CEREALS UNDER IRRIGATION 437 



and tending to form in the lower layers a tough hard pan, 

 impervious to water (Fig. 140). 



473. Prevention of alkali may be accomplished by 

 (1) preventing evaporation and (2) keeping the water 

 table down. Excess irrigation and seepage, with poor 

 drainage, will soon raise the water table, and when less 

 than two feet from the surface, it will be a constant supply 

 of water to take the place of that evaporated, permitting 

 thereby a continual deposit of alkali. The water table 

 can be lowered only by drainage. The drains should be 

 at least three feet deep. Evaporation may be checked 

 by the various means of surface cultivation, mulching, 

 and other methods already mentioned. 



474. Resistant crops. The cereals are all more or 

 less resistant to alkali, barley being the most resistant of 

 all. Alkali tolerance in cereals is discussed fully in another 

 chapter (267, 268). Where the alkali is not yet accumu- 

 lated in much excess, therefore, the more resistant crops 

 may succeed. Different varieties of the same cereal, 

 even, will offer resistance to a different degree. How- 

 ever, very often the excess deposit is far beyond the power 

 of any crop to resist, and either the alkali must be removed 

 or controlled, or cropping be abandoned entirely. 



475. Reclamation. Black alkali land may be re- 

 claimed by the application of gypsum and turning it under. 

 This has been done at Tulare, California, by the State 

 Experiment Station. On spots where previously not a 

 blade of grass would grow, wheat and barley 3 to 4 feet 

 high have been produced, although the surface of the 

 ground at harvest time was covered by a thick crust of 

 white alkali. Gypsum converts the carbonate of sodium 

 into a sulfate, which is still harmful, but much less so 

 than the carbonate. 



