118 SOILS AND FERTILIZERS 



and applying the paper directly to the soil. If the soil is 

 acid, the potassium will be absorbed and an acid or acid salt 

 set free, which will act on the litmus paper, giving it a decided 

 pink color. 



146. The Truog test. — In this test solutions of calcium 

 chloride and zinc sulfide are brought in contact with the 

 soil to be tested and the mixture is boiled. If the soil is 

 acid, a gas called hydrogen sulfide is formed and driven off 

 with the steam. The presence of this gas may be detected 

 by placing a strip of moist lead acetate paper over the mouth 

 of the flask in which the soil and solutions are boiled. The 

 lead acetate paper is rapidly darkened by the hydrogen 

 sulfide gas as it passes out of the flask. Detailed descrip- 

 tions of the methods for making these tests for soil acidity 

 will be found in the laboratory exercises. 



147. Alkali soils. — We have seen that every soil is 

 constantly undergoing decomposition, by which process a 

 very minute fraction becomes soluble every year. Ordi- 

 narily, in humid regions, this soluble matter is leached out 

 by the rain water that percolates through the soil. In 

 those parts of the world where the rainfall is very slight, 

 and yet where decomposition of soil proceeds, there is a 

 tendency for the soluble matter to accumulate in the soil 

 where there is no drainage, or for it to move to places where 

 seepage accumulates. A strong accumulation of such soluble 

 matter is known as alkali because it usually has an alkaline 

 reaction, i.e. it turns red litmus paper blue. 



148. Nature and movements of alkali. — Because of its 

 easy solubility, alkali may move from place to place or up- 

 ward and downward in soils. During periods of drought 

 it is carried upward by the capillary rise of the soil water, 

 while during periods of rainfall it may move downward, 

 where it is out of range of roots. The composition of alkali 

 varies greatly in different regions. The main distinctions 



