Hoagland —34— Plant Nutrition 



the leaves while this is not true of some deciduous 

 fruit tree species. Work on boron, from the point of 

 view of both deficiency and toxicity, constitutes a de- 

 velopment in the study of plant nutrition of important 

 consequence to agriculture in certain regions. 



Discussions of Zinc as a Micronutrient Element : — 



With this general survey before you, I wish to devote 

 the rest of my time to the discussion of one micro- 

 nutrient element that has attracted great interest in 

 California and elsewhere — the element zinc. By de- 

 scribing researches on this element, I think that I shall 

 be able to illustrate the general nature of the micro- 

 nutrient problem in its various aspects, scientific and 

 agricultural. 



Somewhat more than ten years ago an investigation 

 was begun in California of a disease of deciduous 

 fruit trees, known as the "little-leaf" disease. In the 

 particular peach orchard first studied, located on a 

 sandy soil, the trees had made remarkably good growth 

 for several years — then they became affected with 

 the "little-leaf" disease, as it was often called.* No 

 ordinary fertilizer treatment had any beneficial effect 

 and plant pathologists could find no evidence that 

 pathogenic organisms were primarily responsible for 

 the condition of the trees. A micronutrient deficiency 

 was early suspected but the treatment at first applied 

 to test this possibility was not followed by any ben- 

 eficial result. Another special treatment of the soil 

 consisting of a large application of commercial iron 

 sulphate, used for another reason, was successful. 

 Further experimentation showed that the reason why 

 this treatment succeeded was that the iron sulphate 

 contained a considerable amount of zinc as an im- 

 purity and that the properties of the iron sulphate 

 had an influence in preventing too rapid fixation of 

 zinc in the soil. 



*For a more complete review, see W. H. Chandler, 1937. 



