Boron

Boron in the form of borax is the most widely used of the minor elements. Deficiencies of it in a number of crops have been found in the Coastal Plain and Piedmont soils of the Southeast, in New England, the Middle West, the Pacific coast, and other areas. Cork disease of apples, top sickness of tobacco, cracked stem of celery, and heart rot of sugar beets are among the many manifestations of too little boron.

Eight years ago hardly any borax was used as a fertilizer material, but since then its use has rapidly increased. In 1945 about 3,700 tons were used. Because only a few pounds are generally needed to the acre, that amount represents a widespread usage.

Boron is one of those elements for which most plants have a narrow range of tolerance; that is, the amount required is small, and slightly larger amounts may be toxic. The use of more than 20 or 30 pounds of borax an acre will cause injury to some crops, yet as little as 5 or 10 pounds to the acre may save an apple crop from failure due to a deficiency of boron. This narrow range of tolerance, plus the findings of recent research on the influence of the calcium, potassium, phosphate, and nitrate ions on boron availability and utilization, emphasizes the growing need for attention to the practical aspects of the problem. Current research is being mainly directed toward obtaining a better understanding of the relationships between boron and the other elements.

Magnesium

Although magnesium has long been known to plant physiologists as an essential element needed by the plant in fairly large amounts and therefore can hardly be called a trace element, it is only in recent years that agronomists and soil scientists have discovered its importance in agricultural practice. Now it probably ranks second to boron in wide usage in fertilizers. Since 1922, when W. W. Garner, J. E. McMurtrey, Jr., and E. G. Moss, of the Department, showed that sand-drown, a chlorosis of tobacco occurring on the sandy Coastal Plain soils in North Carolina, was due to a deficiency of magnesium, many commercial crops in widely scattered localities have been found to require a magnesium supplement to the regular fertilizer. Examples are vegetable crops in the Atlantic Coastal Plain, potatoes in Maine, apples in New York and Massachusetts, cotton in South Carolina, sugar beets in Michigan, and citrus and tung trees in Florida. As more areas are surveyed, other places may be found to lack magnesium.

For many years more than 300,000 tons of dolomite, a common magnesium-containing limestone, has been used as a neutralizing agent in fertilizers, and has helped to supply the needed magnesium in many areas. But under certain conditions dolomite is ineffective in correcting a deficiency of magnesium, and, consequently, attention is now being directed toward the use of the more soluble forms of magnesium-carrying fertilizers, such as sulfate of potash magnesia and Epsom salts. Also several new forms of magnesium oxide are now available. With the expanded need for the use of magnesium fertilizers and the different compounds now available, further work is required to test the economic effectiveness of the various magnesium fertilizers. with different crops on various soils.

Our experience with tung trees, like the experience of other investigators with apples and other crops, shows the difficulties involved in controlling severe cases of magnesium deficiency with soil applications of magnesium fertilizers. It took 8 pounds per tree of Epsom salts over a 2-year period on 8- to 10-year-old tung trees to correct severe magnesium deficiency. Apparently magnesium deficiency affects the root system to such an extent that it is difficult for applied magnesium to be absorbed. Recent work with apples in Massachusetts, New York, and Maine shows that spraying with approximately 20 pounds of magnesium sulfate in 100 gallons of water corrects the deficiency.

The influence of other elements, particularly calcium and potassium, that are in the soil or in fertilizers on the absorption of magnesium is profound. We have much to learn about the proper balance among calcium, potassium, and magnesium in determining the exact proportions of all the elements for the best plant growth.

Copper and Zinc

Among the minor elements now recognized as essential, copper is present in the plant in the lowest amounts. This small but vitally important amount of copper required by the plant points to its function as a catalytic or enzymatic agent in plant metabolism. Recent work has shown that ascorbic acid oxidase and other oxidative enzymes are copper proteins. The existence of a copper-nitrogen balance has been noted in citrus and demonstrated with tung trees, and this effect on nitrogen metabolism has an important bearing on nitrogen fertilization in copper-deficient areas. That may possibly explain the particular need for copper fertilization on some organic soils. The absorption of copper from the soil is no doubt influenced by the presence of other minor elements like manganese, iron, and zinc, and these relationships are now under study.

Because of the many factors affecting the availability of copper sulfate applied to the soil and the narrow tolerance range of the plant for copper, spraying the plants with dilute solutions of copper sulfate and lime has been found to be effective in correcting copper deficiency.

It is estimated that in 1945 about 12,000 tons of copper sulfate were used as a soil application, whereas 10 years ago hardly any was used. As with the other minor elements, that amount will no doubt be increased in the coming years as more copper-deficient areas become known.

Though a limited amount of work has been done in the last few years on the function of zinc in plant metabolism, steady progress has been made over the country in correcting nutritional diseases due to zinc deficiency, such as white bud of corn, rosette in pecan and apple trees, little leaf of grapes, peaches, and other stone fruits, frenching of citrus, and bronzing of tung trees. In the past 10 years the use of zinc sulfate in correcting these disorders has increased from a few tons to about 3,000 tons annually, most of this being used in the Southern and the Pacific Coast States. A steady increase in the coming years is expected. Probably the bulk of this material is used for spray applications. The influence of other elements, fixation by the soil, leaching effects, and other factors complicate the efficient use of soil applications of zinc sulfate.

Though a somewhat larger amount of zinc than copper is present in plant material, it is the consensus among recent investigators that zinc, like copper, functions as a catalytic or enzymatic agent.