ACID SOILS AND ALKALI SOILS 119 



are between white and black alkali. The former gets its 

 name from the fact that when it accumulates on the surface 

 of the ground, as is very common in a dry time, it has a white 

 appearance. The latter, on the other hand, is black, because, 

 owing to its caustic nature, it dissolves organic matter from 

 the soil, which gives it a black color. 



149. Effect of alkali on crops. — Both white and black 

 alkalis are injurious to plant growth when present in large 

 quantity, but black alkali is much more active in this re- 

 spect, as it attacks plant tissue just as it does the organic 

 matter in soils. White alkali injures plants by withdraw- 

 ing water from the plant cells and causing the plant to 

 wilt. The nature of the salts contained in the alkali, and 

 the species and even the individuality of the plant, de- 

 termine the amount of alkali that is required to destroy a 

 crop. 



150. Tolerance of different plants to alkali. — Some plants 

 are better able to endure the presence of alkali in soil than 

 are others. This is due, in part, to the natural resistance 

 of the plant to the injurious effect, and in part to the rooting 

 habit of the plant. Deep-rooted plants are, in general, 

 better able to resist alkali than are shallow-rooted ones, 

 probably because some part of the root is in a less strongly 

 impregnated part of the soil. 



Of the cereals, barley and oats are the most tolerant. Of 

 the forage crops, a number of valuable grasses are able 

 to grow on soil containing a considerable quantity of 

 white alkali. Timothy, smooth brome-grass and alfalfa 

 are among the cultivated forage crops most tolerant of 

 alkali, although they do not equal the native grasses in this 

 respect. 



The resistance of a number of plants to white alkali, ex- 

 pressed in pounds to the acre to a depth of four feet, is as 

 follows : 



