392 



IRRIGATION PRACTICE 



Ordinarily, alkali is classified as white, black or brown. 

 Black alkali appears as a black, shiny mass, or as black 

 spots, over the soil. White alkali has a clean, white appear- 

 ance like that of salt. Experience has demonstrated that 

 black is far more injurious than white alkali, for it is 

 corrosive and girdles the tissues of the plant near the 

 soil surface and thus destroys the plant itself. White 

 alkali is relatively harmless, and injures plants only as it 



is present in too 

 large an abun- 

 dance. Black or 

 brown alkali is 

 composed chiefly 

 of the carbonate 

 or nitrate of 

 sodium, with 

 perhaps some 

 common salt. 

 The carbonate dissolves the plant tissues with the for- 

 mation of a black mass; it moreover destroys the tilth of 

 the soil by destroying its structure. The nitrate forms 

 brown spots; it also makes soils mushy; but, when the 

 soils containing nitrates dry out, a very characteristic 

 crumbly soil results. The white alkali is composed of 

 the sulfates and chlorides of sodium, calcium and 

 magnesium. 



235. Tolerance for alkali. The tolerance for alkali of 

 plants depends on five factors: (1) the main salt, (2) 

 the concentration, (3) the associated salt, (4) the age of 

 the plant, and (5) the plant itself. Kearney and Cameron, 

 experimenting with seedlings of various plants, have 

 shown that various salts affect growth variously. Sodium 

 carbonate is the most injurious constituent of alkali, 



1 2 3 4 



FIG. 115. Effect of a strong solution of potassium 

 nitrate on protoplasm. 



