78 



The results obtained with carbon black are very 

 interesting. In no case was there an apparent harmful 

 effect, and in several instances there was marked bene- 

 fit to the process of nitrate formation. Carbon black 

 materially reduced the toxic effect of vanillin and 

 cumarin; and of vanillin, cumarin and dihydroxy- 

 stearic acid. Indeed, the combined eft'ect of lime and 

 carbon black was to overcome quite largely the toxicity 

 of cumarin, vanillin, and dihydroxystearic acid com- 

 bined. 



Discussion 



The evidence here brought together would seem to 

 show that, under the conditions of these experiments, 

 even extremely stable nitrogenous compounds may be 

 decomposed in soils. The recent results obtained by 

 Robbins (8) indicate that such decomposition may be 

 due, wholly or in part, to the action of soil organisms. 

 In view of such decomposition, it is very doubtful if 

 these or similar substances would accumulate in soils, 

 under similar conditions, in sufficient quantities to 

 become harmful to growing plants. 



A very interesting point developing from this work 

 is the indication that certain basic compoLiiids, as qiiin- 

 oline or giianidine carbonate, are nitrified best in acid 

 soils, and that liming of acid soils prevents the decom- 

 position of such substances. Indeed, the addition of 

 lime may not only prevent the nitrification of quinoline 

 and guanidine carbonate, but the combined action of 

 lime and these basic substances may also suppress the 

 nitrification of organic compounds normally occurring 

 in soils. Evidence to support this view may be found 

 in tables I., IV. andV. A possible explanation may be 

 that the organisms which have the ability to decompose 

 these compounds function only in acid soils. Or, it 

 may be that the first stages of decomposition are effect- 

 ed by organisms other than bacteria, and that these are 

 not active in the limed soils. Further, the basic com- 

 pounds may form salts in the acid soils, which 

 salts are more readily nitrified than the origi- 

 nal compounds. If this last view be correct, 

 the suppression of nitrification in the limed soils 

 may be due to the prevention by lime of salt formation 

 between quinoline and guanidine carbonate, and acid 

 bodies existing in soils. Not all of the basic compounds, 

 however, were affected alike by liming, since pyridine 

 was decomposed in the limed soils, and the decompo- 



