STUDIES ON SOIL PROTOZOA 



59 



Methods 



The effect of the various protozoa isolated upon soil bacteria 

 was tested in soil extract and then in soil. The soil tests were 

 made by the addition of the protozoan cultures to sterihzed soils 

 reinoculated with protozoa-free soil. Tumblers each containing 

 200 grams of soil were used for these experiments, and the soils 

 were incubated 30 days at laboratory temperature, which ranged 

 from 20° to 25°C. The moisture contents of the soils were held 

 as closely as possible to the optimum for the growth of plants 

 and micro-organisms. (The optimum moisture content is gen- 

 erally considered to be very close to the moisture equivalent of 

 the soil.) Inoculations of the soil and soil extract cultures were 

 made by the addition of one cubic centimeter of an active soil 

 extract culture of the protozoan under investigation. 



The experiments reported here include the tests which have 

 been made with six types of protozoa, the ciliates, Colpoda 

 cucullus, and Balantiophorus elongatus, and the four flagellates 

 which have been previously mentioned (Part II) as the most 

 abundant types in soil: — Monas sp., Dimorpha radiata (?) and 

 two others designated for convenience as flagellates A and B. 



Results with Colpoda cucullus 



In soil extract, as would be expected, the number of bacteria 

 is greatly reduced in the presence of Colpoda cucullus. In soil, 

 on the other hand, no such limiting action could be detected, 

 which further substantiates the view that this organism is not 

 active under ordinary soil conditions. 



TABLE XII 

 Effect of Colpoda cucullus upon the number of bacteria in soil extract 



