CHAPTER X 

 IMPORTANCE OF MICROBES IN SOIL FERTILITY 



Relationships of Microbes to Soil Processes. — In conclu- 

 sion, it may not be amiss to summarize the results presented in the 

 previous chapters dealing with the occurrence, abundance, and 

 activities of various microbes in the soil, and point out their impor- 

 tance for plant growth. The very existence of higher plants and, 

 therefore, also of animals, including man, depends upon the activi- 

 ties of the soil microbes. Consequently there is little likelihood 

 of overestimating the importance of microbes in soil processes; 

 the growth of soil microbes is intimately concerned with the avail- 

 abihty of nutrients which determine the fertility of a soil. Any- 

 thing that hastens the decomposition of the soil organic matter 

 also favors an increase in soil fertility. A fertile soil is distin- 

 guished from an infertile soil not by the fact that it contains 

 more nitrogen, phosphorus, and potassium, but by the fact that 

 the nutrients present in the soil are liberated with greater rapidity 

 in the fertile than in the unfertile soil. 



Liming and cultivation of soil improve the physical condition; 

 they provide a more favorable reaction for the activities of the 

 numerous soil bacteria and admit larger quantities of oxygen which 

 are necessary for the growth of the aerobic organisms. These 

 treatments are of great importance in soil fertility, not only because 

 they produce in the soil a favorable physical and chemical condi- 

 tion for plant growth. They also create more favorable conditions 

 for the activities of the microbes which effect more rapid liberation 

 of the soil nutrients. 



Also part of the response in plant growth following the use of 

 artificial fertilizers is brought about indirectly. The fertilizers 

 increase microbial activities which lead to more rapid transforma- 

 tion of the soil constituents. 



Even a superficial examination indicates that there are numer- 



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