SELMAN A. WAKSMAN 321 



increase in bacteria is accompanied by a considerable increase in the evolution of 

 ammonia (and carbon dioxide). (5) These treatments result in the complete or al- 

 most complete destruction of the protozoa. The natural assumption was that protozoa 

 by destroying the bacteria in the soil are responsible for the limitation of the fertility of 

 the soil. The destruction of the protozoa by partial sterilization of soil results in the 

 removal of the limiting agents and thus leads to an improvement of soil fertility.' 



However, further investigations have shown that many other organisms, espe- 

 cially the fungi, actinomyces, nematodes, and other invertebrates, are also active in 

 the transformation processes in the soil and are also influenced very markedly by 

 partial sterilization of soil. 



The soil is a complex medium, harboring, too, many organisms that are responsible 

 for numerous processes. By modifying the conditions of the soil, as by drainage and 

 aeration, liming, and addition of available energy, in the form of organic matter, we 

 can modify markedly not only the nature of the population, but even its activities, 

 and direct them in such a manner as to have the results beneficial to the growth of 

 cultivated crops. 



The following references may be consulted for more detailed analyses of soil mi- 

 crobiology: 



Russell, J.: Tlie Microorganic Population of the Soil. Longmans, Green & Co., 1924. 

 Waksman, S. A.: Principles of Soil Microbiology. Baltimore: Williams & Wilkins Co., 1927. 

 Waksman, S. A., in collaboration with Barthel, Chr., Cutler, D. W., and Bristol-Roach, 



B. M.: Methoden der mikrobiologischen Bodenforschiing. Abderhaldcii's Handbiich dcr 



biologischen Arbeitsmethoden. Abt. XI, Tail 3. 1926. 



' For a more extensive discussion of micro-organisms in relation to soil fertility, see chap, xxvi 

 by J. G. Lipman in this volume. 



