Hoagland —32— Plant Nutrition 



Practical Aspects of Micronutrient Problems: — I 



have been speaking of highly controlled experiments 

 with micronutrient elements. The elaborate precau- 

 tions necessary to prove that these elements are essen- 

 tial might at one time have made us highly sceptical 

 that deficiencies of chemical elements required in such 

 extremely small quantities would ever be manifested 

 under natural conditions in the field. It is now certain 

 that soils are not invariably capable of supplying 

 enough boron, zinc, copper, and manganese to main- 

 tain healthy growth of plants. This knowledge has 

 come mainly during the past ten years. Within this 

 period thousands of cases from many parts of the 

 world have been reported of crop failure or plant 

 disease resulting from deficiencies of micronutrient 

 elements. Boron deficiencies are surprisingly com- 

 mon in some regions. These statements do not imply 

 that most soils are deficient in any of these elements 

 but the areas involved are large and important enough 

 to warrant the view that the recognition of micro- 

 nutrient deficiencies constitutes a development in ap- 

 plied plant nutrition of major significance. 



When I refer to deficiencies of boron, copper, 

 manganese, or zinc in the soil it is not a question of 

 absolute deficiency in total quantity of the element 

 present in the soil, but rather a physiological deficiency 

 arising from the insufficient availability of the element 

 to the plant ; in other words, not enough of the element 

 can be absorbed and distributed in the plant for its 

 physiological needs at each successive phase of growth. 

 There are various possible reasons for the lack of 

 availability but it would take me too far afield to 

 discuss them, other than to mention presently certain 

 evidence pertaining to zinc and to add here that some 

 common agricultural practices may influence the ability 

 of the soil to supply micronutrient elements to the 

 plant. Boron deficiencies, for example, may in time 

 appear in a heavily limed soil. 



