STUDIES ON SOIL PROTOZOA 177 



substances are due to some action of the antispetics on the soil 

 itself rather than to a simpUfication of its micro-organic popula- 

 tion. 



VIII. RESUM^ 



Discussion 



The results of the foregoing experiments appear to establish 

 quite definitely that protozoa in the soils which have been 

 studied do not have a detrimental effect upon the bacterial 

 flora. It is difl&cult to see how the action of an important phag- 

 ocytic agent could have escaped detection by the methods 

 employed unless the factor is unable to increase in soils which 

 have been previously sterihzed with steam or partially sterilized 

 with volatile antiseptics when again introduced into these 

 soils with an inoculum of normal soil. This restricted power 

 of growth would be very different from the properties of micro- 

 organisms in general, either of animal or plant nature, and 

 there is no evidence, as far as we are aware, that a group of 

 organisms with such peculiar characteristics exists in the soil. 

 As has been poiiited out, the soil protozoa, at least those types 

 which appear in liquid cultures, grow better in soil which has 

 been previously subjected to steam sterilization just as do the 

 bacteria. Aside from the evidence that soil does not contain 

 a biological factor which is inimical to the bacterial flora, the 

 facts that volatile antiseptics do not exterminate the soil pro- 

 tozoa, and that partial sterilization of soil under conditions 

 unfavorable for the action of protozoa (e.g., at 37°C.) is followed 

 by the characteristic rise in the number of bacteria, would ap- 

 pear to cast serious doubt upon the theory of Russell and Hutch- 

 inson as an explanation for the effect of volatile antiseptics upon 

 the soil bacteria. 



Cunningham (1914) has recently pubhshed the results of 

 some work which he thinks proves that protozoa act as a limit- 

 ing factor upon the bacterial flora in soil. The fact that his 

 data on this point are derived from only two experiments, one 

 of which gave negative results, would preclude his conclusions 



