A. G. Norman 169 



seen by direct microscopic examination of soil and a large number of 

 genera of protozoa have been isolated from soil. The role of these two 

 groups in the composite activity of the complex microbial population 

 of which they are a part is completely obscure, and, since the abandon- 

 ment of the "protozoan" theory of soil fertility and the censuslike 

 studies almost twenty-five years ago, both have been largely ignored. 



A soil is a product of its environment and the populations which 

 develop within the various horizons of a soil are differentiating charac- 

 teristics no less valid than those exhibited by the organic or inorganic 

 components of the soil. They are, however, far less readily determined 

 or simply described and, moreover, they are dynamic and in a sense 

 easily mutable. Particularly is this the case in cropped soils. In virgin 

 soils or under permanent sod there is no doubt a greater degree of 

 population stability. 



Soil is a medium in which many types of organisms appear to be able 

 to retain viability for long periods though inactive. This is as true of 

 vegetative cells as it is of bacterial endospores or the spores of fungi or 

 actinomycetes. Perhaps the retention of water by clay surfaces provides 

 a shell that is protective to organisms in contact with the clay. There is 

 no ready way of determining whether an organism isolated from soil 

 was active immediately prior to isolation or was in an inactive, dormant 

 condition. The picture of any soil population that can be obtained is 

 a highly indiscriminate one, and accordingly it is not easy to ascertain 

 the ecological responses to cropping or even to major changes in the 

 physical, chemical, or nutritional status of a soil. 



Even uncropped or untilled soil does not provide a biologically stable 

 environment. Even though the carbonaceous energy supply in such 

 soil is only occasionally supplemented, the physical environment is far 

 from constant. There are seasonal and diurnal temperature changes, 

 and, perhaps more importantly, there are the continually occurring 

 erratic fluctuations in moisture content imposed by rainfall and evapora- 

 tion. In the higher moisture range, particularly in fine textured soils, 

 gas diffusion may be impeded, which results in alteration of the com- 

 position of the soil atmosphere and, in the extreme case of waterlogging, 

 in the establishment of reducing conditions and the dominance of 

 facultative and anaerobic bacteria. 



