The various factors affecting the mineral content of a mixed hay.
Better Soils, Better Food
by KENNETH C. BEESONNUTRITION begins with the soil. The truth of that state- ment has been investigated only recently, but it has long been a factor in the thinking of men. Charles A. Browne in his recent book, 4 Source Book of Agricultural Chemistry, has organized in an interesting way the evolution of the concept from the time of Democritus (about 360 B. C.) to Liebig (1803-73).
Democritus, for example, suggested a cycle of indestructible elements from the soil to the plant and to the animal and back again to the soil. Aristotle (384-322 B. C.) taught that plants absorbed through their roots the necessary material for their growth. In turn, bones, blood, milk, hair, and other parts of animals were derived from constituents of the plants. But long before these speculative ideas began to take form in the minds of men, the practical farmer had observed that his animals responded differently to forages from different localities. Through experience he had learned not only what localities were best suited to his needs but he had learned that some were definitely undesirable.
The nutritional problems of animals in relation to soil conditions, however, did not become acute nor were they the subject of scientific inquiry until nearly the middle of the nineteenth century, when their importance was emphasized as the increase in world population was pushing man out into new lands. Many of the new lands were found to be incapable of supporting animal life, particularly when the animals were not permitted to graze large areas and select their food. The need for new lands has continued to create problems of this nature that must be solved. There are still vast areas in the world, and in the United States in particular, that will be suitable for many agricultural purposes when it is learned how to correct their natural shortcomings.
Troubles in grazing animals, related to deficiencies or to toxic quantities of mineral elements in soils, have been recognized in the United States for more than 50 years. Many of the ailments have been investigated extensively. A deficiency of phosphorus in soils and forage was among the first to be recognized and described.
More recently an appreciation of the role of the trace elements or micronutrient elements, such as cobalt and copper, has developed. They occur in soils in minute quantities. An acre of topsoil, 1 foot deep, for example, ordinarily contains less than 20 pounds of any micronutrient element. A ton of hay normally contains about 2 grains, or one four-thousandth of an ounce, of cobalt. Where such small quantities are involved, an understanding of their importance was not realized until the refinement of laboratory methods made such studies possible.
It is evident that a deficiency of phosphorus, next to iodine, is the most widespread of any mineral nutritional trouble in grazing animals. Recent work has emphasized that phosphorus may generally be deficient in virgin soils throughout the Atlantic and Gulf Coastal Plains, a fact that becomes apparent as more and more of those undeveloped lands are used for profitable grazing. John Foster, of the North Carolina Agricultural Experiment Station, for example, has stated that the raising of beef cattle is rapidly assuming an important place in the agriculture of the Coastal Plain, where the available forages appear adequate for the protein and energy requirement of the animals, but that limiting factors seem to be deficiencies of phosphorus and possibly some micronutrient elements, such as cobalt.
Deficiencies of phosphorus were observed many years ago in the Coastal Plain regions by investigators in Florida, Alabama, and Texas. The work of W. H. Black, J. M. Jones, and their co-workers in southeastern Texas is one of the more recent contributions to the solution of the problem. According to them, an abundance of vegetation is available in that area. Consequently, there are greater numbers of cattle there per section than in any other range area of the State. The fact that a nutritional deficiency does exist is evident from the small size of the animals, reproduction failures, and malformed bones that are easily broken, and a persistent craving by many cattle for bones, dirt, wood, and other materials.
In Tennessee there is a recognized relationship between the supply of phosphorus in the soil and the health and production of animals and the economic status and well-being of the people. In few parts of the country are the contrasts between good and poor soils more striking. The reason lies in the fact that in Tennessee some of the best agricultural soils of the East are closely associated with much less productive and often actually deficient soils.
Conditions in Minnesota and Montana are probably fairly typical of the Northwest. In Minnesota much has been done in locating deficient areas, and experiments designed to correct the troubles are now under way. The work in Minnesota has emphasized the study of the effect of superphosphate over a large part of the State. Much more work is required to demonstrate the practicability of this on the more extensive range areas farther west.
Since 1937 the importance of cobalt in animal nutrition has been repeatedly demonstrated throughout the world. Several areas in the United States are now believed to have soils deficient in it. One of the most extensive is in the Northeast, where the problem is being studied by men at the New Hampshire Agricultural Experiment Station.
