RELATIONS OF PROTOZOA TO BACTERIA 155 



The fact that some of the protozoa feed upon bacteria served as the 

 basis for a theory designated as the "protozoan theory of soil fertility" 

 (812). According to this theory, the capacity of protozoa to consume 

 bacteria is responsible for the limited fertility of certain soils. The bac- 

 teria were viewed as the sole agents responsible for the liberation of 

 nutrients in the decomposition of soil organic matter and for the trans- 

 formation of these nutrients into forms available to higher plants. The 

 protozoa, because of their capacity to digest bacteria, were looked upon, 

 therefore, as the agents injurious to soil fertility. The increased fer- 

 tility which results from the treatment of soil with heat and with cer- 

 tain chemicals was believed to be due to the destruction of the protozoa, 

 considered as the "natural enemies of the bacteria." 



Subsequent investigations did not support this theory. When proto- 

 zoa were added to cultures of bacteria responsible for certain specific 

 processes they did not exert any detrimental effect upon the particular 

 reactions brought about by the bacteria, despite the fact that they fed 

 upon and thereby considerably reduced the numbers of these bacteria. 

 In many cases, the effect of protozoa upon bacterial activities may actu- 

 ally be considered beneficial (163). This was found to be true for such 

 processes as the fixation of atmospheric nitrogen, the liberation of 

 ammonia from proteins, and the formation of carbon dioxide from car- 

 bohydrates. 



Failure to confirm the protozoan theory of soil fertility was due pri- 

 marily to the fact that several assumptions were made that were not 

 fully justified, namely, (a) that bacteria are the only important soil or- 

 ganisms responsible for the decomposition of the soil organic matter j 

 (b) that protozoa, by consuming some of these bacteria, are capable of 

 restricting bacterial development and, if so facto, organic matter de- 

 composition. The fact was overlooked that the soil harbors, in addition 

 to the bacteria, many fungi and actinomycetes capable of bringing 

 about the decomposition of plant and animal residues, resulting in the 

 liberation of ammonia, and that this could take place even if all the bac- 

 teria were completely eliminated from the soil. 



The favorable effect of partial sterilization of soil upon fertility still 

 remains to be explained. Various other theories have been proposed, the 

 most logical of which is one based upon a soil condition designated as 



