RELATIONS OF PROTOZOA AND BACTERIA 145 



The favorable effect of partial sterilization of soil upon fertility still 

 remains to be explained. Various other theories have been proposed, the 

 most logical of which is one based upon a soil condition designated as 

 "microbiological equilibrium" (943). It has also been suggested (498) 

 that the phenomenon is due to the disappearance of the bacterial antago- 

 nists in the soil as a result of partial sterilization. 



In many cases, however, protozoa are responsible for bringing about 

 extensive destruction of bacteria. This may find a practical application 

 in the purification of water and sewage. The action of the protozoa is 

 due in this case to the actual ingestion of the bacteria (440, 595, 743). 



The idea (157) that protozoa may favor soil processes because of the 

 stimulation of bacterial development and hence the accelerated trans- 

 formation of soil materials is not always justified. The assumption is 

 usually made that these processes take place in the soil in a manner simi- 

 lar to those brought about in artificial culture media, a generalization 

 that may be justified only in very special cases. No consideration is given 

 to the fact that the presence of numerous other organisms in the soil 

 may modify considerably the activities of the protozoa. The use of arti- 

 ficial media gives only a one-sided conception of the significance of pro- 

 tozoa in soil processes. Although the more recent claim concerning the 

 function of protozoa in the soil (157) is based upon more direct experi- 

 mental evidence, it is still inadequate, because it gives insufficient con- 

 sideration to the numerous elements involved in the complex soil 

 population. 



The protozoa make up only a small portion of the soil population, 

 both in numbers and in the actual amount of cell substance synthesized. 

 Their ability to reduce bacterial numbers in normal soil is not very sig- 

 nificant. The indirect method of studying protozoa in solution media, 

 whereby the types observed and the activities obtained are quite differ- 

 ent from those occurring in the natural soil, has been largely responsible 

 for the exaggerated importance attached to these organisms. 



One may conclude that the protozoa, by consuming some of the bac- 

 teria, keep these organisms at a high state of efficiency, thus assisting in 

 the breakdown of the plant and animal residues in the soil. In other 

 words, the rate of energy transformation brought about by bacteria and 

 even the total amount of change produced in the substrate are increased 



