CHAPTER 



/ Role of Soil Microorgan- 

 isms in Nutrient Availa- 



bility 



A. G. NORMAN 



S 



'oil microbiology is a field of endeavor about which 

 considerable diversity of opinion has been expressed by workers whose 

 interests lie in the study of factors involved in plant growth. Many com- 

 pletely ignore the soil microbial population, apparently because it has 

 been established by nutrient culture studies that, under certain peculiar 

 circumstances not normally achieved in soil, the presence of micro- 

 organisms is not essential for optimum plant growth. Others refer to 

 the soil population only when unsatisfactory plant growth is obtained; 

 it is then convenient to be able to ascribe the growth deficiency to 

 "immobilization" of this or that essential nutrient by the soil micro- 

 organisms. 



To a degree the views of some of the earlier soil microbiologists may 

 have contributed to this attitude. Particular groups of microorganisms, 

 studied in pure culture, were found to bring about reactions the product 

 or products of which in soil might advantageously afTect plant growth. 

 These were "beneficial" organisms. Others, capable of effecting trans- 

 formations that might result in reduction in solubility, availability, or 

 loss of some nutrient, were "harmful" organisms. The great bulk of 

 the population not having obvious specialist functions was largely passed 

 over. It was implied that in some way or other the role of the soil popu- 

 lation with respect to plant growth could be expressed by a summation 

 of the positive (beneficial) and negative (harmful) components. 



In this review consideration will be given only to the activities of 

 microorganisms that afTect the supply of major and minor plans nu- 



