656 PLANT GROWTH AND PLANT COMMUNITIES 



organic products. Although plant residues form the primary source of 

 support for the soil population, the accumulating evidence suggests 

 that soluble products, such as organic acids, may remain at detectable 

 levels in soils even under aerobic conditions (Schwartz et al., 1954), 

 possibly as a resultant of adsorption phenomena with inorganic col- 

 loids. In general, however, the energy material supporting the soil 

 microflora is largely insoluble, so that utilization depends on produc- 

 tion of extracellular enzymes. The organisms supported are to be 

 found in clumps and aggregations in the immediate vicinity of the 

 fragment— not in dense colonial form, as on the surface of agar media, 

 but nevertheless in communities linked together by common nutritional 

 capabilities. These constitute the micro-habitats of the soil about which 

 conventional bacteriological procedures supply no revealing informa- 

 tion. Only if there is a high degree of nutritional uniformity can it be 

 assured that there are many similar micro-habitats. 



One of the great dilemmas of soil microbiology is the question of 

 what constitutes an adequate and representative sample microbiologi- 

 cally. Should the sample be taken from a large composite, or would this 

 tend to obscure differences of importance? Alternatively, should sev- 

 eral very small samples be examined independently? If so, what weight 

 is to be placed on differences between replicates? The very taking of a 

 sample changes it to a degree. A sample, once taken, is destroyed and 

 cannot be re-used. 



About two decades ago much attention was given to apparent 

 short-term or even rhythmic fluctuations of numbers of microorganisms 

 in soil, and although within the context of the experiments these fluctu- 

 ations seemed to be supported by the statistical analysis of the data, 

 nevertheless their significance in terms of a discontinuous system of 

 micro-habitats presented difficult problems of interpretation (Gray, 

 1938; James and Sutherland, 1940 ) . Respirometric studies on a number 

 of replicate soil samples do not suggest rhythmic changes or great ec- 

 centricities in behavior, nor, under constant environmental conditions, 

 are there unexplainable peaks or troughs in oxygen uptake or carbon- 

 dioxide evolution. Instead, after an early peak there is attained a steady 

 state, trending only slowly downward. Any sort of physical disturbance 

 of a soil is followed by a brief rise in microbial activity. The initial 

 respiratory behavior of freshly taken soil samples is to be discounted 

 because of this phenomenon. No doubt this arises because of the rup- 

 ture and redistribution of microbial communities and the establishment 

 of new micro-habitats. 



The biochemical events in uncropped soil are reasonably well un- 

 derstood as a whole. Because of the versatility of many soil organisms, 

 dissimilatory processes may lead to the same end-result, even though 

 there may have been different sequences. Some biochemists consider 



