360 NATURE AND PROPERTIES OF SOILS 



in general. The litmus paper test is simple and rapid. It 

 can be used with equal facility in the laboratory and field. 

 While its readings may not correlate very definitely with the 

 actual amount of lime that should be applied, it gives a basis 

 for an estimate that in practice should include a number of 

 factors besides so-called soil acidity. One objection to the 

 method lies in the difficulty of obtaining sensitive litmus paper. 

 Again the intensity of the color change is not great and in the 

 hands of an inexperienced person may seem insignificant. In 

 spite of its limitations, it is one of the best practical qualita- 

 tive tests for soil acidity now available. 



The zinc-sulfide test is much more striking than the litmus 

 test and thus is more easily interpreted. On account of the 

 marked change of color there is always a temptation to read 

 into this test a quantitative value which it does not possess 

 to any greater degree than does the litmus paper method. 



The zinc sulfide test is not as rapid as the litmus test, nor is 

 it a satisfactory field method. Moreover, it is more complex 

 and requires a much more extensive technique. Again it does 

 not distinguish between a neutral and an alkaline soil. Lit- 

 mus paper, on the other hand, indicates alkalinity and acidity 

 with equal facility. The zinc-sulfide test is not a method 

 suited for those inexperienced in laboratory procedure. The 

 deductions from the two tests, however, should be approxi- 

 mately the same. 



196. Resume. — Soil acidity is a more or less unfavorable 

 biological condition, which develops in soils due to the lack or 



presence of sufficient favorable bases in the carbonate or bicarbonate 

 forms. A soil, however, may be alkaline and yet fail to effervesce. 



Potassium sulfo-cyanate test. — A new test has recently been proposed 

 in which a sample of soil held in a test-tube is treated with an alcoholic 

 solution of potassium sulfo-cyanate (KSCN). If the supernatant liquid 

 turns red, soluble iron is present, the degree of color indicating the 

 amount. It is assumed that the soluble iron is a comparative measure 

 of the active aluminum in the soil and that aluminum is the toxic 

 constituent. 



Comber, N. M., A Qualitative Test for Sour Soils; Jour. Agr. Sci., 

 Vol. 10, part 4, pp. 420-424, 1920. 



