48 JAMES M. SHERMAN 



were also added to this soil culture. The flora was made more 

 complex by the addition of dilutions of various soils which repre- 

 sented 1/1,000,000 of a gram, and which contained no protozoa. 

 It would seem that the addition of these dilutions would seed 

 the soil culture with those types of bacteria which predominate 

 in normal soils. The protozoa-free soil so prepared was pro- 

 tected from contamination and its freedom from protozoa was 

 verified at frequent intervals. 



In all of the experiments in this part of the work at least one 

 kilogram of soil was used in each pot. For most of the trials 

 larger amounts were used according to the size of the pots. 

 The moisture content of the soils was maintained at about two- 

 thirds of their water holding capacities. 



For the determination of the number of bacteria the soils were 

 plated on Heyden agar and the counts made after ten days 

 incubation at laboratoiy temperature. 



The number of bacteria in soils containing protozoa and free of 



protozoa 



Many determinations have been made of the number of bac- 

 teria in sterilized soils which were reinoculated with normal soil 

 and with the special protozoa-free soil previously described. 

 Before the results from these tests are discussed, it should be 

 recognized that the method is open to severe criticism. A com- 

 parison is made of two soils containing quite different flora. It 

 is not to be expected that the flora of the artificial soil used 

 approaches in complexity the bacterial flora of the normal soil. 

 The number of bacteria found in these soils would probably be 

 different even though neither be influenced by any detrimental 

 factor. It is logical to believe that the greater number of bac- 

 teria would be found in the soil containing the more complex 

 flora, since it would seem, the greater the variety of bacteria the 

 greater would be the efficiency of the flora in the utilization and 

 destruction of its own by-products. 



If this view is correct, the greater number of bacteria should 

 be found in the soil inoculated with normal soil unless the micro- 



