Microbiol:;: 



proposed (1) that this be accomplished by use of the tenn rhizoplane. 

 The rhizoplane is defined as the external surfaces of plant roots 

 together with any closely adhering particles of soil or debris. 



Because of differences in terminology and in documentation of results, 

 as well as because of marked differences in laboratory procedures and 

 techniques, it often is very difficult to compare the rhizosphere data 

 of one laboratory with that of another. In the literature one finds 

 that rhizosphere populations are variously expressed. Such data may 

 be given as (a) bacteria per gram of gross sample, that is, the roots 

 plus adhering soil, (b) per gram of soil in the gross sample, (c) per 

 gram of root in the gross sample, (d) per gram of root surface scrap- 

 ings, or (e) as bacterial numbers per milliliter in successive washinr^s 

 of the root sample. Webley and associates (5) have pointed out that 

 the choice of a basis for expressing results, even when using the sario 

 laboratory data, may determine whether twofold or sevenfold differences 

 are shown for microbial populations in the rhizospheres of two plant 

 species. 



Today we need not concern ovirselves with the very considerable informa- 

 tion concerning microbial populations in the rhizosphere. I am assiming 

 that most of you are not interested in census-type information concern- 

 ing families, genera, and species of bacteria foimd in the rhizosoheres 

 of various plants. At the beginning I did make the remark that microbes 

 are commonly ten to a hundred times more numerous in the rhizosphere 

 than in the surrounding soil. This means that bacterial coimts as high 

 as several billions per gram of gross sample material can, at times, be 

 encountered. 



We might add that gravimetrically the microbes do not constitute a very 

 important part of the total root sample. On the basis of presently 

 available information, it can be estimated that even at the highest 

 level of rhizoplanal populations thus far encoimtered, the bacteria 

 present constitute somewhat less than one percent by weight of the gross 

 sample of roots and microbes. Elsewhere I have estimated that the total 

 bacterial mass in soil ranges from .03 to .28 percent, or to make an 

 even rougher approximation, that it amounts to something like one thou- 

 sand pounds of microbial tissue per acre. 



Certain workers have evaluated the relative importance of the microbial 

 population in the formation of carbon dioxide by plant roots. Lunde- 

 gardh (6) found that nearly one-half the carbon dioxide arising from 

 plant roots growing in unsterilized sand is produced by their accom- 

 panying microorganisms. Other workers (7,8) comparing carbon dioxide 

 production of sterile roots with that of roots possessing their normal 

 complement of microflora have presented quite similar estimates. 



Ro ot Materials Ava l Jablf as Nutrimts 



Wiat root matnrjalr, are available a:; iiutrjontr. to tho microorganisms 



con;rrorat';<] Jn tho rh r/ontihnrc!? 



