61 



The later increase in numbers would seem to be due 

 to the fact that si)eciric organisms find the compound 

 a very favorable food source. With the exhaustion of 

 the compound and perhaps its decomposition products 

 the organisms which fed ui)on them decrease in num- 

 bers. In view of these results it would seem advisable 

 to re-investigate the effect of steam, carbon bisulfide 

 and other agents which have been found to produce 

 large increases in the number of microorganisms in 

 the soil, bearing in mind the possibility that the increase 

 may be due to the fact that the compounds may serve 

 as food sources to bacteria or the treatment of the soil 

 may make food supplies available. (Compare with 

 Greig Smith's (9, 10) suggestions). 



These results would also seem to be of considerable 

 significance to those who are considering the soil toxin 

 theory of soil fertility. They show that the disappear- 

 ance in the soil of organic material toxic to higher 

 plants may in some cases be accomplished by micro- 

 organisms and apparently by specific microorganisms. 

 This view assigns the important role of destruction in 

 the soil of such compounds as vanillin and cumarin 

 to bacteria and not to the oxidizing action of the plant 

 roots as might be inferred to be the case from the work 

 of Schreiner, Reed and Skinner (23, 24, 25, 26, 29.) 

 The persistence of the compounds on the other hand, 

 such as occurred with vanillin in some of the soils used 

 by Skinner (28), would appear to be due to the absence 

 of suitable organisms or to the presence of conditions 

 which inhibit their acting on the compounds in ques- 

 tion. 



Since the disappearance of these four compounds is 

 due to biological factors it can be inferred that the 

 addition of a given organic compound may produce 

 no harmful effects in one soil and decidedl}'' harmful 

 effects in another depending on the presence and action 

 of suitable organisms. This is probably the explanation 

 for the varying and in some cases apparently conflict- 

 ing results obtained with the same compound by dif- 

 ferent workers as summarized in the early part of this 

 paper. Not only may this be inferred but it can be 

 conceived that the same soil under some conditions of 

 temperature, oxygen, soluble salt and water supply, 

 etc., may allow the persistence of a harmful compound 

 and under other conditions may eliminate it rai)idly. 

 What those conditions are can probabl}^ be discovered 

 by a study of the physiolog}^ of the organisms involved. 



