764 PRINCIPLES OF SOIL MICROBIOLOGY 



imagined in an undisturbed condition, even for a very brief period of 

 time, when the moisture content, aeration and temperature are not 

 changed appreciably, one could speak of a condition of equilibrium. 

 A microbiological equilibrium is distinctly different from that of a 

 chemical reaction; in the latter, equilibrium is reached when the reaction 

 goes both ways at an equal rate. A microbiological equilibrium in the 

 soil may occur when the changes in the numbers and activities of 

 the various groups of organisms are constant, possibly when as many 

 new cells are formed in a given period as are destroyed in that time. 

 An ideal condition of equilibrium may never be reached under field 

 conditions more than for a few brief seconds. Under carefully controlled 

 laboratory conditions, an equilibrium can be readily demonstrated, 

 although it takes a long period of time before it is established. 



When a soil is brought into the laboratory and kept at constant 

 optimum moisture and temperature, there is at first a rise in the num- 

 ber of microorganisms, particularly if the soil has been partially or 

 fully air dried before being placed in the incubator. The numbers 

 of microorganisms and the rate of their activities, using the carbon 

 dioxide production of the soil as an index, rise at first, soon drop 

 rapidly and then more slowly, until the rate of change in the numbers 

 and activities becomes constant. 



But even under soil conditions, one may speak roughly of a certain 

 equilibrium which becomes established between various groups of 

 microorganisms, in the competition for the available energy, as shown 

 by Hiltner and Stormer. The quantitative and qualitative composi- 

 tion of the soil flora and fauna were found to depend on the amount of 

 energy and nitrogen available as well as the forms in which these are 

 present in the soil. Any modification in the amount and form of 

 energy and nitrogen brings about a modification not only in the number 

 but also in the kind of microorganisms developing in the soil. Such 

 modifications are brought about either by introducing fresh energy 

 and nitrogen materials, air drying and cultivating of soil, partial sterili- 

 zation, growth of plants, etc. These modifications can be looked 

 upon as resulting in a shift in the condition of equilibrium. 



The differences in the energy and nitrogen metabolism of the various 

 soil microorganisms combined with their relative resistance to the 

 action of disinfectants and their rates of multiplication are the basic 

 factors governing the phenomenon of soil microbiological equilibrium. 



The fungi consume a large amount of the available energy for struc- 

 tural purposes. They produce, therefore, large quantities of carbon 



