irrigated, and the manner of applying it. In 

 the Fresno drainage-district of California it has been shown 

 that some of the drainage-water contains not more than 25 to 

 30 grains per gallon of objectionable salts, and such waters 

 could of course be used on pervious lands with the precautions 

 above noted. 



"Black .llkali" ITaters. A- reganK however, waters con- 

 taining any large proportion of carbonate of soda, it must be 

 remembered that even very dilute solutions of salsoda serve 

 to puddle the soil and thus render it difficultly tillable. When 

 such waters are used it is nece^ary to forestall injury either 

 by the use of gypsum in the reservoir or ditch, or by annually 

 using on the land a sufficient amount of gypsum to transform 

 the carbonate of soda into the relatively innocuous sulfate. 



Variations in the Saline Contents of Irrigation ll'aters. 

 When irrigation waters are derived from dee]) welK there is 

 little if any variation of their saline contents to be expected, 

 and a single analysis \\-ill serve permanently. But in the case 

 of relatively shallow well^. from which the water must be 

 raised by pumping, it not un frequently happens that after a 

 series of seasons of short rainfall, saline waters are brought 

 up by the pump and may seriously injure crops and orchards. 

 Again, in the case of streams and rivers whose flow becomes 

 very small in summer, the saline content may increase to sev- 

 eral times the amount carried at the time of high water. 

 Both kinds of cases occur in southern California, in Arizona, 1 

 1 Bull. Ariz. Exp't Sta. No. 44. 



