CHAPTER XXX 



Soil as a Habitat for Microorganisms Causing Plant and Animal 



Diseases 



The majority of soil microorganisms carry on in the soil processes 

 which are of prime importance to the growth of higher plants. Some- 

 times the microorganisms compete with higher plants for specific nutri- 

 ents; some of them may also produce substances which are directly 

 injurious to higher plants. In addition to the injury which saprophytic 

 soil microorganisms may cause to plants by carrying on certain physio- 

 logical processes, the soil harbors organisms directly parasitic to plants or 

 animals. 



Influence of saprophytic soil microorganisms upon plant growth. 

 Microorganisms take part in four soil processes which directly affect the 

 growth of higher plants. (1) They decompose the soil organic matter 

 and liberate the nitrogen and minerals necessary for the growth of 

 higher plants. They also produce considerable quantities of C0 2 , 

 which is essential for the growth of plants. (2) They oxidize and other- 

 wise transform the various minerals introduced into the soil (ammonium 

 salts, sulfur, etc.), or formed from the decomposition of the organic 

 matter (as NH 3 , H 2 S, etc.), into forms readily available to plants. (3) 

 They synthesize organic matter from inorganic compounds and thus 

 compete with higher plants for the available nitrogen and minerals. 

 This process may become useful in the absence of a growing crop, 

 since the soluble materials are prevented from being leached out. 

 (4) They reduce, under proper conditions, various oxidized substances 

 like sulfates and nitrates to substances which may be directly toxic to 

 higher plants. 



The growth of the extensive group of leguminous plants is directly 

 affected by the symbiotic bacteria, so much so that these plants become 

 almost independent of the soil nitrogen and, therefore, of all processes 

 affecting the available nitrogen in the soil. The growth of a large 

 number of trees and other plants depends to a large extent upon the 

 fungi forming mycorrhiza on their roots, as shown elsewhere. What- 

 ever the nature of the phenomenon, whether it is a case of symbiosis or 

 of mutual parasitism, there is no doubt that the fungi favor in some way 



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