In troubles of this kind it is hard to establish a definite relationship from the soil to the animal, but it has been demonstrated in New Hampshire that ailing animals respond to administrations of cobalt; that the largest proportion of the farms involved occurs on soils developed on materials derived principally from granite, and that there are definite and significant differences in the cobalt content of the forage from good and poor areas. In some cases, however, the cobalt content of poor hay is not acutely low, as measured by the rather uncertain standards that we have.
Troubles in grazing animals due to a lack of cobalt have been known to the scientists in Florida for some time. The situation there (where the first research of this kind in the United States was done) emphasizes that multiple deficiencies may often be encountered. Thus, cobalt and phosphorus, cobalt and copper, or copper and iron have been noted as being simultaneously deficient in forages in those areas. Recognition of this possibility may be helpful in explaining discrepancies in the known facts associated with other trouble areas of the country.
Deficiencies of cobalt have been reported from Michigan and Wisconsin; possible deficiencies have been reported from eastern Pennsylvania, Cape Cod, and northeastern New York. Most of these areas have not yet been studied in sufficient detail to permit generalizations as to the soil, plant, and animal relationships.
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 con- tent of a mixed forage in two ways: Through an increase in the phos- phorus 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 com- position 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 up 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 rclatxonshlp will be discussed in connection with thé 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, sweetclover, 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.
The calcium concentration in plants used for human foods is of more interest than the phosphorus content because of the greater possibility of deficiencies of calcium in the diet. Consequently, increasing the calcium concentration in such crops has been considered by several investigators. For example, the workers in some experiment stations in the Southeast organized a broad cooperative project to study the relation- ships between soils and the composition of certain vegetables, particularly the leafy vegetables used for greens. They noted some interesting effects of calcium and nitrogen fertilizers on the calcium content of turnip greens. For example, very small increases (0.06 percent of calcium) in turnip greens resulted from applications-of gypsum to the soil. On the other hand, nitrogen applied to the soil brought a relatively large decrease in calcium (0.36 percent of calcium). Thus, more than six times as much calcium was lost as a result of fertilization of the crop with nitrogen than was gained through fertilization with gypsum. Also, in the experiments, it was shown that soil type had a greater effect on calcium and phosphorus than did minerals applied as fertilizers.
The application of nitrogenous manures has generally been associated with a reduced calcium concentration in the plant. There is some evidence that ammonium sulfate as a nitrogen carrier may be more effective in this respect than sodium nitrate. Effects of nitrogen on the calcium concentration in mixed forages and pasture herbage may be associated with changes in botanical composition, as well as with changes in the chemical composition of individual species. This is due to the effect of heavy applications of nitrogen in encouraging growth of grasses at the expense of legumes. Potassium or magnesium applied as a fertilizer may also be associated with a reduction of the calcium concentration in plants. The effect of magnesium may be small and is ordinarily of little practical importance except in the case of a plant, such as timothy, that normally has a low level of calcium.
Applications of most of the micronutrient elements to soils even in very small quantities will be followed by an increased absorption of the element by the plant. One exception may be iron, and reported changes in copper concentration in the plant have been rather small. Relatively large increases in cobalt and manganese concentration in plants can be obtained by adding these elements to the soil. In New Zealand and elsewhere, cobalt deficiency is often corrected by the use of cobaltized superphosphate, limestone high in cobalt, or simply crude cobalt salts.
Interest has grown in the effect of certain micronutrient elements like boron, on the absorption of other elements, particularly calcium and nitrogen. It appears from work published by R. Q. Parks, C. B. Lyon, and S. L. Hood of the United States Plant, Soil, and Nutrition Laboratory in Ithaca, N. Y., that boron exerts a profound effect on the absorption of nearly every other plant nutrient. At normal levels of boron supply, an increase in the boron content of the nutrient solution caused an increased concentration of boron, organic nitrogen, magnesium, manganese, calcium, iron, cobalt, and sulfur in tomato plants grown in a greenhouse. The concentration of molybdenum, phosphorus, zinc, copper, and potassium decreased in the same plants. The work has not as yet had practical application, but it is of considerable fundamental importance in studies of ion absorption by plants.
Overliming is a common experience on a soil with a low buffer capacity. The troubles arising from too much limestone are often due to its effect on some of the micronutrient elements like boron and manganese in the soil. Symptoms characteristic of deficiencies of these elements appear in the plant as a result either of chemical reactions that make the compounds of these elements unavailable to plants or through adverse ratios of calcium to these elements in the soil or the plant.
