ACID, OR SOUR, SOILS 387 



is the case. Consequently the base that had produced 

 the blue color is absorbed from the litmus, leaving the 

 acid compound, which is red. Cameron concludes that 

 the test is unreliable, and proposes to extract the soil 

 with water, boil it in order to expel carbon dioxide, and 

 then test the reaction of the solution. 



Much litmus paper that is sold is of very poor quality ; 

 but when good paper is used and the test is carefully 

 made, the general experience has been that it is a fairly 

 good, although not an infallible, guide to the need of a 

 soil for lime. Red coloration due to absorptive action is 

 probably an advantage rather than a source of error in 

 the test, as a soil strongly absorptive of bases is likely 

 to need lime. This coloration does not necessarily in- 

 dicate the presence of free acid, but merely need of lime. 



294. Ammonia test. — In this test the soil is stirred 

 with a dilute solution of ammonia hydroxide. After 

 settling, if the supernatant liquid on standing takes on 

 a dark chocolate or a black color it is said to be acid. 

 This method, which has been proposed by Miintz, 1 is 

 not of general application and would not always be re- 

 liable in the case of soils of arid regions. The depth of 

 color is not a guide to the degree of acidity, since many 

 acid soils are low in organic matter. 



295. Zinc sulfide test. — A test recently proposed by 

 Truog 2 consists in mixing the soil to be tested with a 

 small quantity of calcium chloride and a very little zinc 

 sulfide. Water is added and the mixture is heated to 



1 Wheeler, H. J., Hartwell, B. L., and Sargent, C. L. Chemi- 

 cal Methods for Ascertaining the Lime Requirements of Soils. 

 Rhode Island Agr. Exp. Sta., Bui. 62, pp. 65-88. 1899. 



2 Truog, E. A. New Method for the Determination of Soil 

 Acidity. Science, N. S., Vol. 40, pp. 246-248. 1914. 



