CHAPTER XVIII 



ALKALI SOILS 



It has already been shown that soils are acted upon by 

 a great variety of weathering agents which gradually 

 render soluble a portion of the most susceptible constitu- 

 ents. This soluble material becomes a part of the soil 

 solution and may come in contact with the roots of any 

 crop growing on the soil In humid regions, ^where a 

 large quantity of water percolates through the soil, this 

 soluble matter has little opportunity to collect. In arid 

 regions, however, where loss by drainage is slight, these 

 salt-s may often collect in large amounts. During periods 

 of drought they are carried upward by the capillary rise 

 of the soil water, while during periods of raiiJaU they may 

 move downward again in proportion to the leaching action. 

 At one time the lower soil may contain considerably more 

 soluble salt than the upper ; at another time the condition 

 may be reversed, in which case the solution in contact 

 with plant roots may contain so much soluble matter 

 that vegetation is injured or destroyed. This excess of 

 soluble salts usually has a marked alkaline reaction, but 

 in any case it produces what is termed an alkali soil. 



303. Composition of alkali salts. — The materials dis- 

 solved in the soil water consist of all the substances found 

 in the soil, but as the rates of solubility of these substances 

 vary greatly there is accumulated a much larger quantity 

 of some substances than of others. Carbonates, sulfates, 



391 



