CHAPTER XXIX 



Influence of Environmental Conditions, Soil Treatment and 



Plant Growth upon Microorganisms and Their 



Activities in the Soil 



When a soil is kept at the same moisture and temperature for a 

 considerable period of time the microorganisms are found to reach in 

 that soil a certain condition of equilibrium, as pointed out elsewhere 

 (p. 740). Winogradsky found that a change in the physical, chemical 

 and physico-chemical condition of the soil brings about a change in the 

 soil flora and fauna. The nature of the change depends upon the 

 treatment. The addition of organic matter stimulates the development 

 of certain specific groups capable of decomposing this particular organic 

 substance. A change in the soil reaction brought about by liming, by 

 addition of sulfur or of ammonium salts favors the development of organ- 

 isms more adapted to that soil reaction. An excessive rainfall or reduc- 

 tion of pore space by mechanical means modifies the composition of the 

 soil atmosphere and stimulates the development of anaerobic organisms 

 in preference to aerobes. Changes in temperature and aeration also 

 lead to changes in the quantitative and qualitative relationships of the 

 soil population. 



Whether we accept the idea of Winogradsky concerning an autoch- 

 tonous or native soil flora or not, one thing is certain that a soil, 

 under a given set of conditions, can liberate a definite amount of energy, 

 which is sufficient for the activities of a definite number of microorgan- 

 isms, the qualitative composition of the population depending upon the 

 soil conditions. Any change in these conditions will modify the popu- 

 lation both quantitatively and qualitatively. The changes in the soil 

 population may take place from day to day and even from hour to hour, 

 under field conditions. 1 Samples of soil taken at long intervals of 

 time may not give, therefore, a true picture of the actual soil popula- 

 tion and its changes in the soil. It is important to study the soil flora 

 and fauna at frequent intervals and establish the interrelationships 



1 Cutler, D. W., and Crump, L. M. Daily periodicity in numbers of active 

 soil flagellates. Ann. Appl. Biol., 7: 11-24. 1920; Cutler et al., 1923 (p. 32). 



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