IMPROVING ALKALI SOILS 83 



97. Alkaline Soils. When a soil contains enough 

 alkaline salts as sodium sulphate, sodium or potas- 

 sium carbonate or chloride, to be destructive to vege- 

 tation, it is called an ' alkali ' soil. These soils are 

 found in semi-arid regions, and wherever conditions 

 have been such that the alkaline compounds have not 

 been drained from the soil. Occasionally calcium 

 chloride is the destructive material. Chlorine in any 

 ordinary combination is destructive to vegetation 

 when present to the extent of more than T part per 

 1000 parts of soil. Of the various alkaline com- 

 pounds potassium carbonate is one of the most in- 

 jurious. Sodium sulphate is a milder form of alkali. 

 When evaporation takes place, the alkaline compounds 

 are deposited as a coating on the surface of the soil. 

 Many soils supposed to be strongly alkaline, because 

 a white coating is formed on the surface, simply con- 

 tain so much lime carbonate that a deposit is formed. 

 Excellent soils have been passed over as ( alkali ' soils 

 when in reality they are limestone soils. 



98. Improving Alkali Soils, 27 When a large tract 

 of alkali is to be brought under cultivation the amount 

 and kind of prevailing alkaline compound should be de- 

 termined by chemical analysis. It frequently happens 

 that drainage followed by deep and thorough- cultiva. 

 tion is all the treatment necessary. If the prevailing 

 alkali is sodium carbonate a dressing of land plaster 

 may be applied so as to change the alkali from sodium 

 carbonate to sodium sulphate, a less destructive form, 

 the reaction being 



Na 2 C0 3 + CaS0 4 = CaCO 3 + N 



