38 Microbiological Population of the Soil 



not only serve as a reservoir for the further activities of microorgan- 

 isms, but also act as binders for the soil particles. 



Many of the groups of microorganisms found in the soil are cos- 

 mopolitan in nature, whereas others are of only limited occurrence. 

 Some are found in a number of soil types, and others only in certain 

 soils and under specific environmental or cultural conditions. Among 

 the bacteria, the Bacillus subtilis and the B. mijcoides groups are 

 cosmopolitan in nature, whereas the Rhizobium legwninosarum is 

 limited largely to soils in which specific legumes are growing. Azoto- 

 bacter chroococcwn is found only in soils that have a pH above 6.0, 

 whereas A. indicum can withstand much more acid reactions. Fungi 

 are more abundant in acid soils, and actinomycetes in alkaline. Many 

 organisms are controlled by the nature and abundance of the organic 

 matter, by climatic conditions, aeration, and reaction, and by the 

 specific vegetation. The mycorrhiza fungi and the various plant- 

 pathogenic fungi and bacteria are particularly influenced by vege- 

 tation. 



The associative and antagonistic effects among microorganisms 

 are often believed to exert a controlling influence upon the specific 

 nature of the soil microbiological population. The inhibition of 

 many bacteria, notably of the spore-forming rods and cocci, by anti- 

 biotic substances produced by fungi; the feeding of certain fungi 

 upon nematodes and protozoa; the feeding of many protozoa upon 

 bacteria; the attack of many bacteria and fungi upon insect larvae; 

 the ability of various phages to attack bacteria— all contribute to the 

 modification of the soil population. 



The addition of large amounts of organic matter, especially fresh 

 plant and animal residues, to the soil completely modifies the nature 

 of its microbiological population. The same is true of changes in 

 soil reaction which are brought about by liming or by the use of acid 

 fertilizers, by the growth of specific crops, notably legumes, and by 

 aeration of soil resulting from cultivation. The results of fertiliza- 

 tion and liming upon the microbiological population, as determined 

 by the agar plate method, are brought out in Table 4. Liming of 

 soil favors the bacteria and actinomycetes, but not the fungi. Acid 

 mineral fertilizers, like ammonium sulfate, favor the fungi, but not 

 the other two groups. Manure favors all groups. 



Often a sequence of forms occurs after a certain treatment, one 

 group of organisms following another. The addition of cellulose- 

 rich materials, for example, first favors the development of fungi, 

 notably species of Chaetomiiim, Fusarium, Aspergillus, Penicillium, 



