ACID SOILS AND ALKALI SOILS 121 



and carries it out through the drains. This is, by all means, 

 the most effective way of removing alkali. 



Gypsum has been used for converting black alkali into 

 white alkali, which it does by inducing chemical changes in 

 the alkali. This may well be used when black alkali land 

 is to be drained. 



Scraping consists in allowing alkali to accumulate at the 

 surface of the soil and then removing it with a scraper. This 

 is never a very effective treatment. 



Flushing is accomplished by removing the surface incrusta- 

 tion with a rapidly moving stream of water instead of a 

 scraper. like the former method it is not usually an 

 adequate treatment. 



153. Control of alkali. — Instead of actually removing 

 alkali its injurious action may often be kept in check by keep- 

 ing it well distributed through the soil and not allowing it 

 to accumulate near the surface. This may be done by con- 

 trolling evaporation and by the cultivation of alkali-tolerant 

 plants. The methods usually employed for retarding evap- 

 oration of moisture are generally applicable for controlling 

 alkali. 



Cropping with alkali-tolerant plants naturally suggests 

 itself as a means of combating alkali where it does not exist 

 to such an extent as to interfere with all crop production. 

 As these plants remove considerable quantities of alkali in 

 their ash, they also serve as a means of alkali removal. 



QUESTIONS 



1. Distinguish between positive and negative acidity in soils. 



2. Describe three ways in which soil acidity may be injurious to 

 plant growth. 



3. State three ways by which the growth of plants on soil tends 

 to make it become sour. 



4. What is the effect on soil acidity of a continued use of am- 

 monium sulfate ? 



