Effect of Protozoa upon Soil Bacteria 273 



Of these theories, oiiK tlie last two deserve careful consideration 

 iroin the point of view of soil microbiological processes and their 

 effect upon plant growth. 



Effeci^ of Protozoa upon Soil Bacteria 



"The protozoan theory of soil fertility" was suggested by Russell 

 and Hutchinson. It was based upon the belief that the capacity of 

 protozoa to consume some of the bacteria is responsible for the 

 infertility of certain soils. 



The results of later and more detailed investigations on the rela- 

 tion of protozoa to bacteria, however, fail to support this theory. 

 When protozoa are added to cultures of specific bacteria concerned 

 in known important soil processes, such as ammonia-forming and 

 nitrogen-fixing bacteria, they are able to feed upon these bacteria 

 and to bring about considerable reduction in their numbers. This 

 capacity is not necessarily accompanied by a detrimental effect upon 

 the specific processes for which these bacteria are responsible; the 

 effect of the protozoa may actually be beneficial. It has, therefore, 

 been suggested that the presence of protozoa in the soil, even if 

 accompanied by the consumption of bacteria, may result in keeping 

 the latter at a level of maximum efficiency. 



The theory that protozoa play a controlling part in soil fertility 

 was based upon the changes in bacterial numbers and activities as a 

 result of partial sterilization. When the protozoa were destroyed by 

 heat or chemicals, the bacteria were found to multiply rapidly. This 

 was believed to lead to more active decomposition of the organic 

 matter, to greater liberation of nitrogen, and to improvement in soil 

 fertility. This explanation was based upon several assumptions 

 which were not fully justified. It was assumed, for example, that 

 bacteria are the only important soil organisms responsible for the 

 decomposition of organic matter; actually it has been repeatedly 

 shown that fungi, actinomycetes, and other organisms are also ca- 

 pable of bringing about this process. It was further assumed that 

 protozoa, by consuming some of the bacteria, especially those de- 

 composing organic matter and forming ammonia, restrict bacterial 

 development and, ipso facto, organic matter decomposition. The 

 fact was overlooked that fungi and actinomycetes of the soil could 

 bring about, just as well as the bacteria, the decomposition of soil 

 humus and liberate the nitrogen as ammonia, a process which could 

 thus take place even with the elimination of all the bacteria. 