These visual symptoms appear only in cases of acute deficiency. In less serious deficiencies, retarded growth in various degrees may be the only symptoms. Under subacute conditions only the amount of the micronutrient element absorbed by the plant may be limited. It is the latter condition that may be of greatest importance to the nutritionist. We have no clear evidence yet as to the lowest level that elements such as iron, manganese, and copper can reach in the plant before deficiency symptoms appear. It is certain that the content of iron and manganese can vary tremendously and that copper can vary within a limited range before their absence becomes evident. Apparently cobalt can be entirely absent without any visual physiological disorder being produced in the plant.
The addition of liming materials, consequently, is a practice that farmers and technicians should study carefully, particularly in areas that are subject to overliming troubles. Evidence is at hand that liming will reduce materially the manganese content of forage without reducing yields. It has long been known that the iron content can be lowered, and it is reasonable to assume that cobalt would be lowered under some conditions. The evidence with respect to cobalt is still incomplete.
The micronutrient element content of plants (especially manganese and cobalt) may also be lowered through the excessive use of nitrogenous fertilizers. That is to be expected because of the effects of nitrogen in producing more rapid and greater vegetative growth. If such growth occurs in soils containing minimum quantities of an element, such as cobalt, that is not essential to the growth of the plant, a low concentration of the element in the plant will result.
A sound program of fertilization, then, can increase the nutritive value of a mixed forage in two ways—by the introduction of highly nutritious forage plants, such as the legumes and the elimination of less desirable plants, and by an increase in the individual plant of such mineral nutrients as calcium, phosphorus, cobalt, and other of the micronutrient elements.
We can assume that many other constituents, some that are unknown and consequently not subject to laboratory measurements, may be modified by fertilization. These factors, both known and unknown, all contribute to the nutritive quality of forages. Therefore, many workers in an attempt to evaluate the nutritive quality of one forage against another have fed the two forages to animals. The growth response has then been used as an over-all measure of nutritive value.
This over-all measure, which has been termed a bio-assay of soil fertility, is subject, however, to many limitations. A successful or positive result from such an experiment might actually be considered fortuitous.
Two situations that would result in negative conclusions when positive ones would have been justified will be discussed. Many other combinations of circumstances could be considered.
If a forage (assume a pure species) is produced at different fertility levels of the soil, it is assumed on the basis of the foregoing discussion that there may be some measurable difference in their nutritive quality. It may be that these differences are due to constituents unknown with respect at least to their ability to influence animal growth or health, or it may be that some constituent such as protein is the variable. Suppose that a particular fertility level of the soil is associated with a forage containing 20 percent of protein as compared to 15 percent in a forage from a lower fertility level. The two forages are fed to lambs and the growth response is taken as the criterion of the nutritive values of the two forages. But the difference in protein levels would probably not be reflected in a difference in growth of the two groups of lambs, because the optimum protein requirement for this animal is about 10 percent, and both forage samples exceed this. Thus, a negative result would be obtained from the experiment, although there was an actual difference in the composition of the forages.
A second set of circumstances that might give rise to negative results is based on suboptimum, rather than superoptimum, concentrations of a nutritive constituent. Assume that each forage in this case contains 6 percent protein but that there exists a difference in the phosphorus content, one forage containing less than the minimum required for growth of sheep. The difference in phosphorus levels in this case would probably not be detected because of the limiting factor in both forages, the low protein content.
Both these circumstances could be repeated in the relationship between any two or more constituents of a plant that are required in some minimum quantity by animals. It is obvious that the possible combinations resulting in negative conclusions are manifold. The examples illustrate furthermore a basic requirement of such bio-assays, that the nutrients, known or unknown, be fed at suboptimum levels if differences are to be detected and also that nutrients other than those under test be fed in adequate amounts.
Requirements of this kind imply that specific nutrients be under consideration rather than all of them at one time. It is probable that future effort in the field of evaluation of the nutritive quality of plants will be directed largely toward this kind of an approach, which will necessarily be predicated largely on preliminary laboratory examination of the composition of the plant.
E. W. Crampton, D. A. Finlayson, and their co-workers at Macdonald College in Quebec have contributed materially to our knowledge in this sphere through their numerous publications. They conclude that some factor or factors other than quantitative differences in total protein, energy value, fiber, or minerals (calcium and phosphorus) of mixed forages are responsible for observed differences in nutritive value. In the experiments upon which their conclusions are based, however, the fertilized forage contained nearly four times more white clover than the unfertilized forage did. A corresponding reduction in weeds and undesirable grasses occurred in the fertilized forage. Hence it is not possible to differentiate between the effect of fertilizers on the botanical composition of the mixed herbage and on the chemical composition of individual species. In other words, one cannot conclude from these experiments that the nutritive value of individual forage species has been altered by fertilizing the soil.
