CHAPTER XXVIII 

 Soil Microbiological Equilibrium 



influence of air drying and partial sterilization upon the 

 activities of microorganisms in the soil 



Microbiological equilibrium in the soil. The large numbers of micro- 

 organisms harbored in the soil vary greatly morphologically and physio- 

 logically. Conditions favoring the activities of one group of organisms 

 may be distinctly injurious to others. An acid reaction, for example, 

 may be favorable to the development of fungi but inhibitory to a 

 number of bacteria and actinomyces. The presence of an excess of 

 lime will favor bacteria, including nitrogen-fixing, nitrifying and other 

 groups, but may depress the development of fungi. Aeration has a 

 favorable influence upon some organisms but not upon others. The 

 very metabolism of some organisms depends upon the activities of 

 others which provide the substrate for them, as in the case of ammonia 

 forming, nitrite and nitrate bacteria, etc. 



The soil flora is so complex and the resulting activities are so various 

 that one group of processes can hardly be separated from another and 

 studied by itself. The addition to the soil of nitrogen compounds, 

 carbon compounds, or minerals does not only stimulate the activities 

 of one or more groups of organisms, but may bring about a series of 

 changes in the soil, the resultant or the end of which is hard to foresee. 

 Not only are the various organisms affected in different ways by differ- 

 ent soil treatment, but they themselves exert stimulating or injurious 

 influences upon the activities of one another. Some aerobic organisms, 

 for example, use up the excess of oxygen, and thus create in the soil 

 conditions favorable for the activities of the anaerobic forms and, 

 ipse facto, unfavorable for the activities of other aerobic organisms. 

 Some break down complex carbon compounds, like the celluloses, 

 making the energy available for other forms, like the nitrogen-fixing 

 organisms; others may compete with the latter for this available energy 

 in the presence of available nitrogen. Some break down the complex 

 nitrogenous substances liberating the ammonia, which can be utilized 

 by higher plants or can serve as a substrate for nitrifying bac- 



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