318 THE PRINCIPLES OF SOIL MANAGEMENT 



carbonates, and soluble phosphates, both of which are 

 precipitated by the change. 



182. Retarding evaporation. As evaporation of 

 moisture from the surface of the soil is the cause of rise 

 of alkali, it is important to reduce evaporation to a 

 minimum, either in drained or in undrained land. 

 Especially where irrigation is practiced without drainage, 

 it becomes desirable to use as little water as is necessary 

 to produce good crops, and to conserve this to the utmost 

 by checking evaporation from the surface of the soil. 



The methods used for checking evaporation are the 

 maintenance of a soil or other mulch, and of a good 

 tilth. (See page 195.) In handling alkali spots in the 

 semi-arid region, it is very important to reduce evapo- 

 ration to the smallest amount practicable. 



183. Cropping with tolerant plants. Certain alkali 

 soils that are strongly impregnated with alkali may be 

 gradually improved by cropping with sugar-beets and 

 other crops that are tolerant of alkali, and which re- 

 move large amounts of salts. This is more likely to be 

 efficacious where irrigation is not practiced. 



184. Other methods. Numerous other methods of 

 disposing of alkali or ameliorating its effects have been 

 used or proposed. Among these are the following: 

 (1) "Leaching," which consists of flooding the surface 

 of the soil for the purpose of carrying the soluble salts 

 down to a depth of three or four feet, where they will 

 not effect the roots of ordinary crops. If natural drain- 

 age exists, this plan is effective and without danger; 

 otherwise evaporation must be reduced to the smallest 

 possible amount. (2) Removal of alkali by scraping the 



