358 NATURE AND PROPERTIES OP SOILS 



tests give them an advantage over the somewhat questionable 

 lime-requirement determinations. As the amount of lime 

 applied is at best only an estimate, a simple test, rationally 

 correlated with the many other factors that must be consid- 

 ered, may prove as satisfactory as a more complicated pro- 

 cedure. 



193. Qualitative tests for acidity— litmus paper. — Per- 

 haps the oldest test for acidity is the use of litmus paper.^ 

 This may be used alone or in connection with some sensitiz- 

 ing agent. Potassium nitrate, a neutral salt, is often utilized 

 in this capacity. As has already been explained (par. 141), 

 the addition of such a salt, especially to a soil lacking in ac- 

 tive bases, results in a marked selective absorption and the 

 development of a hydrogen ion concentration. In using litmus 

 paper and potassium nitrate it is assumed that the selective 

 absorption and basic exchange is an approximate measure of 

 the so-called soil acidity. 



The procedure is as follows: A small amount of the soil 

 to be tested is placed in a small dish or other container and 

 moistened with a neutral potassium nitrate solution. A thick 

 batter is produced by mixing. The soil is then smoothed 

 down and one end of a strip of neutral litmus paper is care- 

 fully applied. The reddening of the paper is an indication 

 of acidity, while the rate of the reaction is a rough measure 

 of the degree. The portion of the paper not in contact with 

 the soil may be used for comparison when the change is slight. 

 The unused end may even be moistened with distilled water 

 to make the comparison more accurate. 



194, The zinc-sulfide test. — Another qualitative test 

 based on the same general principles has more recently been 



^Barlow, J. T., Soil Acidity and the Litmus Paper Method for Its 

 Detection; Jour. Amer. Soc. Agron., Vol. 8, No. 1, pp. 23-30, 1916. 



Karraker, P. E., The Value of Blue Litmus Faper from Different 

 Sources as a Test for Soil Acidity; Jour. Amer. Soc. Agron., Vol. 10, 

 No. 4, pp. 180-182, 1918. 



