STUDIES ON SOIL PROTOZOA 57 



by the plate culture method. This observation supports the 

 theory of Russell and Hutchinson. It must be remembered, 

 however, that the bacterial flora contained in the soils with and 

 without protozoa were quite different; the flora of the protozoa- 

 free soil was not so complex as that of ordinary soil. It was 

 shown that, under the conditions of these experiments, the apparent 

 number of bacteria was afl'ected appreciably by the number of 

 kinds of bacteria present. That the difference in the two flora 

 was an important factor in the disparity between the apparent 

 number of bacteria in the soils with and without protozoa can 

 hardly be doubted; the assumption that it was the only factor, 

 however, would not be justified. 



If the bacterial flora in these tests was limited by the protozoa, 

 these organisms must be able to grow actively at low tempera- 

 tures (0°C.), at high temperatures (37°C.), and in soils of very 

 low moisture content (Tables VIII, IX, and X). It would also 

 have to be acknowledged that the harmful protozoa are capable 

 of multiplying as rapidly in soil as the bacteria (Table XI). 

 Such characteristics would be different from those of any protozoa 

 now known. Admitting these possibilities, it must be concluded 

 that the weight of the evidence is opposed to the view that 

 the soil protozoa are harmful to the bacteria. 



An objection that may be raised to this work is that sterihzed 

 soil was used as a substratum. It may be claimed that the 

 particular kinds of protozoa which are believed, by some workers, 

 to act unfavorably in the soil are not capable of development in 

 sterihzed soil. Such a possibility must be recognized, though 

 there is no reason for this belief, while the evidence we have on 

 this subject (see Part III) is opposed to such a view. 



A very essential part of this work is the study of the bacterial 

 processess in soils with and without protozoa. This phase of 

 the work was undertaken by another worker in this laboratory 

 the results of whose work will probably be ready for pubhcation 

 in the near future. It may be stated now, however, that the 

 data obtained are in perfect accord with the foregoing observa- 

 tions. 



