Lecture 2 — 29 — Micronutrient Elements 



indications that silicon, aluminum and other elements 

 may belong in the essential list. (See plates 10 and 



11)- 



It is of interest to compare the needs of various 

 types of organisms for micronutrient elements, to the 

 limited extent present data make this possible. Ac- 

 cording to existing knowledge, animals (as represented 

 by the rat, for example) have an essential require- 

 ment for manganese, zinc, and copper. Boron has 

 usually been regarded as non-essential, but the diffi- 

 culties of providing a boron free ration and the scarcity 

 of investigation of this element from the point of view 

 of the animal leaves the question open. The fungi 

 carefully investigated, especially Aspergillus species, 

 have requirements for zinc, copper, manganese, and 

 molybdenum, but apparently none for boron. Another 

 element, gallium, is assigned an essential role for these 

 organisms by Steinberg (1938). Molybdenum has 

 a role in some nitrogen fixing bacteria. Iodine and 

 cobalt, which have a function in animal metabolism, 

 have not yet been shown to be indispensable for higher 

 plants. Far more investigation is desirable of various 

 lower organisms with reference to their requirements 

 for micronutrient elements. 



The quantities of boron, manganese, copper, zinc 

 or molybdenum needed to insure good growth of a 

 higher plant are extremely small, varying in concen- 

 tration in a culture solution from less than .01 milli- 

 gram in a liter to a few tenths of a milligram. In a 

 culture solution, however, not only the initial concen- 

 tration of the element, but also the total volume of 

 solution supplied for a given number of plants requires 

 attention, as well as interrelations among the micro- 

 nutrient elements. 



Functions of Micronutrient Elements : — The func- 

 tions of these elements effective in minute amounts in 

 plant growth and metabolism are still in a large meas- 

 ure obscure, although various suggestions can be made 



