342 Recent Developments in Soil Microbiology 



Interrelationships of Members of the Soil Population 



The interrelations of members of the soil population, on the one 

 hand, and of higher plants and other soil microorganisms, on the 

 other, have received considerable attention. Of particular interest 

 are the antagonistic and associative effects among microorganisms. 

 The antagonistic effects have received recognition by those interested 

 in combating soil-borne plant diseases. The specific effects of fungi, 

 bacteria, and actinomycetes in depressing various disease-producing 

 fungi, such as cotton root rot, various root diseases of cereals, and 

 damping-off diseases of other plants, have been ascribed to the pro- 

 duction of toxic substances by saprophytes or to the competition 

 with the parasites for the available food. In some cases, the depres- 

 sion of the parasite has been brought about by controlling the 

 activities of specific soil saprophytes. Potato scab may be controlled 

 by addition of sulfur, which is oxidized by specific bacteria to sul- 

 furic acid; the resulting acidity becomes unfavorable to the actino- 

 myces producing the scab. 



An attempt has been made to interpret the ability of certain or- 

 ganisms to produce antibiotic substances in terms of survival of 

 certain microorganisms in the struggle for existence in nature, and 

 especially in the soil. One cannot, of course, deny the fact that 

 certain substances produced by some organisms are toxic to others, 

 and may thus tend to control the development or even the survival 

 of the latter in the soil. If one considers, however, the artificial 

 conditions under which antibiotics are produced by various selected 

 strains of organisms, the fact that these antibiotics are selective in 

 their action upon other organisms, and that these can readily develop 

 strains which are resistant to the action of antibiotics, one wonders 

 how effective these substances are in controlhng the soil population 

 under natural conditions. Penicillin, produced by various species of 

 Penicillium and Aspergillus, offers a good illustration. It is produced 

 only in highly specific media, of which the soil is hardly a type. 

 It is readily destroyed by various organisms inhabiting the soil. It 

 has but little activity upon the fungi and most of the bacteria living 

 in the soil. 



Effect of Changing Conditions 



The numerous bacteria and fungi living in the soil will not always 

 react in a similar manner to a change in conditions of nutrition and 

 environment. Many of the important soil bacteria, such as the 



