Nutrient-Element Balance

by C. B. SHEAR and H. L. CRANE

BALANCE is a fundamental part of nature's law. But man's activities have a way of upsetting nature's balance and usually it takes a serious unbalance to make us see the situation and try to correct it.

Our increased concern over our own diets exemplifies an unbalance. The variety of foods that comprise the average normal diet should provide enough of the essential minerals and vitamins to meet our normal requirements. But not so. Many of the soils on which food crops are grown do not supply the plants with sufficient minerals to meet our needs or to enable the plants to synthesize vitamins in quantities to meet our demands. Further, and worse, we are not satisfied to use many plant products, particularly the cereals, in the form in which nature gives them to us, but demand that they be processed and "purified." These purifying processes remove some of the nutrients, and unless we know which and how much of the nutrients are removed, and supply them in our diets from other sources, we do not get enough of them.

Much the same situation exists with respect to the nutrition of our crops. The soils upon which plants depend for their food materials developed from minerals, which originally contained a fairly well-balanced supply of the essential mineral nutrients for the plants that became native to each soil. Plants with different nutrient requirements developed on soils having different powers to supply nutrients. Many soils, in their virgin state, do not furnish a balanced nutrient supply for agricultural crops. Man, however, has not restricted his cultivation to the soils best suited to the crops he wants to grow, nor has he been careful to protect the soil from erosion and leaching. Furthermore, he has continued to crop the soil year after year and has failed to return to the soil all of the nutrients removed in the crops produced. The organic matter of the soil, one function of which is to act as a reservoir of slowly available nutrients, has also been allowed to be depleted.

When America was a predominantly agricultural country, a high proportion of the products of the land was returned to the soil in the form of manure, thus helping to maintain the original soil fertility. As the country became more industrialized and the farm mechanized, less and less of the material produced on the land was returned to the soil. The farmer came to depend more and more on chemical fertilizers to maintain crop production.

The so-called complete chemical fertilizers that he used generally contained only three "plant food" elements, nitrogen, phosphorus, and potassium. Even they were not supplied in the quantities or proportions in which they were removed in the crops. In the early days of commercial fertilizers, the materials used in making them were of low grade, and contained substantial quantities of other elements of value in crop production. When fertilizers of high analysis were developed, it became necessary to use purified materials in compounding them, with the result that they contain few or no impurities. Calcium, in the form of lime, was applied in some localities as a means of correcting soil acidity, but it was not generally considered a plant nutrient, but rather a soil amendment.

The other chemical elements that the plant physiologist has determined in the laboratory to be essential for normal plant growth were thought to be required in such small quantities that they would never be of practical concern to the farmer. It has been only within the last two decades that deficiencies of such elements as magnesium, manganese, zinc, boron, and copper have been observed and diagnosed on plants growing on our principal agricultural soils, even by experienced horticulturists and agronomists. Only more recently have we recognized that the prevailing practice of returning to the soil only nitrogen, phosphorus, potassium, sulfur, and sometimes calcium was hastening the depletion of the other plant nutrients through increased crop production.

The explanation of this depleting effect of the so-called "complete fertilization" now seems quite simple. Continued application of only part of the necessary plant food materials maintained crop yields at the level that the available supply of the other nutrients in the soil would permit. As each succeeding crop was removed from the land, the supply of these other essential nutrients became less and less. The effect of their diminishing supply was reflected in reduced plant growth and yields long before their effect was evident in definite symptoms of malnutrition. It was not until these advanced symptoms appeared and their causes determined that the failure of the "complete fertilizer" to maintain the original producing power of the soil was recognized. Man, by his lack of knowledge regarding nature's complex balance, had now brought about a condition that made it necessary for him to determine the balance required for his crops and find means of obtaining and maintaining that balance. This was no simple task.

The first method of attacking the problem was to try to determine the quantities of the many essential elements in the soil. The method produced valuable information about the chemical composition of the soil, but it soon became evident that the amounts of the elements extracted from the soil by the various solvents employed in. the laboratory did not necessarily represent the amounts of those elements available to a plant growing in that soil. Evidently the plant had its own way of obtaining its nutrients from the soil.

Perhaps the answer lay in "asking" the plant what it needed and what it was able to get from the soil. The only means of "asking" the plant was through chemical analysis of the plant, or some part of it.