172 Mineral Nutrition of Plants 



each other and with inorganic or organic colloids with respect to cation 

 distribution. 



EFFECTS OF THE CROP ON THE SOIL POPULATION 



Although the main topic of this paper is the relationship between soil 

 microorganisms and plant nutrition, it is pertinent to inquire just what 

 effects may be impressed upon the microbial population of the soil by 

 the presence of a crop. In other words, this amounts to an inquiry as 

 to whether the population of an uncropped soil would be modified in 

 any way by the presence of a crop. All techniques of study show that 

 in the immediate vicinity of the root hairs, rootlets, and roots there is 

 a great concentration of organisms, primarily bacteria, and that the 

 root system of plants is, therefore, virtually encompassed by a zone in 

 which microbial activities are presumably intense and are not neces- 

 sarily identical with those occurring in soil more remote from the roots 

 (2, 4). The biochemical activities of the rhizosphere population are 

 somewhat obscure and yet highly pertinent to the general topic under 

 consideration. It seems to be established that the population of the 

 rhizosphere is not merely a more concentrated and more active version 

 of that to be found in the soil away from the roots. Qualitative as well 

 as quantitative differences are found. Bacteria greatly predominate over 

 fungi or actinomycetes; gram negative rods constitute the bulk of the 

 population; sporeformers are few. The high concentration of organisms 

 in this zone is particularly striking. Although there is great variation 

 in figures reported by different workers, according to particular cir- 

 cumstances, the rhizosphere contains 5-50 times as many bacteria 

 countable on plates as are found outside this zone. The question might 

 therefore be asked as to whether there is much point to the study of 

 the general soil population; perhaps the rhizosphere population is the 

 only significant one in so far as the growth of the plant is concerned. 

 Such an answer would ignore the fact that the root zone constitutes 

 only a fraction of the soil under any crop other than a permanent sod. 

 It would ignore those biochemical activities that precede the establish- 

 ment of a particular crop on the land and its invasion by roots. It 

 would ignore the fact that no new organisms are ordinarily introduced 

 when a crop is planted. It would ignore the fact that when a crop is 



