STUDIES ON SOIL PROTOZOA 37 



recently received serious consideration, probably because of the 

 lack of evidence that these organisms exist in an active, free- 

 living condition in the soil. 



The phagocytic theory of soil fertility 



Interest in the soil protozoa was given a great stimulus in 

 1909 when Russell and Hutchinson (1909) of the Rothamsted 

 Experimental Station announced their theory which involves the 

 protozoa as a factor detrimental to the soil bacteria, and, there- 

 fore, to soil fertility. This theory, commonly known as the 

 phagocytic theory of soil fertility, was proposed in an effort to 

 explain the phenomena associated with the partial sterilization 

 of soil with heat or with volatile antiseptics. The increased 

 yields of crops obtained after partial sterilization is explained, 

 by the sponsers of this theory, on the view that the soil protozoa 

 prey upon the bacteria and thus act as a limiting factor on the 

 microflora of the soil. The process of partial sterilization is 

 thought to destroy the protozoa while the bacteria are greatly 

 reduced, but not exterminated. When the protozoa are sup- 

 pressed, the bacteria which remain are allowed to multiply un- 

 hindered and so attain numbers greatly in excess of those found 

 in normal soils. A greater number of bacteria results in the 

 elaboration of a greater amount of plant food, hence larger crops 

 are produced. 



No direct proof has been produced in support of this theory, 

 but Russell and Hutchinson and their associates (1909), (1912), 

 (1913) have presented much evidence of an indirect nature which 

 indicates strongly that some biological factor, detrimental to 

 bacteria, does exist in the soil. 



Other views on the partial sterilization of soil 



In opposition to the protozoan theory of Russell and Hutchin- 

 son, several other explanations have been advanced to account 

 for the beneficial effects of volatile antiseptics upon the higher 

 and lower forms of plant life in the soil. Koch (1899) claims 

 that the antiseptic acts as a stimulant directly upon the bac- 



