Important Problems for Future Study

Since the amount of sunshine, and not soil fertility, seems to be so important a- factor in determining the vitamin content of plants, one may be led to conclude that there is little that can be done to bring about an increase of these important nutritional factors. Experimental work carried out at many stations, both State and Federal, shows that there is at least one other way open for improvement, e. g., through plant breeding. For example, there are wide, and fairly consistent, differences in the vitamin C content of various tomato varieties grown commercially at the present time. Some varieties produce fruits that contain much less vitamin C than others. Even the best varieties in use at the present time do not contain the maximum amount of vitamin C that should be obtainable through breeding. Thus, some species of tomatoes, not marketable because their fruits are too small, contain 3 or 4 times as much vitamin C as the existing commercial varieties. By suitable breeding programs, it appears possible to incorporate the tendency to be rich in vitamin C into tomatoes which are commercially acceptable. The results promise to be of much greater importance than could be obtained through any reasonable modification of the mineral supply of the soil.

It is apparent that the studies of the effect of fertilization and soils on the nutritional quality of plants have not yet produced sufficient data obtained under a variety of conditions to permit many conclusions to be drawn. Thus, most of the recognized factors, such as soils, climate, and plant species that modify the influence of fertilizers have been studied in little detail.

The diagram (top page 494) summarizes some factors that would affect the mineral composition of a mixed hay—climate, the properties of soils, and the character of the plants. It is evident that within any one group one must deal with a highly complex and relatively little understood. system. The water relationships in soils, for example, are known to affect profoundly the mineral supply to the plant, but the magnitude and nature of these effects are probably greatly modified by the other factors, such as the physical character of the soil and the nature of the mineral compounds present. The chart is intended to stress both the direct and indirect effects of climate on the mineral composition of the plant. The direct effect is assumed to be much greater than the indirect effect that reaches the plant through the soil. In the same way the plant has an important effect on the nature of the soil, and this effect will be reflected again in the plant.

Several scientists have observed that a plant may differ to a greater extent in relation to the soil and environment in which it is growing than in relation to any fertilizer treatment on each soil. When the cause of the striking variability of the composition of plants grown in different soils is understood, an important step toward better fertilization programs will have been taken. When one considers the complexity of the biological system under which we produce our food crops, he will recognize as natural the variable results obtained by the superimposition of a fertilizer on a set of soil conditions. The problem is one that requires detailed study of each of these factors and its effect on the others as well as on determinations of the over-all or net effects as found in each soil as a unit.

Paul Macy, during work at Cornell University, has discussed in an instructive way the relationship between the sufficiency of a nutrient and its percentage content in the plant. His concept can be employed to support the observation, for example, that fertilization often does not change materially the mineral content of a plant. This is presented in a chart. When the yield under any set of conditions lies between A and Al, the application of fertilizers should, theoretically, result only in an increase in the yield. Beyond the point A', the addition of fertilizers should result in both an increase in yield and an increase in the concentration of the mineral elements of the fertilizer in the plant. At the point E no additional yield is obtained as a result of fertilization, but the increase in the concentration of minerals in the plant continues. It is recognized, of course, that on a different soil another level such as B—B' will prevail. Likewise, it is conceivable that other relationships as C—F or even D—G would be found to hold for some soils. The important point is the recognition that any response to fertilization will be a function of the soil characteristics as well as the fertilizer itself.

This concept is of great importance to the animal nutritionist, although it was first considered with respect to problems in plant physiology. It means that the problem of yields is closely related to composition. For example, it is often borne out by experience that the phosphorus concentration of a plant will remain at a low level until growth requirements for the phosphorus are met.

It is evident, of course, that the limiting effects of other fertilizer elements than the one under consideration must also be taken into account. The effect of a deficiency of potassium associated with a high phosphorus content of the plant is an example. The potassium deficiency may limit the growth of the plant causing it to enter the region of "luxury consumption" of phosphorus (E—G, in the figure). A clarification of these factors is of utmost importance in solving the problem of improving the nutritional quality of plants grown for food.

More information is needed concerning the use of fertilizers in widening the choice of crops to be grown. Even if no practical means of altering the composition of a particular plant is found, it is of great practical importance to be able to improve the over-all nutritive quality of a forage crop through introduction of different plant species.

Such a solution may be practical in many of our cobalt-deficient regions. Thus, investigations carried on in the Plant, Soil, and Nutrition Laboratory have shown that different grasses vary greatly in their ability to absorb cobalt from the soil. Likewise, the legumes seem to have a much higher content of cobalt, even in deficient regions, than do the grasses or other forage plants such as reeds.

A knowledge of the relationship between the fertility level of the soil and the character of the botanical population will result in two very important practical things: It will indicate to us the advantage, nutritionally, or fertilizing and liming in order to grow crops that are naturally higher in nutritive factors; we will be able more readily to indicate the characteristics of soil types as they influence the nutritive adequacy of plants.

The need for standardizing the nutritive value of plants in terms of animal health is obvious. Certainly any limited number of laboratory determinations is inadequate with our present fund of information. An adequate evaluation of known factors simply cannot be made by laboratory methods. In undertaking planned animal experiments, however, it must be realized that animal performance is subject to as many variables as is soil fertility. Furthermore, there are differences in the nutritive value of rations which are not measurable in terms of growth performance but which require refined physical, biochemical, and histological techniques. There is before us a real opportunity to contribute to human welfare through the combined experience in the field, the laboratory, and with the experimental animal.

THE AUTHOR

Kenneth C. Beeson, senior chemist at the United States Plant, Soil, and Nutrition Laboratory of the Agricultural Research Administration, Ithaca, N. Y., has been with the Department since 1930. He has published papers on fertilizer technology, particularly on the problem of chemical reactions occurring in mixed fertilizers, and is now undertaking studies of the relationship of soils to the occurrence of nutritional troubles in animals. Mr. Beeson is a graduate of the University of Iowa.