388 SOILS: PROPERTIES AND MANAGEMENT 



boiling. A strip of moistened lead acetate paper is held 

 over the mouth of the flask for two minutes while the 

 boiling proceeds. If the soil is acid, the paper will be 

 darkened on the underside; if the soil is not acid, no 

 darkening will occur. 



This method is evidently designed to test the need of 

 the soil for lime as well as actual acidity, for the absorp- 

 tion of calcium from the dissociated chloride would leave 

 free hydrochloric acid. The action of this acid on zinc 

 sulfide would generate hydrogen sulfide, thus blackening 

 the lead acetate paper. 



A somewhat similar principle is involved in the proposal 

 to use a solution of potassium nitrate in the litmus paper 

 test. 



296. Litmus paper and potassium nitrate. — This is 

 performed in the same manner as the former litmus 

 paper test, except for the substitution of a saturated 

 solution of potassium nitrate instead of distilled water 

 for moistening the soil. 



297. Acid test for carbonates. — In this test a dry 

 sample of the soil is treated with a few drops of dilute 

 hydrochloric acid. Effervescence indicates the presence 

 of carbonates or bicarbonates in sufficient quantities to 

 insure an alkaline soil, although sometimes lime may 

 still be beneficial. 



Whitson and Weir x have objected to this method on 

 the ground that the displacement of air in the pore spaces 

 of the soil by the dilute acid may be mistaken for evolu- 

 tion of carbon dioxide. In the hands of an experienced 

 and careful operator this would not necessarily invalidate 

 the method. 



1 Whitson, A. R., and Weir, W. W. Soil Acidity and Lim- 

 ing. Wisconsin Agr. Exp. Sta., Bui. 230, pp. 7-11. 1913. 



