Soils — Their Effects on Nutritional Values of Foods
WHEN NATURE PUTS different species of plants into different places so that grasses grow luxuriantly in some areas, and trees, shrubs and brush in others, she is demonstrating the different plant-nourishing qualities that exist in the soils. Nature produced forests, built primarily of cellulose (woody fibers) with little value as food for warm blooded animals, on the well-watered calcium-deficient soils of the eastern United States. She produced tall grasses on the younger soil of the eastern prairies where the rainfall was moderate; she grew short grasses on the still younger semiarid soils of the western plains.
The American bison, a beast noted for its massive bony structure and great strength, was the animal product characteristic of the plains area soils on which, a generation or so later, wheat was grown that was high in protein and in the mineral elements required to support the manufacture of protein by plants. Protein is an essential food substance, basic in the functioning elements of all animal bodies. The starch also present in the grasses represents only stored energy, and it plays a part in the building of excessive and abnormal fatty layers in domestic animals, fat that is bad for the pocketbook and for the health of the consumer who buys the animal as meat.
Organized knowledge about the locations and relationships of plants, animals, and human beings and their interactions with their environments is the basic outline of the expanding biological science called ecology. Among the chief environmental factors is the pattern of the fertility of the soils which fundamentally maintain all life.
Nature’s array of various interrelated life forms has been telling us for countless generations that the soil of each region is best suited to supplying the needs of the specific plant and animal life which thrives in that region. Each of nature’s crops (plant or animal) represents a survival of the fittest, determined basically, if there are no disturbing factors, by what the local soil provides in nutrients. This fact was emphasized by Darwin a century ago.
The wisdom of the animal body
Animals search out their feeds for quality in terms of health and growth. They test and choose soils according to the nutritive qualities of the plants which grow on them. That there are soil differences which underlie variations in the nutritional values of crops is indicated every time a cow breaks through the fence to get out of her pasture. She is prompted by more deeply seated causes than a mere desire to get to the other side or to escape confinement.
Careful observation of the cow’s misbehavior, as she seeks to eat where she is not supposed to, is very revealing. One finds that usually she is expressing a preference for grass growing on the virgin unharvested soils along the highway or the railroad right-of-way, as compared with the grass growing in the long-used, time-worn soils inside the pasture area. The cow’s wisdom in seeking soil that grows feed of high quality carries an important lesson for men. When men moved their grazing animals such as cows, sheep, and goats to the fenced-in well-watered lands now customary, they failed to realize that animal physiology is such that the animals were not necessarily feeding well and living in good health. There were deficiencies, due to the insufficient fertility of the soil in the enclosed pasture areas, and continuing unplanned use of the land often increased the deficiencies and caused additional ones to appear. In due course, farmers often came to face the need of purchasing protein supplements for animal feeding, even for the abnormal castrated animals that are fed only to be fattened for slaughter.
When it comes to judging the nutritive value of various grasses available for feed, appearances may be deceiving. Thus, in a fenced-in, humid grazing field where the soil has been largely depleted, lush green grasses may grow in certain spots fertilized by a cow’s droppings, and it might seem to a man that these grasses ought to be desirable and especially nutritious to the cow. But the beast, in its natural wisdom, knows better, and avoids the greener grasses which are not really as nutritious as they seem. (See Figure 1.)
Fig. 1. Although the hummocks of grass in this field appeared lush and green, the cow which fed here instinctively preferred the surrounding forage which, as explained in the text, was more nutritious despite its less succulent appearance. (Photo by G. E. Smith, Department of Soils, University of Missouri.)
In current agricultural practice, there is often an attempt to grow dark green grasses of the sort that the cow does not prefer by fertilizing humid soils with chemical nitrogen. We labor under the delusion that a higher concentrations of nitrogen in the fertilized vegetation is proof that the grass must contain higher concentrations of protein, but unfortunately, protein is not fully measured by nitrogen content, which may be but a crude and unreliable chemical symbol of the nutritive quality of feeds. Those who contend that the crude protein in the grass grown by such fertilization with nitrogen duplicates the nutritional quality of the protein which is grown by legumes fail to manifest the wisdom exhibited by the cow.
On the semi-humid soils of the Plains and Western Prairies, the cow’s droppings as fertilizers do not exhibit the disturbing effects that have been mentioned on the growth of the grasses, nor on the cow’s choice of food, and no dark green spots in the pasture are in evidence. On the dryer soils, under only low to moderate rainfalls to change the character of the soil, little organic fertility has been leached away. Plenty of plant-nutritive material is left to balance the added nitrogen. The crops actually growing, on account of the scant rainfall, have not acted to remove important elements of the fertility supply.
