Samuel Merrill, Jr., formerly of the United States Laboratory for Tung Investigations, Bogalusa, La., and S. R. Greer, of the Mississippi Agricultural Experiment Station, Poplarville, Miss., have obtained leaf analyses and growth measurements on tung trees that show- that, when the concentration of all the essential elements with the exception of nitrogen and phosphorus are high, it is necessary to increase the accumulation of both elements in order to increase growth. The application of either of these elements alone did not result in increased growth, even though the leaf content of the applied element increased.

The necessity for maintaining a proper balance among the elements in the leaf is not restricted to the five elements so far discussed. The so-called minor elements iron, manganese, zinc, copper, and boron must be accumulated in the proper proportions to one another and to the other elements if satisfactory crop production is to be maintained. Also, just as is the case with the other nutrient elements, the accumulation of the minor element in the leaves is not dependent alone on their available concentration, but is influenced by the available concentrations of all of the other elements.

Experiments with the tung tree have shown that the appearance of symptoms associated with manganese deficiency may result from a high accumulation of any one of the three major bases, and that these symptoms are the most severe when all three of the bases are high in Proportion to manganese. It has also been demonstrated that the three major bases exert the same influence on iron, zinc, copper, and boron. Thus, conditions promoting increased accumulations of calcium, magnesium, and potassium work in two ways to bring about deficiencies of one or more of the minor elements: First, by depressing their accumulation, and second, by increasing the concentrations required in the leaf to create a proper balance with the other elements.

Because the accumulation of each of the minor elements is affected to a different degree by the accumulation of each of the major bases, an unbalanced accumulation of potassium, calcium, and magnesium may also result in an unbalanced relationship between two or more of the minor elements. In experiments with tung trees, for example, too low a level of potassium in proportion to calcium and magnesium has resulted in an unfavorably high ratio of manganese to copper in the leaves. Other examples of similar phenomena have been observed on a number of different crop plants. For instance, boron toxicity may occur when the level of one or more of the three major bases is unduly low, yet at high levels of the bases the plant may become boron deficient. This deficiency is not the result of depressing the boron content of the leaves below a critical level, but is caused by an unfavorable balance between boron and one or more of the bases.

It has been long known, and demonstrated on numerous crop plants, that an excess of iron in proportion to manganese may induce manganese deficiency and that an excess of manganese in proportion to iron may induce iron deficiency.

S. G. Gilbert and M. Drosdoff, of the United States Laboratory for Tung Investigations, Gainesville, Fla., and H. M. Sell, of the Michigan Agricultural Experiment Station, have demonstrated that increasing the nitrogen content of tung leaves already low in copper will result in severe copper-deficiency symptoms. This was found to be true even though the copper content of the leaves in the fall when the copper-deficiency symptoms were most severe was higher than that of the leaves early in the growing season when the copper deficiency was much less pronounced.

Only a few of the more important relationships between the essential elements have been discussed here. Many other relationships and other instances of decreased growth or other symptoms of malnutrition resulting from improper nutritional balance within the plant have been demonstrated on a number of crop plants. The relationships between the elements and the examples of unbalance which have been given should indicate to the reader that the problem of plant nutrition is by no means a simple one. It should also be evident that the use of deficiency symptoms to diagnose nutritional difficulties is like waiting until the horse is stolen before locking the stable. In order to reap the maximum returns from the land our crop plants must be maintained in a well-balanced nutritional condition. This cannot be accomplished through hit-or-miss fertilization but must result from the integration and application of all of our available knowledge concerning the nutrient requirements of each crop, the nutrient-supplying power of our soils, and the effects of the rate of supply of each element on the accumulation and function of all the other elements in the plant.

It is unfortunate that our present knowledge in these respects is too inadequate for most crops to permit the determination of their fertilizer requirements by means of leaf analysis. It is hoped that such knowledge will be accumulated in the near future. When such knowledge is available, it should be possible for properly trained soil scientists and plant physiologists to make accurate fertilizer recommendations on the basis of complete leaf analyses.

THE AUTHORS

C. B. Shear, a physiologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering, has been working on the nutritional requirements of the tung tree since 1939. He has made special studies of the nutrient-element balance and the determination of the nutrient requirements of plants through leaf analysis. He is a graduate of the University of Maryland.

H. L. Crane, horticulturist, Bureau of Plant Industry, Soils, and Agricultural Engineering, has been in charge of the Bureau's investigations on nut crops since 1935. He is largely responsible for the- research required for the successful development of the pecan and tung industries. In 1946 he visited China as tung specialist on the China-United States Agricultural Mission. Dr. Crane is a graduate of Cornell.