A. G. Norman 173 



harvested the rhizosphere population does not immediately disappear; 

 activities in the vicinity of the roots remain intense though now root 

 decomposition occurs. 



Any concentration of microorganisms in soil is an indication of the 

 presence of available energy material. The high concentration of organ- 

 isms in the vicinity of roots is not easily explained unless normal plant 

 roots in soil lose soluble organic substances. There are, of course, ab- 

 raded root caps and sloughed-off epidermal cells, but these seem in- 

 adequate to account for the distribution of rhizosphere organisms en- 

 countered around roots of all ages and all species, and for the clear 

 qualitative differences that are detectable among the rhizosphere popu- 

 lations of different species. Root excretions of various sorts have been 

 detected in a number of special cases. Toxic secretions have been said 

 to be responsible for "soil fatigue," and for the effect of one crop on 

 another succeeding it, though microbiological and nutritional factors 

 have not always been excluded (14). Root excretions, either toxic or 

 nontoxic, would be likely to be detected in soil only if they were un- 

 available or only slowly available as energy sources for microorganisms. 

 It is now well established that certain inorganic ions may be lost from 

 plants, particularly as maturity is approached, but the technical difficul- 

 ties of studying under aseptic conditions the excretion of organic sub- 

 stances seem to have discouraged investigation. Claims have been made, 

 and some evidence presented, that sugar and carboxylic acids, such as 

 malic acid, may pass out of roots of certain plants, and amino acids out 

 of legumes (5, 8). Certain enzymes, presumably of plant origin, also 

 have been identified in the vicinity of roots. On microbiological grounds 

 there would certainly appear to be support, admittedly inferential, for 

 the view that some energy source, other than cellular debris, originates 

 in the roots of the growing plant and causes the development of the 

 microbial mantle which ensheathes the roots of plants in soil. 



EFFECTS OF MICROBIAL ACTIVITY ON SOLUBILITY OF NUTRIENT ELEMENTS 



Microorganisms may affect the nutrient supply to higher plants in a 

 number of ways. Most reviews and textbooks put emphasis on indirect 

 and inadvertent effects on the solubility of many elements. In addition, 

 it is pointed out, quite correctly, that there are a limited number of 



