STUDIES ON SOIL PROTOZOA 181 



9. A great difference may exist in the number of bacteria 

 as determined by the plate culture method, due to a difference 

 in the complexity of the flora, between soils which are free of 

 protozoa. 



10. Experiments with soils containing protozoa and free of 

 protozoa showed that the bacterial flora in the two soils behaved 

 in exactly the same way when exposed to different conditions 

 of temperature and moisture content. 



11. The data obtained indicate that soil does not contain 

 a biological factor which is harmful to bacteria. 



12. Pure culture tests with the ciliates, Colpoda cucullus and 

 Balantiophorus elongatus, showed that these organisms are very 

 detrimental to bacteria in solutions. In soil, since the cihates 

 are inactive, they are unable to affect the bacterial flora. 



13. Pure culture tests with four types of active soil flagellates 

 showed that these organisms were not capable of limiting the 

 number of bacteria when acting in soil. One of the cultures, 

 however, had a very marked limiting action upon the bacteria 

 when tested in soil extract. 



14. Treatment of soil with the ordinary amoimts of volatile 

 antiseptics (1 to 2 per cent) does not appear to simplify the proto- 

 zoan fauna. A complex mixture of cihates, flagellates and amoe- 

 bae is to be found in cultures made from soils partially sterihzed 

 with volatile antiseptics. 



15. As much as 10 per cent of carbon bisulphide and toluene 

 when added to soil fails to extermmate the protozoa entirely. 



16. The active soil protozoa which are at first suppressed by 

 treatment with volatile antiseptics soon begin to multiply so 

 that they are again found in numbers equal to those of untreated 

 soil within one month after treatment. 



17. The maximum number of bacteria in partially sterilized 

 soil is not found while the protozoa are suppressed but after 

 they have again returned to their normal level. It appears 

 that the development of these two classes of micro-organisms 

 subsequent to treatment with volatile antiseptics runs parallel. 



18. The reinoculation of partially sterilized soils with 1 per 

 cent of normal soil fails to decrease the number of bacteria. 



