Although plants contain and need as little as one part per million of zinc, this element is essential to growth of many plants and possibly to all. Its role in plant nutrition was discovered comparatively recently. Zinc deficiencies were first observed in Florida and California. Its most important role seems to be in seed formation. Peas and beans grown in zinc-deficient soils may form small, seedless pods. If some zinc was present in the soil earlier but was exhausted before flower pollination, plants make some growth and rob older foliage of zinc to mature the seeds formed when pods begin to set. Often enough zinc is "borrowed" so that a near-normal crop will be set on stunted plants.
Zinc also plays an important role in cell formation. When zinc is lacking, cells do not divide but continue to enlarge in size. Apparently, without zinc the nucleus is incapable of dividing to form new cells.
Like sulfur, zinc enters into synthesis of such vital products of plant metabolism as protein and plant growth regulators.
An unusual characteristic of zinc should be noted: It seems to be scarcest where organic matter is most abundant. Most other metallic minor elements such as iron and boron are more readily available in the presence of organic matter, but apparently this does not hold true for zinc.
Nearly 400 years ago, borax was shipped from Central Asia to Europe for use as a fertilizer-one of the first chemicals to be used in feeding plants. Despite this early use of a boron-bearing material, it was not until 1915 that boron's essential role in plant nutrition was fully established. Because boron is used in such minute amounts,
modern chemical methods were needed to make the necessary analyses to detect its role. It is unique in that the lack of as little as one or two parts per million in soil may produce deficiency symptoms; and, conversely, if boron is present in concentrations of only 10 to 15 parts per million, it may be toxic. A ton of cut alfalfa will contain less than two ounces of boron drawn from the soil, but those two ounces are vital to alfalfa growth. If boron is not present, terminal buds of the plants die, forcing side shoots to develop. In turn, tips of these shoots die, producing a plant full of short stubs with dead ends.
Boron has other uses in plant nutrition, many of them critical. It enters into cell division, affects flowering and fruiting, stimulates pollen grains into germinating, affects translocation of water in plant tissues and enters into many metabolic processes. Like several other elements, boron is linked with calcium in its effects on plants.