PAPERS OX CHEMISTRY AND PHYSICS 293 



acid, showing a Ph value of 5.0 to 3.5 by the colorimetric 

 method. They were free from ferric iron, since 

 they failed to respond to KCXS additions, although a 

 positive reaction was secured by adding Comber's solu- 

 tion to the soil itself. The intensity of the color is re- 

 duced greatly by substituting an aqueous solution of 

 KCXS because of the excessive ionization of the colored 

 ferric salt. 



Since the intensity of the red color is dependent upon 

 the amount of iron which can react, the iron content of 

 the soil might be expected to affect the results. It has 

 been found, in fact, that this is true. Five soils (numbers 

 1620, 1717, 2015, -1471, 8153), showing a faint pink color 

 with KCXS, were moistened and treated with iron filings. 

 After standing two days they were dried and then taken 

 up with KCNS as usual. The color was intensified con- 

 siderably in each case. 



It has been observed frequently that this test is not- 

 applicable to peat or peaty soils, no color being produced 

 even by very acid peats. This was conceived to be due 

 to a possible iron deficiency. Accordingly several peat 

 soils ranging from alkaline to acid by the Hopkins 

 method were treated with the Comber solution, both 

 with and without the addition of iron filings. All those 

 without iron filings were colorless. In each case the 

 samples with iron filings and having a lime-requirement 

 by the Hopkins method showed a red color, while the 

 neutral and alkaline samples remained colorless. The 

 iron filings were added to the dry soil immediately before 

 adding the KCXS. It is suggested that this test may be 

 made applicable to iron-deficient soils by means of this 

 slight modification. 



SUMMARY 



1. The Hopkins, Veitch and Jones quantitative meth- 

 ods, and the Comber and Truog field tests were studied 

 on 57 soils representing a wide range of Illinois soil 

 types and degrees of acidity. 



2. The three quantitative methods failed to show any 

 consistent agreement with each other from a quantita- 

 tive point of view. 



