STUDIES ON SOIL PROTOZOA 169 



soil with volatile antiseptics; these organisms, being inactive 

 in most soils, do not increase subsequently and so it is obviously- 

 necessary to use a larger sample in order to demonstrate their 

 presence. 



That the presence of protozoa in the partially sterihzed soils 

 used in this work was not due to contamination was shown by 

 holding ten pots of sterilized soil under identical conditions 

 for one month and then taking samples for protozoa. Nine 

 of these pots were found to be free from protozoa, while the 

 tenth contained one small flagellate. 



These results argue strongly against the protozoan theory 

 as an explanation of the phenomena of partial sterilization, but 

 it cannot be said they positively disprove it, since, as was pointed 

 out before, the particular kinds that are detrimental, if such 

 exist, may be very sensitive to volatile antiseptics. 



VII. EXPERIMENTS RELATING TO THE POSSIBLE EXISTENCE IN 



SOIL OF A HARMFUL BIOLOGICAL FACTOR WHICH IS DESTROYED 



BY THE ACTION OF VOLATILE ANTISEPTICS 



Introduction 



The experiments made in his part of the work were planned 

 in an effort to determine whether the beneficial action of vola- 

 tile antiseptics upon the soil bacteria is due to the destruction 

 of a detrimental factor which is antagonistic to them. This 

 problem was attacked in much the same way as was the study 

 of the soils containing protozoa and free of protozoa (Part IV). 

 If normal soils contain a bacterial-limiting factor while partially 

 sterilized soils do not, it would seem that that fact could be 

 quite definitely established by a comparison of the numbers of 

 bacteria found in these soils when subjected to various condi- 

 tions. It should also be easy to demonstrate the presence of 

 this harmful factor by the reinfection of the partially sterihzed 

 soils with a small amount of untreated soil. 



