50 



JAMES M. SHERMAN 



TABLE VI 



The number of bacteria in different types of soil containing protozoa and free of 



protozoa 



After fifteen days incubation 



After sixty days incubation 



Protozoa-free . 

 Normal soil.. . 



210,000,000 

 108,000,000 



157,000,000 

 70,000,000 



21,800,000 

 6,700,000 



The effect of the complexity of the bacterial flora upon the apparent 

 number of bacteria in soil 



As was previously pointed out the weakness in the tests in 

 which the number of bacteria in soils containing protozoa and 

 free of protozoa are compared lies in the fact that the flora of the 

 two soils are different. Whether the complexity of the bacterial 

 flora affects the apparent number of bacteria, as revealed by the 

 agar plate count, is very important in this connection. An 

 experiment was conducted in order to test this point. 



Twelve pots of sterilized soil were divided into four groups of 

 three pots each. The soils in group A were then inoculated 

 with all of the bacteria which developed on six Heyden agar 

 plates from two different soils. Group B was inoculated with A 

 plus a mixture of all the bacteria that developed on beef extract, 

 nutrose, and casein agar plates taken from several different 

 types of soil and from different depths of soil. Group C was 

 inoculated with A and B plus the ''protozoa-free soil" used in 

 the previous experiments. The pots in group D were inocu- 

 lated with normal soil. After incubation periods of fifteen and 

 thirty days at laboratoiy temperature, samples were taken and 

 bacterial counts made using Heyden agar. The results obtained 

 are given in Table VII. 



The results in Table VII show very clearly that the bacteria 



