3 



OccuiTence of Specific Microorgaiiisiiis 

 ill the Soil 



The soil microbiological population has been divided by Wino- 

 gradsky into two broad groups: (a) the autochthonous or native mi- 

 crobes, which are characteristic of the particular soil and which may 

 be expected always to be found there; {h) the zymogenic microbes, 

 or those which develop under the influence of specific soil treat- 

 ments, as addition of organic matter, fertilization, or aeration. To 

 these two groups, another may be added, (c) the transient microbes, 

 comprising organisms that are introduced into the soil intentionally, 

 as by legume inoculation, or unintentionally, as in the case of agents 

 producing animal and plant diseases; these may die out rapidly or 

 may sur\i\e in the soil for \'arying periods, especially in the presence 

 of plant or animal hosts. 



Very few organisms can be identified whfle they are still living in 

 the soil or in the compost. It is necessary to isolate them in culture, 

 and preferably in a purified state. For physiological studies, pure 

 cultures of organisms are absolutely essential. Certain fungi, actino- 

 mycetes, and heterotrophic bacteria can easily be isolated and culti- 

 vated in pure culture by means of ordinary bacteriological pro- 

 cedures and simple media. In the case of other organisms, however, 

 isolation of pure cultures involves considerable skfll, use of special 

 techniques, and expenditure of much time. This is true, for ex- 

 ample, of the autotrophic bacteria, most of the protozoa, and certain 

 fungi. The methods to be used in the isolation and study of differ- 

 ent microorganisms must, therefore, be adapted to the nature and 

 nutrition of the organisms. 



For identification of different microorganisms, known treatises, 

 such as Bergey's manual, or special monographs, such as Waksman's 

 Actinomycetes and Oilman's Soil Fungi, are used. 



59 



