SOIL ACIDITY 359 



developed. This is the zinc-sulfide method. 1 The soil sample, 

 usually 10 grams, is placed in an Erlenmeyer flask and treated 

 with an excess of neutral calcium chloride and zinc sulfide. 

 About 75 cubic centimeters of water are added. The mixture 

 is boiled for one minute to control frothing and to develop 

 uniform ebullition. A strip of moistened lead acetate paper 

 is now laid over the mouth of the flask and allowed to remain 

 there exactly three minutes, the boiling being continued at 

 a uniform rate. The reactions involved in the test are as fol- 

 lows : 



Soil + xCaCl 2 (neutral) ±5 Ca x Soil + xHCl 



2HC1 + ZnS = ZnCl 2 + H 2 S 



H 2 S (Expelled by boiling) + Pb(C 2 H 3 2 ) 2 = PbS 



(black) + 2C 2 H 4 2 



The selective absorption and basic exchange of the soil de- 

 velops actual acidity, which produces hydrogen sulfide from 

 the zinc sulfide. The gas is driven off against the lead acetate 

 paper, producing a black color. The principle involved is the 

 same as that already explained for the litmus test, a different 

 means being employed for measuring the actual acidity de- 

 veloped. 



195. Comparison and criticism of qualitative tests. — A 

 comparison and criticism of these two methods will amply 

 show the advantages and disadvantages of qualitative tests 2 



1 Truog, E., New Method for the Determination of Soil Acidity; 

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



Truog, E., Testing Soils for Acidity; Wis. Agr. Exp. Sta., Bui. 312, 

 1920. 



2 There are a number of other qualitative tests for acidity, of which 

 the following may be mentioned: 



Ammonia test. — In this test the soil is placed in a bottle and treated 

 with a strong solution of ammonia. After shaking, the soil is allowed 

 to settle, the depth of the color developing in the supernatant liquid 

 being considered as indicating the degree of acidity. This color depends 

 on the amount and character of the soil organic matter rather than on the 

 acidity. 



Acid test for carbonates. — In this test a sample of the soil is treated 

 with a few drops of dilute hydrochloric acid. Effervescence indicates the 



