248 SOILS. 



Forty grains per gallon is usually assigned as the limit for 

 potable as well as irrigation waters. But if most or the whole 

 of such mineral contents should consist of the carbonates and 

 sulfates of lime and magnesia, the water while unsuitable for 

 domestic use may be perfectly available for irrigation, since 

 these salts are either beneficial or harmless in the amounts 

 likely to be introduced by the water. But if most or the whole 

 of such forty grains should consist of " alkali salts " proper, 

 viz., the sulfates, chlorids and carbonates of potash and soda, 

 or if they should contain even small amounts of the chlorid 

 and magnesium, they might render the water either wholly 

 unsuitable for irrigation, or if used it would be needful to 

 take the mineral content into consideration, by regulating its 

 application accordingly. 



It has been found in California that practically the upper 

 limit of mineral content for irrigation water under the ordinary 

 practice lies below seventy grains per gallon in all cases; for 

 when this strength is reached, even though such water may 

 bathe the roots of almost any plant with impunity, yet acci- 

 dental concentration by evaporation is so certain to happen, 

 that injury to crops is practically almost unavoidable. 



In South Dakota and other parts of the American semi-arid region, 

 waters containing seventy grains and even more of alkali salts per 

 gallon are annually used during the short irrigation season. This can 

 be done harmlessly because the aggregate amount used is only small, 

 and the more abundant rainfall of that region annually washes the salts 

 out of the soil. But where almost the full amount of water required 

 by crops must be supplied by irrigation, the total amount of salts thus 

 introduced would speedily render the land uncultivable. 



According to the observations of Means and other explo- 

 rers 1 of the U. S. Dep't of Agriculture, waters of much 

 higher mineral content are used for irrigation both in F.gypt 

 and in the Saharan region, some going as high as 8000 parts 

 per million, or 214 grains per gallon. The cultivators are 

 said to be very skilful in the use of these waters, applying 

 them only to plants of known resistance, and in certain ways. 

 These ways include doubtless a good deal more time and pa- 



1 Bull. No. 21, Bureau of Soils ; also circular No. 10, ibid. 



