ACID, OR SOUR, SOILS 389 



298. Plants as indicators of acidity. — In addition 

 to these chemical tests for acidity there may also be 

 mentioned what is perhaps the most reliable indication 

 of the need of lime, namely, the failure of a soil to produce 

 red clover, and the presence of those weeds that have 

 previously been shown to thrive on sour soil (par. 289). 

 When a soil bears this relation to the plant growth it may 

 safely be assumed that those plants included in the list of 

 crops that are injured by sour soils will yield better if the 

 soil is limed than if it is not so treated. The crops adapted 

 to sour soils may not be injured. 



299. Quantitative determinations of acidity. — A num- 

 ber of quantitative methods for determining the degree 

 of acidity or the lime requirements of soils have been 

 devised. Only a few of these need be mentioned. 



300. Potassium nitrate method. 1 — The soil is shaken 

 with a normal solution of potassium nitrate for three 

 hours, and then allowed to stand overnight. An aliquot 

 portion of the supernatant liquid is boiled in order to 

 expel carbon dioxide, and when cool it is titrated with 

 a standard solution of sodium hydroxide. 



This method does not estimate either the free acid 

 or the lime requirement of the soil. What it does is 

 to give the absorptive power of the soil for potassium 

 when in equilibrium with a solution containing the 

 acid with which the potassium was originally in com- 

 bination. There is a substitution of bases during the 

 contact of the nitrate solution with the soil, and a 

 partial decomposition of these salts during the titration 

 with alkali. 



1 Official and Provisional Methods of Analysis. Association 

 of Official Agricultural Chemists. U. S. D. A., Bur. Chem., 

 Bui. 107 (revised), p. 20. 1908. 



