Lecture 2 — 35 — Micronutrient Elements 



Experiments were extended by other California 

 investigators to citrus orchards, in which the trees 

 showed the symptoms of the disease known as "mottle- 

 leaf," a disease of frequent occurrence in California. 

 This disease had been studied for about twenty years 

 without any good clue as to its cause. Many hypo- 

 theses had been advanced but no one of them was 

 consistent with all the observed facts. It is now 

 clear that the mottle-leaf disease is also caused by 

 zinc deficiency. There are practical difficulties in 

 correcting this condition by addition of zinc salts 

 to the soil, but spraying the trees with appro- 

 priate zinc compounds is effective and is at present a 

 common commercial practice. Zinc deficiencies in field 

 grown crops are now known in many parts of the 

 world. They often occur in Florida and other south- 

 ern states, from which independent evidence became 

 available of zinc deficiency as the cause of certain 

 nutritional diseases of crops, for example, pecan "ros- 

 ette". In Australia a disease of pine trees has been 

 traced to zinc deficiency. In Hawaii pineapple plants 

 grown in certain soils produce abnormal, distorted 

 blades, recently recognized to be the result of zinc 

 deficiency. The condition is easily remedied by zinc 

 sprays. (See plate 12). 



When a plant growing in soil suffers from lack 

 of zinc, obviously this means that the plant cannot 

 absorb enough zinc for its needs. Yet the total amount 

 of zinc present in the soil may be ample. To illustrate, 

 in the California peach orchard to which I have re- 

 ferred, the trees became severely diseased from lack 

 of zinc and nevertheless, the mass of soil within the 

 root zone contained enough total zinc to meet the 

 requirements of the trees for many centuries. The 

 reasons for unavailability of zinc in such cases may 

 be various, and include several types of chemical fixa- 

 tion. In some of these the zinc is fixed in the crystal 

 structure of soil colloids. It is also of interest to sug- 

 gest the possibility that soil microorganisms sometimes 



