168 Mineral Nutrition of Plants 



trients in soils. Direct effects other than nutritional, or indirect effects 

 such as those that relate to soil aggregation, will not be discussed. 



THE NATURE OF THE SOIL POPULATION 



As will be developed later, the vegetation supported by the soil and 

 the microbial population within the soil each exert influences on the 

 other. The relationship is peculiarly complex and cannot be readily 

 described or denned in a single term. The activities and requirements 

 of the plants mesh with the activities and requirements of the micro- 

 organisms at many points. At some points these two populations may 

 be competitive; at others the activities of the one may help to satisfy 

 some requirement of the other; elsewhere they may be compatible or 

 inert. 



As Truog has pointed out, the "living phase" of the soil, its mi- 

 crobial population, is diverse in character. Many of the specialist or- 

 ganisms in soil, some autotrophs and some heterotrophs, were dis- 

 covered fairly early in the development of soil microbiology. Because 

 almost all of these were bacteria, the impression has developed that in 

 so far as the growth of plants is concerned, the bacterial flora of soil is 

 the significant flora and that other groups of microorganisms, being 

 more or less inert in their relationships to plant growth, can be safely 

 ignored. Soil microbiology, however, is certainly not a subfield of 

 bacteriology. Plating experiments and pseudoquantitative ecological 

 studies on the soil population undoubtedly show that the bacteria are 

 numerically dominant. They are, however, individually small in size 

 and may not, therefore, occupy so dominant a position with respect to 

 the total mass of the population as their profusion might suggest. This 

 may particularly be the case in soils or horizons of soils in which there 

 have been recent additions of residues of crops or other vegetation. 

 Filamentous forms — fungi and actinomycetes — may then in mass con- 

 stitute a major part of the active population. The available quantitative 

 information about such forms is of dubious value. Plate counts of or- 

 ganisms with an indeterminate habit of growth are largely meaningless 

 because they represent as individuals both spores and hyphal fragments. 

 In certain soils, and particularly those of lacustrine origin, algae may 

 not infrequently be quite numerous. Protozoan forms can often be 



