CRITERIA OF ESSENTIALITY OF INORGANIC 

 MICRONUTRIENTS FOR PLANTS 



With Special Reference to Molybdenum 



hy Daniel I. Arnon, Ph.D., 



Division of Plant Nutrition, University of California, Berkeley, California 



It is now generally accepted that the inorganic requirements 

 of higher plants are no longer fully represented by the classical 

 list of ten elements. In the past few decades manganese, boron, 

 copper, and zinc and, very recendy, molybdenum have been rec- 

 ognized as additional essential nutrients for plant growth. Chem- 

 ically different as these five elements are, they are characterized 

 as plant nutrients by the common feature of being required in 

 minute amounts. The methods and circumstances surrounding 

 their discovery give rise to the question whether other elements 

 need not be regarded as indispensable to plant life. The ques- 

 tion of the completeness of the list of essential elements is impor- 

 tant from the standpoint of plant physiology as well as agricul- 

 tural practice. It is clear, however, that an answer to this question 

 must be preceded by the recognition of certain well defined cri- 

 teria of essentiality. 



Suggestions for considering a given element essential generally 

 emanate from two sources, agriculture and biochemistry. In agri- 

 cultural practice it is sometimes observed that the addition of a 

 given element to the soil results in improved plant growth. Inter- 

 esting and even important as such observations may be in terms of 

 a particular soil-crop system, they seldom permit of unequivocal 

 interpretation, when a micronutrient is involved. For example, 

 in the investigation of the little-leaf disease of fruit trees in Cali- 

 fornia a large application of iron sulphate to the soil resulted in a 

 marked recovery. The favourable response from adding iron sul- 

 phate was not obtained in a second trial. Further experimenta- 

 tion disclosed that the particular lot of iron sulphate which was 

 used successfully contained a large zinc impurity and that zinc 

 deficiency was responsible for the little-leaf disease (!)• 



