282 ILLINOIS STATE ACADEMY OF SCIENCE 



In the Comber test, advantage is taken of the fact that 

 iron and aluminum occur in acid soils in a combination 

 that can be broken up by reaction with a salt, as KCNS, 

 and it is essentially a test for ferric iron in such form. To 

 2 or 3 grams of soil in a test tube is added an excess of 4 

 per cent KCNS in 95 per cent alcohol. After shaking 

 thoroly and allowing to settle, the supernatant liquid be- 

 comes red within 15 minutes if the soil is acid. The red 

 color is due to the formation of ferric thio-cyanate. 



The Truog test, like those of Jones and Hopkins, relies 

 upon reaction with a neutral salt to liberate acid-reacting 

 substances. The soil is mixed with zinc sulfide and bar- 

 ium chloride. Distilled water is then added and the mixt- 

 ure boiled. A strip of filter paper, saturated with lead 

 acetate, is held over the flask and is blackened by the for- 

 mation of PbS from the H 2 S liberated from the flask. 

 The intensity of blackening is presumably proportional 

 to the acidity of the soil. 



EXPERIMENTAL 



Fifty-seven samples of surface soil (0" — 6^") were 

 selected from the stock samples collected in various parts 

 of the state, representing twenty-six soil types. These 

 were selected to represent a wide range of lime-require- 

 ment as previously determined by the Hopkins method. 

 For the presentation of data, these soils were classi- 

 fied into five groups, each group containing soils similar 

 in physical and textural characteristics. These groups 

 are as follows: 



I. Sandy soils (8 types.) 



II. Light colored silt loams (5 types). 



III. Dark colored silt loams (4 types). 



IV. Clay loams and clays (5 types). 



V. Black soils, high in organic matter (4 types). 



The results of the tests by each of the five methods are 

 presented in Tables I to V inclusive, each value repre- 

 senting the average of closely agreeing duplicate deter- 



