288 Irrigation and Drainage 



tion of water and air, it is only too plain that where 

 conditions are persistently maintained which allow the 

 formation of the salts without permitting them to be 

 removed by any cause whatsoever, there must come a 

 time, sooner or later, when the amounts produced and 

 accumulated in the soil shall reach the degree of con- 

 centration which is intolerable to cultivated crops. 

 Under the natural conditions of rainy countries, there 

 is usually a sufficient amount of leaching to permit 

 the white and black alkalies to be borne away in the 

 country drainage with sufficient completeness to pre- 

 vent their effects attracting general attention, and if 

 the same processes obtained in irrigated countries, it 

 is plain that in these, too, the difficulties would not 

 arise. The conclusion is irresistible, therefore, that some 

 method must be devised by which, periodically at least, 

 sufficient water is applied to irrigated fields to pick up 

 and carry out of the country the soluble alkali salts 

 which are fatal to cultivated crops. 



In the old-time irrigation of the Nile valley, the 

 greater part of the land was under basin irrigation, 

 and thus thoroughly washed during some fifty days 

 every year. Lands not so treated were the lighter 

 sandy soils near the Nile, protected by only slight 

 banks from inundation, and these dykes usually gave 

 way as often as every seven or eight years, so that 

 they, too, were occasionally thoroughly flooded. Un- 

 der this system of washing and drainage, the fields of 

 the Nile were kept free from alkalies for thousands of 

 years. But at the present time, w r hen what are called 

 more rational methods are being applied, but with no 



