74 THE SOIL POPULATION AND ITS DISTRIBUTION 



The magnitude of the effects of different factors in bringing about 

 changes among the microorganisms of the soil, including numbers, 

 types, and activities, varies greatly. Cultivating a soil shows only 

 slight effects on the population. Addition of inorganic fertilizers 

 increases the activities slightly more. Of the various soil treat- 

 ments in humid regions, the introduction of organic substances 

 affects the numbers and activities of microorganisms more than 

 any other. 



The soil in its natural environment is constantly exposed to 

 fluctuating conditions of temperature, moisture, pressure, and air 

 movements, numerous factors being active at any one time. 

 Furthermore, addition and removal of organic and inorganic 

 materials, through the agency of man, animals, and plants, are 

 always occurring. These factors are undoubtedly responsible 

 for the perpetuation of the active biological conditions in the soil. 

 They exert their action either directly upon the microbes, or 

 only indirectly, by first influencing the plant and the soil con- 

 stituents, and these in their turn modifying the development of the 

 microorganisms. 



The composition of the soil microbial population, both qualita- 

 tively and quantitatively, is thus a resultant of numerous factors, 

 which can be traced to the soil and atmospheric agencies, as well 

 as to the nature of the plant and animal populations. 



LITERATURE 



1. Buchanan, R. E., and Fulmer, E. I. Physiology and biochemistry of 



bacteria. Volume 2. The Williams & Wilkins Co. Baltimore, 1930. 



2. Conn, H. J. A bacteriological study of a soil type by new methods. 



Soil Science, 25, 1928: 263-272. 



3. Henrici, a. T. Molds, yeasts, and actinomycetes. John Wiley & Sons, 



Inc. New York, 1930. 



4. Jordan, E. 0., and Falk, I. S. The newer knowledge of bacteriology and 



immunology. Chapters 23, 24, 26. The University of Chicago 

 Press, 1928. 



5. Russell, E. J. Soil conditions and plant growth. 5th ed. Chapter 6. 



Longmans, Green & Co. London, 1927. 



6. Russell, E. J. The microorganisms of the soil. Longmans, Green & 



Co. London, 1923. 



7. Waksman, S. a. Principles of soil microbiology. Chapter 1. Williams 



& Wilkins Co. Baltimore, 1927. 



