The Use of Minor Elements

by MATTHEW DROSDOFF

SOIL SCIENTISTS use names like "minor elements," "trace elements," and "secondary elements" for boron, copper, zinc, manganese, and iron. The substances are secondary in name and quantity only; they are highly essential to plant growth although they are generally needed in very small amounts. Many a story is told of how a minute bit of iron or copper or zinc that has washed off a bucket carelessly left outside has saved a tree.

Scientists began investigating the practical problems of plant nutrition more than a hundred years ago, but only in the past two decades has proper attention been paid the minor elements. Now, in the past few years, their use in fertilizer practice has rapidly expanded, and all but a few States have reported soils deficient in one or more of the minor elements to the extent that without their use in fertilizers plants of good quality cannot be grown on these soils.

Considerable study is in progress on the need of other elements, especially molybdenum, and before another decade passes some may have to receive consideration in crop production. Other investigations have been directed toward the practical aspects of the problem, rather than as at first determining the need of plants for the minor elements.

Several important recent developments have helped considerably to speed progress in the application of the research. One is the symptom diagnosis, that is, the recognition of certain abnormalities of the foliage, fruit, or other plant parts as being associated with certain minor-element deficiencies. Another development is the widespread use of foliar diagnosis, or chemical analysis of leaves as a guide to the mineral-element nutrition of the plant. If a particular element is not supplied by the soil in quantities sufficient for normal plant growth or the elements are out of balance, the condition can be detected before any plant symptoms develop by a chemical analysis of the leaves.

A third factor is the increasing awareness that such mixed materials as soils cannot always supply enough minor elements for normal plant growth. As might be expected, the highly leached sands of the Coastal Plain, especially in Florida, were among the first to show deficiencies of the minor elements under field conditions. Somewhat more unexpected was the discovery that the organic soils of the Florida Everglades required the addition of such elements as copper for the satisfactory production of vegetables.

These developments led to the investigation and ultimate discovery of minor-element needs on similar soils elsewhere. Meanwhile, other types of soils were found that required one or more of the elements for the successful production of certain crops. For example, some soils were so high in lime that the minor elements present were relatively unavailable to certain crop plants. Much evidence has been accumulated on the harmful effects of over-liming in limiting the availability of certain of the elements. Some soils that formerly had an adequate supply of minor elements have become so depleted by heavy cropping through the years that it has become necessary to replenish the supply of these elements through fertilization.

Scientists have found that certain minor elements may be present in amounts sufficient to produce a good crop but not enough for a high-quality product. A notable example is the external or internal cork of apples due to boron deficiency. Although certain elements like cobalt and iodine have not been shown to be necessary for normal plant growth, they are needed by animals and humans. The quality of a feed crop, therefore, is dependent on the presence in adequate amounts of these elements. An instance of great economic importance is the salt sickness of some Florida cattle that feed on forage low in cobalt.

The trend away from organic fertilizers and manures and toward the use of more concentrated materials has accentuated the need for supplemental elements. Manure and compost usually contain a balanced amount of minor elements, and farmers who still use large amounts of those materials are less likely to encounter deficiencies of minor elements.

Many of the practical problems concerned with the use of minor elements are in a sense parts of the one major problem, that is, how to supply the elements to the plant in the most efficient and economical manner. This involves the fundamental consideration of the requirements of the plant, the nature of the soil and the soil-fertilizer interaction, and the relationships between the different elements. Though a number of crops on many soils require supplements of the minor elements, many do not. Only certain soils and often only specific crops on these soils require minor-element fertilizers.

The importance for optimum plant growth of a proper balance among the minor elements and between the minor and major elements is now generally recognized. Though much information has been obtained in recent years, the need for more knowledge of this subject represents one of the great challenges to future agricultural research.

Following are some pertinent facts about the most important minor elements. Although magnesium and sulfur are needed by plants in fairly large quantities, they are included here because it is only in recent years that their importance in fertilizers has been recognized.