CHAPTER VII 



TRANSFORMATION OF MINERAL SUBSTANCES IN 



SOIL THROUGH THE DIRECT OR INDIRECT ACTION 



OF MICROORGANISMS 



Relationships of Microorganisms to the Elements 

 Occurring in Nature. — Practically all the elements which are 

 essential, in large or small amounts, for the growth of cultivated 

 or uncultivated plants, as direct nutrients or as catalysts, are 

 subject to the action of microorganisms in the soil. The trans- 

 formations of carbon and nitrogen considered in the preceding 

 pages are particularly prominent. Some of the other elements 

 are made available for plant utilization through the direct action 

 of microorganisms, and still others may be subject to their indirect 

 action. 



Compounds of carbon and nitrogen, available to higher plants, 

 are apt to become very scarce in nature unless the great variety 

 of organic materials produced by living things are constantly acted 

 upon by microorganisms and kept in circulation. The elements 

 hydrogen and oxygen, which are present in great excess in the 

 atmosphere and in the lithosphere, in either combined or free 

 forms, seldom become limiting factors in the growth of plants. 

 However, these elements are constantly subject to the activities 

 of microorganisms in one way or another, since they occupy an 

 important place among the constituents of the microbial nutrients 

 and of the microbial cells, and take an active part in the numerous 

 oxidation and reduction processes carried on in the soil by microbes. 

 It may be sufficient to call attention to only two processes to indi- 

 cate to what extent these two elements are important in the activi- 

 ties of microorganisms and in the various soil transformations, 

 and also what role microbes play in the changes that these two 

 elements undergo in nature. 



In the absence of sufficient uncombined oxygen, as in the case 

 of peat bogs, where the supply of gaseous atmospheric oxygen is 



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