A.G.Norman 181 



The problem in the case of elements such as potassium or calcium 

 which have cationic properties and fixed valences is relatively simple. 

 Such elements presumably undergo immobilization by microorganisms 

 when they are present in living tissues and then are not freely available 

 to plants. On death of the tissues they can be immediately reutilized. 

 Even so, there are circumstances in which the supply of such elements 

 may be affected by practices which involve microbiological activity. On 

 certain plots at the Long Ashton Research Station in England, apple 

 trees under clean cultivation develop acute potash deficiency symptoms 

 which do not occur if a grass cover is maintained. 



Other cationic elements of variable valency may similarly be in- 

 corporated into microbial tissues but in addition may undergo biologi- 

 cal oxidations or reductions that may completely change the availa- 

 bility of the element. Manganese is a particularly good example of this 

 group of elements. Exchangeable manganese may be oxidized by 

 heterotrophic organisms to manganic oxide and manganese dioxide 

 (9), both of which are presumably unavailable to plants. Little is 

 known of the microbiology of the oxidative process and still less of the 

 reducing conditions that might make oxidized manganese again avail- 

 able. There is some evidence that temporary reduction can be effected 

 by the addition of thiosulfates, which concurrently are biologically 

 oxidized to polythionates and eventually to sulfate (13). It is to be 

 noted that this class of transformations is not related quantitatively 

 to the needs of the microbial population in the synthesis of cellular 

 material, as is the case with nitrogen and phosphorus. 



Iron also may undergo oxidative or reductive transformations in 

 soil. In this case autotrophic organisms are known which are capable 

 of effecting specifically one oxidative step but iron oxidation is probably 

 not limited to autotrophs. 



No doubt it will be found that other micronutrients such as copper, 

 zinc, and molybdenum, whether or not essential to the growth of 

 microorganisms, similarly undergo transformations that are brought 

 about by microorganisms, and which have effects on availability. In- 

 deed, there is already some evidence of this nature in the case of zinc 

 (j). Nothing is known as to the nature of transformations that may 

 be undergone in soil by ampholyte elements such as boron, aluminum, 



