132 



Journal of Agricultural Research 



Vol. XVI, No. 4 



It is evident from these results that while the increased osmotic pres- 

 sure exerted by the salts added to a soil plays an important part in the 

 retarding of the bacterial activity, it is not the only factor nor probably 

 the main one. The principal factor is probably a physiological one 

 caused by the action of the substance upon the living protoplasm of the 

 cell, changing its chemical and physical properties so that it can not 

 function normally. However, we do not find a relationship between the 

 toxicity of the compound and its power to precipitate colloids. It 

 appears, therefore, that while the precipitation of the colloidal cellular 



F 



CaCQ, 





CaSOf. 



>. \ 'h h. > 'h 'h >,}>, y^ 



^ 



I I I I 





Fig. s. — Graphs showing the percentages of nitric nitrogen produced in loo gm. of soil to which had 

 been added aXio"' mole of the various salts, the untreated soil being counted as producing loo per 

 cent. 



material often causes death of the organisms, it is not necessarily the 

 determining factor in the toxic action of these salts. 



As can be seen from figure 4, it is not necessarily those compounds 

 which become toxic at the lowest concentration which have the greatest 

 far-reaching effect upon the bacterial activities of the soil. This con- 

 dition holds for both the ammonifying and nitrifying organisms. It re- 

 quires in almost every case more of the specific salt to reduce ammonifi- 



