Correcting Deficiencies in Plants Through Fertilizers

Fertilizers are used mostly to increase yields; their use over the years has proved to be a satisfactory and economically sound practice. We are confronted now with the problem of whether fertilizing our soils to improve the nutritive quality of the crops, as well as the yield, is a matter of practical importance. The problem has two aspects: The modification of the mineral content of the plant and the modification of the organic constituents, including the proteins, carbohydrates, fats, vitamins, hormones, and other ( probably unknown) constituents in the plant. Any attempt to improve the plant with respect to one group of these factors may, of course, modify those in the other group, at least with respect to their relative proportions. For the purpose of this discussion the two groups will be treated separately, although their interrelationships are obvious.

Applications of phosphates to a soil may increase the phosphorus content of a mixed forage in two ways: Through an increase in the phosphorus content of individual plant species, and through an increase in the proportion of grasses and legumes that are normally high in phosphorus in the mixture. The latter way—that is, the change in the botanical composition of the forage—is the one commonly experienced. This principle is, of course, the basis for most of the pasture-improvement programs throughout the country.

Agronomists and others generally assume that an application of fertilizers containing a particular element should result in an increase in the content of that element in the plant. The assumption is probably correct with respect to phosphorus, but there are definite limitations to any generalizations that may be -made. A survey of recent findings on the subject suggests that relatively small increases in the phosphorus content of plants often are obtained particularly where the unfertilized plant contains a quantity of phosphorus well below the level required by animals. Experience indicates that frequently even very large and uneconomical applications of phosphate on some soils will fail to build tip phosphorus in the plant equal to that normally found in plants from other soils.

Unfortunately, a relatively low phosphorus content in plants may often prove to be the most difficult one to increase by programs of fertilization. That is, the increases that have been obtained are often too small to be of much practical importance to the grazing animal. On the other hand, it has been a common experience of investigators that a plant containing more nearly normal or even relatively high quantities of phosphorus may increase its phosphorus content very appreciably following application of phosphates or other amendments to the soil. A further consideration of this relationship will be discussed in connection with the general problem of nutrient absorption.

The question naturally arises as to whether the application of,phosphates to range lands is always profitable. W. H. Black, from his work in eastern Texas, believes that under conditions there it may prove to be so. Under other conditions, phosphates are best supplied directly to the animals as mineral supplements. In South Africa, scientists suggested that the grains as rich as possible in phosphorus could supply the necessary phosphorus to animals that grazed phosphorus-deficient range.

The application of any element, such as nitrogen, to the soil produces so many effects that it is difficult to generalize with respect to its effect on plant composition. The use of nitrogen as a fertilizer has not been associated with any definite trend in the phosphorus content of the plant. Where potassium has been supplied as a fertilizer, there seems to be general agreement that the phosphorus content of the plant is either unchanged or lowered. The differences in phosphorus where there is a change are often small and of little practical importance. The application of lime alone to the soil has likewise had little effect on the phosphorus concentration in plants in most of the experiments reported, although occasionally a reduction of phosphorus is noted. If the limestone is added with superphosphate, however, a greater increase in the phosphorus concentration in the plant has often been obtained than with either material alone. But this trend is not consistent; sometimes no change in phosphorus is reported, even from very carefully controlled experiments.

Deficiencies of calcium have seldom been reported in cattle and sheep. A study of available data indicates that most legumes will always supply sufficient calcium for animals. L. A. Maynard, director of the Cornell School of Nutrition in Ithaca, has stated, however, that, "Generalizations regarding the adequacy of grass hay in calcium content for animal nutrition are unsafe because of the very large variations that can occur." It is, therefore, of interest to examine the effects of fertilization on the concentration of calcium in plants.

It is recognized that liming acid soils tends to encourage the growth of forage plants relatively high in calcium in both pastures and hay crops. The growth of many of the legumes, such as white clover, soybean, sweet-clover, alfalfa, and red clover, is favored markedly by limestone, and they are able to displace the nutritionally less desirable grasses and other plants. Since those legumes are higher in calcium than the species in the original pasture or hay mixture, the calcium concentration in the forage as a whole will be increased without regard to the composition of the individual species. There is, in fact, very little evidence that any fertility program will result in an important increase in the calcium concentration of individual forages. The addition of lime to certain soils has increased the calcium concentration of some plants under certain conditions, but the changes are not always of great practical importance. This is in marked contrast to the large differences that have been found in the calcium content of the same species of plants grown in different localities.