A. G. Norman 171 



but the maintenance of a condition of asepsis that would rule out the 

 intervention of organisms on the surfaces of the roots has not been 

 indubitably established. There are reserves of phosphorus and sulfur 

 in soils in forms other than organic, so the fact that organic sources of 

 these elements may be available to microorganisms but not to higher 

 plants is not of special significance. There is, however, no reserve of 

 nitrogen in soils other than that in the organic form. It might, there- 

 fore, be supposed that the soil population could successfully compete 

 with the plant for nitrogen and that the supply to the plant would be 

 limited thereby. This is rarely the case, however, as will be discussed 

 later. 



In so far as the mechanism of uptake of nutrient elements is con- 

 cerned, it would seem that the process in plants and microorganisms is 

 essentially similar. The clay fraction of soil is now recognized as being 

 of great significance in plant growth. The physical and nutritional 

 characteristics of a soil are determined to a large degree by the physico- 

 chemical properties of the clay components. The organic fractions, 

 though constituting a vital and dynamic reserve of certain nutrients, 

 modify but cannot fundamentally alter the character of a soil imposed 

 on it by the presence of particular clay minerals. Microorganisms in 

 close contact with the clay colloids can no doubt compete with the clay 

 for cations in the soil solution and by contact exchange can accept ions 

 adsorbed on the clay. Certain soil organisms have indeed been shown 

 to make better growth in a clay-containing medium than in conven- 

 tional mineral nutrient media (S). Although this may not be due ex- 

 clusively to the form of presentation and means of transfer of ions, 

 nevertheless it appears probable that in soil, microorganisms absorb 

 ions by a process similar to that which holds for plant roots, and in this 

 respect they can be considered to be competitive for exchangeable 

 ions. The cation adsorptive capacity of some bacteria has been shown 

 experimentally to be several times that of an equal mass of barley roots 

 (7). This observation may not be of particular significance because not 

 all retentive positions have to be occupied by cations for growth to 

 occur. Viability may be retained even though all cations are replaced 

 by hydrogen or methylene blue. It appears likely, then, that bacteria 

 and plant roots in the same absorption zone are in equilibrium with 



