386 SOILS: PBOPJSBTim AND MANAGEMENT 



While soils may be either sour or alkaline, there are 

 also degrees of sourness. Thus a soil may be so sour as 

 to completely prevent the growth of one kind of plant 

 and yet produce excellent crops of another plant which 

 would have perished if the soil had been more acid. For 

 example, red clover will grow fairly well on soil that is 

 too sour to raise alfalfa. 



292. Qualitative tests for acidity. — A simple test to 

 indicate an acid condition of soil is not so easy of execu- 

 tion nor so infallible in its prediction as might be desired. 

 The object of such a test is to ascertain whether a soil is 

 not well adapted to the growth of certain plants and 

 whether the application of lime would benefit it in this 

 respect. A number of tests have been proposed which 

 will be outlined and briefly discussed. 



293. Litmus paper test. — Blue litmus paper is brought 

 into contact with the wet soil. A rapid and decided 

 change to red is taken to indicate an acid condition of 

 the soil. Carbonic acid, which is always present in soils, 

 is supposed to give only a faint pink color to the litmus 

 paper. Various ways of bringing the paper into contact 

 with the soil have been recommended, among others the 

 interposing of filter paper between the soil and the litmus 

 paper.^ It is also generally pointed out that the acid 

 perspiration on the fingers may lead to delusion. 



A criticism of the test has been made by Cameron,^ 

 who states that the absorbent action of soils for bases is 

 greater than is that of paper, while for acids the reverse 



1 Kellerman, K. F., and Robinson, T. R. Legume Inoculation 

 and the Litmus Reaction of Soils. U. S. D. A., Bur. Plant Indus., 

 Cire. 71, pp. 3-11. 1910. 



2 Cameron, P. K. The Soil Solution, pp. 65-66. Easton, 

 Pennsylvania. 1911. 



