274 Associative and Antagonistic Effects 



When protozoa were found to exert a favorable effect upon various 

 processes brought about by bacteria in controlled laboratory experi- 

 ments, it was assumed that similar action is exerted in the soil. The 

 protozoa were thus found to be not injurious but actually favorable 

 to soil processes. This assumption, however, may also be open to 

 question; no consideration is given to the fact that the presence of 

 numerous other organisms in the soil modifies considerably the 

 activities of the protozoa. The use of artificial media may give a 

 one-sided concept of the significance of protozoa in soil processes. 



Direct microscopic methods of soil examination have revealed the 

 fact that protozoa make up only a small portion of the soil popula- 

 tion, both in numbers and in the total amount of active cell sub- 

 stance. Their ability to reduce bacterial numbers in normal soil is 

 very slight. The indirect method of studying protozoa in solution 

 media, where the types developing and the activities resulting are 

 quite different from those occurring in the soil, has been largely re- 

 sponsible for the exaggerated importance attached to these organ- 

 isms. Certain observations have also been made on the toxic action 

 of different bacteria upon protozoa. In some cases protozoa were 

 able to develop a certain resistance to the action of bacterial prod- 

 ucts. 



It is now generally agreed that partial sterilization of soil brings 

 about the destruction of most parasitic insects and fungi. In this 

 process, a large amount of organic matter is made available for 

 the surviving bacteria (Table 62). These soon begin to develop at 

 the expense of the available organic matter and bring about the 

 liberation of large amounts of nitrogen as ammonia. This ammonia 

 accumulates in the soil, since it cannot be nitrified, because of the 

 destruction of the nitrifying bacteria by the treatment. It favors 

 increased plant growth. 



The effect of organisms destructive to pathogens and their use 

 in controlling various plant diseases offer great practical potentialities. 



Production of Antibiotic Substances by Microorganisms 



Tremendous interest has been aroused in recent years in the sub- 

 ject of antibiotic substances, especially from the point of view of 

 their possible utilization as chemotherapeutic agents. These sub- 

 stances are produced largely by soil-inhabiting microorganisms. 

 They are classified on the basis of the organisms producing them, 

 such as penicillin, actinomycin, or streptomycin, or on the basis of 



