292 ILLINOIS STATE ACADEMY OF SCIENCE 



sist of soil types which ordinarily contain a larger pro- 

 portion of combined organic and inorganic colloidal con- 

 stituents than the soils shown in the latter groups. This 

 statement can be taken as no more than a mere indication 

 of a possible relationship between colloidal content and 

 high results by the Veitch and Jones methods. 



An attempt was made to compare the Comber and 

 Truog field tests with the quantitative methods discussed. 

 In studying Comber's test, 17 shades of color were pre- 

 pared as standards for comparison of the colors obtained 

 with the various soils. These were prepared by adding 

 increasing quantities of FeCl 3 to an alcoholic solution of 

 KCNS. The shades obtained in the Truog test were 

 classified into 13 groups and numbered from 1 to 13. In 

 order to have these "Truog numbers" comparable with 

 the "Comber numbers" they were each multiplied by 

 the factor 17/13 or 1.3. Again in order to increase the 

 scale so that these results could be charted along with 

 those from the quantitative methods, the results of both 

 tests were further multiplied by 20. 



The results obtained in these two tests appear to agree 

 rather closely, but show no correlation with the quantita- 

 tive methods used. Field observations with the Comber 

 test indicate that in general soils which give a red or pink 

 color will not grow sweet clover or alfalfa and will not 

 grow red clover satisfactorily without the application 

 of limestone. Further than this, quantitative deductions 

 can not safely be made. 



In another series of nearly 200 tests, not reported here 

 in detail, the Comber test was compared with the Hop- 

 kins method on soils covering a wide range of types and 

 degrees of acidity and alkalinity. The Comber test con- 

 sistently gave negative results on those soils reacting 

 neutral or alkaline to the Hopkins test, while a red color 

 was produced invariably with those soils having a lime- 

 requirement of 40 or more pounds per two million pounds 

 of soil. In these tests, the color intensity was of no signi- 

 ficance from a quantitative point of view. 



In a further study of the Comber test, samples of a 

 very acid soil (No. 2009) were extracted with water and 

 alcohol respectively. These extracts were both distinctly 



