KINDS OF ALKALI 309 



171. Composition of alkali salts. The materials 

 dissolved in the soil water consist of all of the sub- 

 stances found in the soil, but, as the rates of solubility 

 of these substances vary greatly, there accumulates 

 a much larger quantity of some substances than of 

 others. Carbonates, sulfates and chlorides of sodium, 

 potassium, calcium and magnesium occur in the largest 

 amounts. Sodium may be present as carbonate, sulfate, 

 chloride, phosphate and nitrate. Potassium may be 

 similarly combined. Magnesium is likely to appear as a 

 sulfate or chloride, and calcium as a sulfate, chloride 

 or carbonate. In some soils one salt will predominate, 

 and in other soils other salts will prevail. A base may be 

 present in combination with several different acids. 

 The nature of the prevailing salt influences greatly the 

 effect upon vegetation. Table XLVII gives the composi- 

 tion of the soluble salts from a number of alkali soils. 



172. White and black alkali. Sulfates and chlorides 

 of the alkalies when concentrated on the surface of the 

 soil produce a white incrustation, which is very common 

 in alkali regions during a dry period, as a result of evapo- 

 ration of moisture. Alkali in which these acids predomi- 

 nate is called white alkali. 



Carbonates of the alkalies dissolve organic matter 

 in the soil, thus giving a dark color to the solution 

 and to the incrustation, and for this reason alkali con- 

 taining large quantities of these salts is called black 

 alkali. 



Black alkali is much more destructive to vegetation 

 than is white. A quantity of white alkali that would 

 not seriously interfere with the growth of most crops 



