FORMS OF PLANT FOOD 87 



93. Alkaline Soils. — When a soil contains such an 

 excessive amount of alkaline salts as sodium sulphate, 

 sodium or potassium carbonate or chloride, as to be 

 destructive to vegetation, it is called an ' alkali ' 

 soil. These soils are found in semi-arid regions, and 

 wherever conditions have been such that the alka- 

 line 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 i part per iooo parts of soil. Of the 

 various alkaline compounds potassium carbonate is one 

 of the most injurious. 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 contain so much lime carbonate that a 

 deposit is formed. Many excellent soils have been 

 passed over as 'alkali' soils when in reality they are 

 limestone soils. 



94. Improving c Alkali' Soils. 26 — When a large tract 

 of alkali is to be brought under cultivation the amount 

 of prevailing alkaline compounds should be determined 

 by chemical analysis. It frequently happens that 

 deep and thorough cultivation are all the treatment 

 that is necessary. If the prevailing alkali is sodium 

 carbonate a dressing of land plaster may be applied so 



