Chapter XIII. 



ALKALI. 



In the preceding chapters there have been considered the 

 phenomena which obtain under humid conditions. Under 

 exceptional conditions of prolonged drought there occurs an 

 accumulation of soluble mineral substances at or near the sur- 

 face of the soil. This phenomenon is pronounced in arid and 

 semi-arid regions, 1 and the accumulations of soluble salts oc- 

 curring in such regions is known in the United States as 

 "alkali," in India as "reh," in Africa as "brak," and in other 

 countries by various local designations. The study of the ex- 

 treme conditions producing alkali has added materially to the 

 present knowledge of the processes taking place in soil of humid 

 areas. Moreover, alkali-infested areas are themselves becoming 

 of so much importance with the growing needs for further new 

 lands, that it seems wise to give here an outline of the chemi- 

 cal principles involved in their soil solutions. 2 



Alkali is sometimes a single salt, but usually a mixture of 

 some two or more of the chlorides, sulphates, carbonates, bi- 

 carbonates, and occasionally the nitrates, phosphates and borates, 

 of sodium, magnesium, potassium, and calcium, and occasionally 

 strontium and lithium. In the United States, when the carbonate 

 of sodium is present to an appreciable extent, the salt mixture is 

 known as black alkali, in contradistinction to white alkali, which 

 latter does not contain sodium carbonate. 3 Generally, but not 



1 Occasional occurrence of alkali in humid regions, by Frank K. 

 Cameron, Bull. No. 17, Bureau of Soils, U. S. Dept. Agriculture, 1901, 

 PP- 36-38. This phenomenon should not be confused with the surface 

 deposition of various kinds of saline material from springs, which is 

 fairly common in both humid and arid regions, the world over. 



2 Alkali soils of the United States, by Clarence W. Dorsey, Bull. 

 No. 35, Bureau of Soils, U. S. Dept. Agriculture, 1906. 



8 Black alkali is so called because the caustic solution containing 

 sodium carbonate, in rising to the surface of the soil, dissolves and 

 carries with it organic matter which is subsequently left on the surface 

 in more or less blackish deposits, often ring-like in appearance. It is by 

 no means uncommon, however, to find deposits of "black alkali" which 

 are not black at all, and it is quite common to find "white alkali" so dark 

 in color as to suggest the presence of sodium carbonate, although the 

 latter be absent. 