Soil Quality, Not Plant Pedigree Alone, Determines the Nutrient Balance in a Crop
Those who contend that a given plant does as well on any soil on which it grows as it would on another soil except for the volume or weight of crop produced, have a wrong understanding of the nature of plant inheritance. In one experiment, potassium (potash), recognized as an essential factor for the plant’s production of carbohydrate and needed often for increased yield of plant material, was added in various amounts to all but one of four plots of soil which were otherwise alike. Seeds from the same mother plant were used to give a constant genetic background. Of three crops, the first was not given legume bacteria for nodule production, while the seeds for the other two were treated with bacteria which established prominent root nodules.
In terms of crop yields, the first crop was a bountiful one, varying in quantity with the amount of applied potash. Nevertheless, in terms of total nitrogen, it gave back no more in the total crop (both tops and roots) than what was in the seed originally planted.
The second crop took nitrogen from the atmosphere to increase nitrogen in the plants over the total in the planted seed. However, the third crop did not give more nitrogen than was present in its seeds because removal of the tops and roots of the plants had depleted the soil to such an extent that even with the root nodules, the third crop was not able to utilize atmospheric nitrogen. The third crop gave larger yields of dry matter (plant material with moisture removed) than the second. Although the seeds for the second and third crops were identical in every way, nutritional properties of the crops were quite different because of differing soil characteristics.
In the same experiment, measurements were also made of the amounts of sugar and starch in the various crops. The results (see Figure 2) show that within each group of identical and identically treated seeds, characteristics of the soil had an important effect on the proportions of sugar and starch in the crops. (In the diagram, “B” and “C” indicate the seeds that were inoculated with nodule-forming bacteria; the “A” seeds were not so treated.)
These observations show the serious error of those who argue that soil is of no importance in determining the nutritional value of the crops grown on it. It is the soil which determines nutritional value. The genetic factor present in the seed and transmitted to the growing plant merely sets limits to the range of variations the soil factors can produce. To argue, as have government officials, that the soil, however deficient in natural fertility, is of no significance in determining the quality of the crop is a confession of misinformation, not of scientific knowledge.
Animals, Too, are Greatly Affected by Soils
Experimental work with rabbits has shown that the body growth of the animals and their body physiology, even the maintenance of sexual vigor, varied according to differences in the fertility of the soil on which crops for their feed were grown.
Tests were made with hay grown on soil with no nitrogen, and other hay grown on companion lots, otherwise the same, which were treated with various sized doses of commercial fertilizer nitrogen.
In repeated trials with lots of five rabbits (two males and three females), the animals were offered for choice equal amounts of four different hays along with constant quantities of corn. The rabbits regularly chose to consume the most hay from the check plot which had no nitrogen treatment, and consumed decreasing amounts of the hay crops that had been grown with larger amounts of chemical nitrogen used as fertilizer. Thus the rabbits demonstrated an ability to recognize differences in the nutritive values of the several crops grown on differently fertilized soils. Naturally enough, the animals which chose to eat less hay showed smaller gains in weight than the others. Thus the properties of the soil samples had an indirect but definite effect on the characteristics of the animals fed with the forage grown on them.
The data indicated that the animals were struggling with poor nutrition and body growth and avoided the supposed protein supplement in the form of the artificial high nitrogen content in the hay, which, though it was nitrogen, did not correspond to the high quality protein which they needed. The unbalanced nutritional quality of the feed obtained by some of the rabbits even produced sexual infertility, which was not present when the animals were on a diet which was known to be complete for growth.
The facts that have been outlined will be observed in nature by those who do not have preconceived ideas about plant growth. The professional agriculturist unfortunately often views the effects of soils on the growth of plants with a distant outlook, as if the only problems were those of industrial manipulation of dead materials, with emphasis on the various technologies for economic advantages only. People who approach agricultural research in this way have lost sight of agriculture as a biological demonstration by the forces of nature, where man is more a spectator than a manager in complete control of soil and produce.
Such unrealistic views of agriculture have led to the expressions by high governmental officials of the view that soil is but a chemical and physical agent for the production of larger quantities of crops; they seem unaware that the soil of our planet is a complex material developed through many centuries, having the power of creation, not only for plants, but for everything that lives, moves and has its being upon the earth.
Fig. 2. The three groups of bar graphs show the effects of different soil treatments on three successive crops of soy beans. The height of each bar indicates the total yield of dry matter from a given quantity of seed. The solid section at the bottom of each bar represents the amount of sugar in the crop as determined by analysis, and the cross-lined section represents the amount of starch. Within each group the seeds used were identical, and the soils used were identical except for increasing amounts of added potassium proportional to the numbers, 0, 1, 2, 3, shown at the bottoms of the bars in each group. Note that differences in soil treatment affect not only the total yields but also the relative ratios of sugar and starch to each other and to the total yields. This demonstrates clearly that characteristics of the soil do have a marked effect in determining the nutritional qualities of the foods grown on it.