THE BIOLOGICAL ENVIRONMENT OF ROOTS 663 



in physico-chemical studies or in nutritional studies. Microbial sur- 

 faces contain active exchange sites. The effective exchange capacity in 

 the \ icinity of roots is likely to be much higher than that of the inor- 

 ganic soil colloids alone, and it must surely affect the concentration 

 and array of ions that can enter the free space of the roots. Microbial 

 products may indirectly or directly affect the solubility of nutrient ions. 

 Some cycling of nutrients, particularly nitrogen, may take place in the 

 rhizosphere zone. Root exudates are reported to have a relatively 

 narrow carbon/nitrogen ratio and to contain amino acids which would 

 be quickly deaminated. This nitrogen would either be temporarily im- 

 mobilized as microbial protein or be re-absorbed by the plant. 



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