W. A. Albrecht and his associates at the Missouri Agricultural Experiment Station have also attempted to measure this improvement in the biological factors in forage plants that follow the use of fertilizers, particularly liming materials and superphosphate. Their general approach, like Dr. Crampton’s, has been to use animal growth as an index of the changes in crop quality in response to soil treatment. They assume that the animal should respond to changes in the composition of the forage even though some of the changes cannot be detected even by ordinary chemical analyses.
The greatest danger in work of this kind is the tendency to generalize and oversimplify the situation. For example, in three carefully controlled feeding experiments carried out at the Plant, Soil, and Nutrition Laboratory, S. E. Smith found that in only one was there any indication of an advantage in favor of the fertilized forage.
The work at Missouri has also indicated a variability in results. Thus, in one of the experiments there, the difference in the average weight of sheep that were fed hay from fertilized soils and those fed hay from unfertilized soils was very slight. The daily average gain in the two was 0.1408 and 0.1644 pound per head. Data are not presented to show that this small difference is significant. In fact, allowing for a reasonable variability in the response of the animals, it can be shown that the greater gains could just as well have been made by either group of the sheep if both had been fed identical hays. In view of the additional fact that 50 percent of the hay from unlimed soils and 46 percent of the treated hay was refused by the animals in this experiment, it is manifestly impossible to conclude that the fertilization or liming program had materially altered the nutritive quahty of the forage plant. These instances are cited to show how difficult it is to carry out and interpret these over-all bio-assays of nutritional quality of plants.
It has become obvious lately that soils do not tell the whole story with respect to the nutritive quality of food.
There is the question of sunlight, for instance. Today we know that sunlight is an important factor in the elaboration of vitamins in both plant and animal bodies. The variations in ascorbic acid (vitamin C) in a food, such as the tomato, are amazing. Studies of the causes of such variations have been made by Karl C. Hamner and his associates at the Ithaca laboratory. They concluded that the causes are not so much associated with soil conditions or cultural practices as they are with location. They found that tomatoes of the same variety grown in some parts of the country have three times more ascorbic acid than those grown in another part. Drastic soil treatment failed to modify the vitamin content of tomatoes grown under similar climatic conditions. An explanation for a large part of the variations was found in the amount of light received by the plant. In fact, the effect of variation in the amount of light on a plant is so pronounced that any effect of soil fertility or of fertilizer treatment may be completely obscured. Failure to recognize this point has led to several questionable interpretations in the recent literature of the relationship between fertilization and the vitamin content of foods.
Even the actual amount of sunlight received by the entire plant may not be the deciding factor in determining the vitamin C content of the portion used for food. For example, later and more refined experiments by George F. Somers and Hamner have shown that the vitamin C content of the tomato itself is apparently related to the amount of sunshine that strikes directly on the fruit, for sunshine on the vines apparently has little or no influence on the fruit. Consequently, it is clear that shading of the fruit by an abundant foliage will have an important effect in limiting the vitamin C content. Furthermore, fruit taken from the shady side of the tree or vine or fruit subjected to any other interference from sunlight may be lower in vitamin C than the fruit obtaining direct sunlight. Thus, the total amount of illumination on a field of tomatoes may not necessarily be related to the vitamin content of the fruit from that field because of other factors that are also operating.
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 com- pounds 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 A1, the application of fertilizers should, theoretically, result only in an increase in the yield. Beyond the point A1, 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-B1 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 AUTHORKenneth 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.
ACKNOWLEDGMENT
George F. Somers of the United States Plant, Soil, and Nutrition Laboratory contributed material for the section on soil fertility and the vitamin content of plants,
which was invaluable in the preparation of this article.
FOR FURTHER READING
Browne, Charles A.: A Source Book of Agrcultural Chemistry, Chronica Botanica, volume 8, No. 1, 1944.
Hartman, A. M.: Deficient and Excess Minerals in Forage in the United States, Yearbook of Agriculture, pages 1027-1044, 1939.
McMurtrey, J. E., Jr., and Robinson, W. O.: Neglected Soil Constituents That Affect Plant and Animal Development, Yearbook of Agriculture, pages 807-829, 1938.
Russell, F. C.: Minerals in Pasture—Deficiencies and Excesses in Relation to Animal Health, Imperial Bureau of Animal Nutrition, Technical Communication No. 15 May 1944.
