Miorobiolfl 



SO^E FACTOHS IN TilJE MICROBIOLOGY OF THE HHIZOSPHIilHE 

 Francis E. Clark 



My knowledge of the nonatodes living in soil or in association with 

 plant roots can best be summed up in the admission that I am not at 

 all sure that I can distinguish the south end of a nematode from the 

 north end. Obviously then, it would not be fitting for me to attempt 

 to discuss the rhizosphere microbiology with any special reference to 

 the nematodes. What I propose to do is to discuss the microflora of 

 the rhizosphere, and I hope that you will make such additional entri-;- 

 and interpretations as are needed with respect to the microfaima. 



In preparing this discussion, I have scanned through several reviews 

 on the microbiology of the rhizosphere. Several such reviews do 

 exist (1,2,3). I do not intend to abstract that literature in any 

 complete or orderly manner. I do intend to exercise considerable 

 freedom in selection of material in order to show some of the general 

 problems faced by the students of the rhizosphere. My point of view, 

 at times, may appear to be that of a soil micro-ecolorrist rathrr than 

 that of a soil bacteriologist. 



As a beginning, let us briefly orient our thinlcing about vjhat we mean 

 by the rhizosphere. By definition the rhizosphei-e is that region of 

 the soil under the immediate influences of plant roots (U). Hicro- 

 biologically, the plant roots are in their net effect stimulatory, so 

 much so that there occurs perhaps ten times, possibly even one hundred 

 times, as many bacteria in the soil adjacent to roots as in soil apart 

 from roots. This, of course, should not be surprising. The soil 

 bacteria are not uniformly distributed throughout the soil, but are 

 localized largely according to available food supply. In the rhizo- 

 sphere, where the root, together with its excretory products and debris, 

 constitutes a food source, the site of most intensive localization can 

 be expected to occur at the surface of the plant root. 



This fact immediately leads us into a technical difficulty. The rhizo- 

 sphere, as already noted, is defined as soil under the influence of 

 plant roots, Rhizosphere samples, however, most commonly include the 

 plant roots with their closely adhering soil. Increasingly pronounced 

 rhizosphere effects upon the microbial populations can be demonstrated 

 by including less and less soil and more and more roots in the bull-c 

 samples taken for study. The frequent employment in the literati.u:'e of 

 the phrase "the microorganisms associated with the root surfaces, " and 

 of the terms "outer rhizosphere" and "closer rhizosphere" indicates an 

 unwillingness by many workers to ascribe to the surrounding soil the 

 microbes present on root surfaces. Inasmuch as the root surfaces are 

 in fact responsible for the bulk of the microorganisms found present 

 in combined soil and root samples, it would appear more logical to 

 speak of this microflora actually on the roots in ternis of the root sur- 

 faces and not in terms of the surrounding soil, I have previously 



