52 JAMES M. SHERMAN 



on agar was the chief means employed to obtain bacteria free 

 of protozoa, there can be no doubt but that the protozoa-free 

 soil used in these experiments contained a higher percentage of 

 bacteria capable of development on agar plates than did the 

 normal soil. 



The relation of the number of kinds of bacteria in soil to the 

 total number is in itself a large problem. Whatever the explana- 

 tion for the variation in the numbers of bacteria, as determined 

 by the plate culture method, with the complexity of the flora 

 may be, it appears very clear that the differences in the numbers 

 of bacteria in the soils with and without protozoa obtained in 

 the foregoing experiments were due in large part, at least, to the 

 complexity of the bacterial flora itself. This casts doubt upon 

 the belief that protozoa act as a limiting factor, but it is possible 

 that the reduction in bacterial numbers in group D was in part 

 due to these organisms. 



The effect of temperature upon the development of bacteria in soils 

 with and without protozoa 



If the soil protozoa act as a limiting factor upon the bacteria 

 it should be possible to demonstrate that fact by the subjection 

 of soils with and without protozoa to conditions that would 

 inhibit the growth of the protozoa but not that of bacteria. 

 Russell and Hutchinson explain the fact that the soil contains 

 more bacteria during the winter and early spring months than 

 in the summer on the view that the protozoa are not active at 

 such low temperatures. It seems, therefore, that observations 

 on the development of bacteria in soils which contain protozoa 

 and others free of protozoa when exposed to winter weather 

 should throw some light on the subject. 



Four pots of sterilized soil were inoculated, two with bacteria 

 alone and two with ordinary soil. The soils were kept at labo- 

 ratory temperature for two months after inoculation and then 

 placed out of doors during the months of December, January, 

 February and March. The soils remained frozen throughout the 

 greater part of this period. Bacterial counts were made just 



