Diversity of Amino Acids in Legumes According to the Soil Fertility
AGRICULTURE IS CONCERNED with the synthesis of food. Our ultimate goal in this industry has always been the increase of production, i.e. greater numbers and more pounds, per acre. Too often only such physical attributes of the products—even of people—are of prime consideration when some other criteria are of more fundamental importance. We neglect the quality of our food products and continue to measure our output only in bushels and tons per acre.
In accordance with the long-held belief that a specific crop is of value because it produces much bulk, we have imported many exotic plants in the hope of maintaining a high level of food production. While watching the delivery of bulk, we have kept up the synthesis of caloric compounds by plants, but much of their capacity to synthesize proteins has been lost. For these latter, or body-building, substances, more than good weather is necessary; plants, like animals, can be said to be, and to behave, only according as they are nourished via the soil.
When the soil fertility declines, our attempts to adapt crops to this lower level of plant nutrition become a fallacy in terms of the demands of the animal diet. Of the many requirements of any diet, protein presents itself for first consideration. In the production of healthy animals the major problem is this one of obtaining sufficient protein of the quality commensurate with nutritional demands. Just as the furnace must be constructed prior to its service in consuming fuel, so must the animal use proteins to build its body prior to any consideration of its expenditure of energy. In the animal the mere hanging on of fat is much of a luxury performance to which we have all want only subscribed. In agriculture we must become concerned with the biosynthesis of the building stones of the body, namely, the amino acids, making up the proteins and not be content to adopt as our criterion the photosynthesis of the carbohydrates composing the plant bulk.
While this plant bulk may reflect other factors of the environment, we have been able to trace many of our nutritional problems to the effects of the ash constituents coming via the plant. These soil-borne nutrients control plant metabolism more than we yet appreciate. Biosynthesis requires these inorganic elements, not only to catalyze various reactions within the plant, but also to fashion and to build its structure. In turn, animals depend on the plants to synthesize the protein constituents for them. Herein lies the vital function of the soil. According as the different soils deliver divergent quantities of the inorganic elements, so we experience the pattern in the ecological array of the plant species. Each species represents a different organic composition according to the differences in the soil fertility.
In some recent studies, lespedeza was grown on five outlying experiment fields with five different soil types representing the five major soil regions of Missouri. The protein quality of this crop in terms of the different amino acids was assayed by using the newer microbiological techniques. The diversity in the plants’ contents of these constituents of the protein molecule manifests itself in going from one soil to another, as shown in Table 1. Here, in terms of the quality of the protein produced through biosynthesis by the plant, we have a more significant yardstick by which to measure our agricultural production according to the different soils, to say nothing of the different products themselves.
In order to determine what fertility elements might be the cause of these diversities, alfalfa was grown on a single soil given treatments of the separate trace elements, manganese and boron, and a mixture of these with some others, as supplements to the common fertilizer elements calcium, phosphorus, and potassium. Wide diversity in the amino acid array in the protein could scarcely be expected when relatively small amounts of these trace elements are applied on the surface of the soil. Yet the quality of the alfalfa protein in terms of its constituent amino acids was modified by these soil treatments, as shown in Table 2. While a marked diversity manifested itself in the case of each amino acid, the methionine content varied most widely of all the amino acids measured in this study. Seemingly these results substantiate the hypothesis that these two trace elements namely, manganese and boron, function in the conversion of the carbohydrate into protein.
The data in these two tables illustrate well the wide variations in concentrations of these amino acids because of (a) differences in the crops and (b) differences in the fertility of the soils. Since the need to grow protein is greater than that of growing carbohydrates, both for man and animals, here is the suggestion that we should use a more critical measure of our agricultural production—the quality of it according to the fertility of the soil.
A more critical examination of the final crop products is needed. We need to measure not only their physical attributes as bushels and tons but also the amount and quality of their protein, thereby giving fuller consideration to the fertility of the soils on which the products were grown. The diversity of the amino acids within these crops demonstrates clearly that the fertility level of the soil determines our agricultural production in terms of the protein output, which is much more significant than its commonly considered control in terms of only bushels and tonnages. When the national food problem is now looming larger, we believe it is high time to adopt this newer criterion by which to view and direct the creative business that is agriculture.
