Rothamsted Symposium on Trace Elements 32 



Other factors also contribute to the complexity of soil-plant 

 interrelations. A chemical analysis of a soil offers no reliable 

 guide to its supplying power for a nutrient. In a California zinc- 

 deficient soil, quantities of this element were found sufficient to 

 meet plant requirements for many centuries. The zinc was, how- 

 ever, held so tenaciously by the soil as to be unavailable to the 

 plant. The significance of the unavailability of elements present 

 in the soil is also illustrated by the kind of negative evidence 

 sometimes obtained in soil trials with micronutrients. The addi- 

 tion of several hundred pounds of zinc sulphate per acre failed to 

 produce a favourable response in zinc-deficient trees owing to the 

 fixation of the added zinc in the soil. There is also the problem 

 of soil microflora. For example, the application of small amounts 

 of molybdenum to the soil may benefit the plant indirecdy by 

 favouring the nitrogen-fixing bacteria (5, 6). 



The inconclusiveness of soil experiments with regard to the 

 essentiality of micronutrients is in sharp contrast to the decisive- 

 ness, whenever available, of biochemical evidence on the specific 

 function of a micronutrient element. This has been forthcoming 

 from the studies of enzyme systems and is best illustrated in the 

 case of copper which was found to be the prosthetic group of sev- 

 eral oxidases such as polyphenol and ascorbic acid. The demon- 

 stration of the essential role of these enzyme systems in plant 

 metabolism constitutes also conclusive evidence of the indispensa- 

 bility of copper. It is by no means implied that the discovery of 

 one or more enzyme systems with copper as the prosthetic group 

 exhausts the function of copper in the green plant. What can 

 be asserted is that if copper is required for at least one essential 

 enzyme system, copper is indispensable to plant life for that reason 

 alone, regardless of other functions it may also perform. 



The history of micronutrients reveals, however, that for every 

 element, the biochemical elucidation of its function is either as 

 yet largely unavailable or, when it was forthcoming, it invariably 

 followed rather than preceded the acceptance of the micronutrient 

 as essential. Important physiological advances and agricultural 

 application of our knowledge of the indispensability of boron, 

 manganese, copper and zinc have occurred either in advance or 

 in absence of any knowledge of their functions within the plant. 

 It seems reasonable, therefore, to expect that, in the future as in 

 the past, the plant physiologist will be called upon to assess and 

 elaborate on the evidence from the field, by conducting critical 



