284 Irrigation and Drainage 



strong, the alkalies will necessarily be brought to the 

 surface and become concentrated there, hence in posi- 

 tion to do the greatest harm to growing crops. 



If thorough tillage is practiced early, so that but 

 little water is evaporated except that which passes 

 through the roots of the crop, then the salts cannot 

 become concentrated in a narrow zone, but, on the 

 contrary, will be left all through the soil where the 

 roots which are taking water are distributed. In those 

 cases, therefore, where the general soil water is not 

 too highly concentrated to permit normal growth, 

 crops may prosper so long as the surface is kept 

 shaded and thoroughly tilled. 



It must be observed, however, and kept in mind, 

 that the roots of plants cannot withdraw moisture from 

 a soil without at the same time tending to concentrate 

 the salts in solution in the zone where the roots do 

 their feeding ; hence, that if alkali waters are being 

 used for irrigation, and in the long run if the purest 

 waters are being used under conditions of no drainage, 

 sooner or later the soil of the root zone must become 

 so highly charged with the alkali salts that reduced 

 yields are inevitable. 



USE OF LAND PLASTER TO DESTROY BLACK ALKALI 



Hilgard long since pointed out that in regions 

 where the water contained sulphate of lirne in solu- 

 tion, there sodium carbonate was absent, or existed in 

 such small quantities as not to be harmful to crops, and 

 he early saw and recommended that where fields were 



