Boron, zinc and copper are termed micronutrients because they represent a small portion of the plant by weight. Boron is only about .006% of a plant’s makeup by weight. It plays a significant role in cell division, amino acid production, nodule formation and pollen grain viability.
Absorbed as boric acid or negatively charged borate ions, alfalfa is the only crop usually noted for boron deficiency in Illinois. Such alfalfa is usually stunted and yellowed on the upper leaves since the boron nutrient does not usually move from old to new tissue.
Zinc, absorbed as positively charged ions, makes up only about 0.02% of the plant by weight. Within the plant, the nutrient is used for plant enzyme activity and metabolism. Like boron, zinc is not very mobile and it will very rarely appear deficient on the lower parts of plants.
Finally, copper within the plant is only about .002% of its weight. It is used for enzyme activity, photosynthesis, and like zinc and boron, nodule formation in legumous plants.
So what is the answer? Is it necessary to apply these materials or do they exist at adequate levels within the soil already? Copper deficiencies are usually rare to non-existent within the state. Zinc deficiencies are nearly non-existent with soybean production, and very, very rare in corn production. Boron had better be used carefully if used at all (except in previously-noted cases of boron deficiencies in alfalfa).
Unfortunately, boron has the nasty habit of causing toxicity, yellowing or browning of the leaf margins, and crinkled leaf edges, if it is not evenly applied and if dry conditions prevail.