266 Associative and Antagonistic Effects 



3. Production by certain organisms of specific substances which 

 are essential for the growth of other organisms. These are fre- 

 quently designated as growth-promoting substances or as vitamins. 



4. Utilization and destruction by various microorganisms of the 

 metabolic waste products of other organisms. In this process, the 

 former organisms create conditions which are favorable for the 

 continued growth of the latter. 



5. Dependence of certain organisms upon others for carrying out 

 life activities; this association becomes one of symbiosis. Miscel- 

 laneous associative relationships exist among microorganisms; for 

 example, the living together of algae and Azotobacter, the former 

 synthesizing carbon compounds and the latter fixing nitrogen. This 

 is true of associations of leguminous plants with root-nodule bacteria, 

 of coniferous trees and other higher plants with mycorrhiza fungi, 

 of insects with fungi and bacteria; in the association between an 

 insect and an actinomyces, the latter provides some growth substance 

 for the former. 



Antagonistic Effects 



Antagonistic interrelationships are also very common among mem- 

 bers of the soil population, whereby one organism, directly or indi- 

 rectly, affects injuriously the activities of another organism. These 

 interrelationships may also be briefly summarized: 



1. Competition among microorganisms for available nutrients. 

 This may occur between organisms belonging to the same group, 

 as between two types of bacteria, or between organisms belonging to 

 different groups, as between bacteria and fungi. 



2. Creation by one organism of conditions which are unfavorable 

 for the growth of another, as by changing the reaction of the medium 

 to acid, by the production of inorganic (nitric, sulfuric) or organic 

 (citric, oxalic, fumaric, butyric, lactic) acids. 



3. Production by one organism of specific substances which are 

 injurious to growth of other organisms (Table 60). Here belong 

 such well-defined compounds as alcohols and quinones, as well as 

 the numerous antibiotics. These substances are frequently classified 

 as soil toxins, the exact nature of most of which still remains un- 

 defined (Table 61). 



4. Direct parasitism of one organism upon another. Here belong 

 the various effects of fungi upon bacteria, of bacteria upon fungi, of 

 fungi and nematodes upon insect larvae. One of the significant as- 



