THE BIOLOGICAL ENVIRONMENT OF ROOTS 655 



The soil environment 



The study of the microbiology of soils presents some special diffi- 

 culties, not sufficiently recognized, which are basic to the discussion of 

 ecological relationships in soil. These difficulties arise because of the 

 peculiar nature of soil as an environment. The science of bacteriology 

 rests solidly on the classical, pure-cultural methods of the pioneers, 

 just as the technology of the fermentation industries similarly depends 

 on the exclusion of all but the selected organism. Whenever the bac- 

 teriologist has been confronted with situations involving mixed popula- 

 tions, he has been less successful in distinguishing the organisms that 

 are of major significance from the hangers-on whose presence is ines- 

 sential. Consider, for example, the state of our knowledge of the micro- 

 biology and ecology of the rumen flora, or that of ocean waters. As an 

 environment, soil possesses the peculiarity that, except when water- 

 logged, it lacks continuity, and even more importantly, largely lacks 

 substrate uniformity. The micro-environment of most soil organisms is 

 a water film. Fungal mycelia may not be so limited, if in a zone of high 

 humidity. Calculation of the internal surface of soils makes it evident 

 at once that at field capacity or lower moisture content, the water film 

 cannot be of sufficient volume to accommodate clumps, clusters, or 

 colonies of bacteria, except in micro-pools or contact rings between soil 

 particles. The effective discontinuity of the water film means also that 

 there is no substrate uniformity. Not all soil water in a particular hori- 

 zon contains the same nutrients, though there may be a measure of 

 physico-chemical uniformity by reason of the equilibrium condition 

 arising from the presence of the same array of clay minerals. Even 

 this, however, might be only an average condition of innumerable 

 micro-habitats— similar but not identical. 



In surface soils there is little continuity or uniformity in the dis- 

 tribution of the primary energy sources of vegetative origin, but as de- 

 composition proceeds, and synthesized microbial tissue is substituted 

 for plant residues, the nutritional dissimilarities between micro-habitats 

 are reduced. A common type of energy material may be progressively 

 substituted for more dissimilar materials. In lower horizons or illuvial 

 zones also, there may be a greater measure of substrate uniformity, 

 though at the same time the microbial diet is leaner. 



Fluctuations in the moisture level, by changing greatly the volume 

 available for colonization by microorganisms, have disproportionate 

 effects on the microbial world of the soil. This is probably the most 

 potent single factor influencing soil-microbial activities, because in ad- 

 dition to these spatial considerations, moisture changes are accom- 

 panied by changes in the concentration of inorganic ions and of soluble 



