A MODIFIED TEST FOR SOUR SOILS. 



By NORMAN M. COMBER. 



{Deparlmcnt of Agriculture, The Universilij, Leeds.) 



It was ruuently suggested' by the writer that tlie neutral salt test for 

 sourness iu soils could be adapted for rapid qualitative ])urposes by using 

 an alcoholic solution of potassium thiocyanate. This reagent becomes 

 coloured in contact with soils deficient in strong bases, because of the 

 presence of iron among the weak bases which are brought into solution 

 from such soils by neutral salts. 



This thiocyanate test appears to have been used quite extensively 

 in this country, in Denmark and Scandinavia: and one attempt- has 

 been made in America to nuike it a quantitative lime-requirement method. 



The use of this test by farmers and others not in close contact with 

 a chemical laboratory is very restricted on account of the difficulty of 

 obtaining either the alcohol or the thiocyanate. Also, the reagent is 

 expensive and highly ))oisonous. A minor inconvenience is the necessity 

 of drying the soil: freshly sampled soils often (!ontain enough water to 

 destroy the colour. 



Some less restricted modification of the test was therefore sought , and 

 it was found that an aqueous solution of potassium salicylate is a useful 

 substitute for an aU'oholic solution of the thiocyanate. 



In the absence of large amounts of mineral acids, salicylic acid and 

 its salts give a violet colour with traces of ferric salts. In aqueous solution 

 this test for iron is far more delicate than the thiocyanate te.st. Aqueous 

 solutions of ferric salts are found to give the violet colour with salicylic 

 acid in dilutions too great to admit of coloration by thiocyanate. 



When, therefore, a solution of potassium salicylate is applied to a 

 soil which yields iron to neutral salt solutions, the appearance of this 

 violet colour in the solution might be expected. Accordingly the action 

 of this reagent was examined in the first instance on twelve soils known 

 to be sour apd to respond to the thiocyanate test, and on twelve soils 



1 Thia Journal, 1920, 10. 



- Carr, Jown, Ind. Enxj. Chcm. VX1\, 13, Xo. 10. 



