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The Soil Nitrogen Supply Is Still a Big Problem
Published in Victory Farm Forum, October 1946
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Among the many soil problems that have come on the stage to hold public attention, perhaps that of maintaining its nitrogen has been the one actor most persistently before us. Even though soil erosion has held such a prominent place in our general public as well as our agricultural thought during the last decade, that fact does not mean that the decreasing nitrogen in the soil has not also been playing for attention right along with it. Our mental gaze needs to be fixed on the deficient nitrogen while this may be prompting the erosion, since both are playing villainous roles simultaneously on the same agricultural stage. We need to see not only the erosion, but also the declining soil nitrogen. It is this that has reduced the soil’s capacity to grow its own cover quickly as protection against erosion. At the same time there is need for some extra soil nitrogen, to render services in sustaining more than just any kind of plants merely for soil covers. Vegetative covers of our soils must now be plants producing more proteinaceous foods.
Protein Deficiencies
In our food shortages, it is the proteins of which the deficiencies are most acute. Proteins are some of the many possible chemical combinations of amino acids. These building stones out of which proteins can be constructed are characterized by their contents of the element nitrogen linked with carbon, hydrogen, oxygen, and, in some cases, with phosphorus and sulfur. The amino acids are forms of nitrogen-containing, chemical provokers of growth. They are synthesized from the different chemical elements into these life-carrying combinations only by plants, and not by the animal body. This synthesis of nitrogen into amino acids is one of the signal services for animals and humans by the plants since neither man nor beast can synthesize them so far as we now know. It is from these highly elaborated compounds that man and animal, as higher life forms in the biotic pyramid, must construct the particular proteins necessary for their body growth and repair.
The efficient utilization and elaboration of nitrogen by plants, both legumes and non-legumes, calls for a liberal supply of calcium, phosphorus, and other mineral fertility coming from the soil, as well as for the necessary supply of nitrogen. The physiological demand for calcium by the plants has been almost blanketed out and put into lesser significance in our thinking by our believing that lime was beneficial for crops only through its reduction of the degrees of soil acidity. Proteins, especially such as meats, eggs and milk, have long been considered a major problem in the struggle for food. We have not, however, led our thinking about meats to connect them as proteins with the nitrogen, with the calcium, with the phosphorus and with other items of soil fertility. We have been content to think of meat as coming from the animals. We have not thought of the animal’s source of protein going back to the soil’s source of nitrogen and other fertility supplies. Food shortages putting meat into less, and cereals and vegetables into more prominence in our diet will remind us forcefully how essential the nitrogen in certain particular protein combinations is, and how serious the shortages of it in some of them may be.
Knowledge Still Lacking
Unfortunately, the word “protein” is still not a completely known term. We still have no chemical measures of each of the many proteins. In measuring the amounts of protein we determine the element nitrogen, multiply it by 6 ¼ (the cereal chemist multiplies it by 5.7) and call that mathematical result the amount of the proteins. We know that proteins are chemical combinations of different amino acids, but which of them and how much of each amino acid you and I must ingest daily is still not so definitely known, even though there have been cataloged from the 23 known proteins, 8 for man and 10 for animals, as specifically essential.
We know nitrogen quantitatively only in terms of this simple approach of analytical chemistry. We still know little in terms of its synthesis within the animal body and less of its synthesis within the plants. Consideration of protein merely as nitrogen by analyses for this element through combustion of the substances in sulfuric acid, and the multiplication of the amount of this ash form by a mathematical factor is evidence of the pronounced ignorance of its chemical behaviors in plant physiology, to say nothing of them in animal and human nutrition.
Different Compositions
As combinations of several different amino acids, proteins suggest different possible compositions varying widely according as the plants are of different species and according as they are nourished differently by the soil. When, for example, by using different ratios of the potassium to the calcium–both in exchangeable forms on the colloidal clay–Dr. Graham reduced the crop yield of soybean forage by 25 per cent, but yet synthesized in it more protein as measured by the total nitrogen, and yet increased the concentration of phosphorus by 100 and of calcium by 200 per cent through the modification of only the ratio of these two fertility elements, can we feel that the physiology of the soybean plants was a constant behavior and that it was building the same kind of protein in the larger as it was in the smaller soybean crop? The need to differentiate between the kinds of proteins synthesized by the plants under varying ratios of the elements of fertility and under the variable supplies of different items of fertility in the soil poses one of the nitrogen problems that is high on the research addenda. This is especially true if we are to shift the protein-providing responsibility in our diet away from meats and milk and more toward the foods of distinctly vegetable origin, and thereby a step closer to the soil fertility as the foundation of the entire biotic pyramid.
Plants are not so directly synthesizers of the amino acids and their combinations but rather indirectly via their carbohydrates. The biosynthesis, or synthesis by the plants probably through the use of stored chemical energy, of the proteins is very probably a sequel only to the photosynthesis of the carbohydrates by sunshine energy. Then, too, plants are providers of calcium, phosphorus, and other nutrient elements. These elements are the soil fertility that controls not only the photosynthesis and the biosynthesis, but also, apparently, the construction of the catalyzing agents, like the vitamins, hormones, etc., that are provided by the plant for us as essentials along with other compounds classified more distinctly as foods. Dr. S. H. Wittwer1 has recently shown that the soil fertility, particularly the nitrogen, is closely connected with the elaboration of some of the vitamins in spinach, for example. If these catalyzers which we call vitamins are connected with the nitrogen as soil fertility, this is another problem for soil research that will connect the soil with nutrition of the animals and humans through a very fine chemical thread.
More Study Needed
Nitrogen viewed through the simple chemical evaluation and multiplication by an arithmetical factor is no longer complete satisfaction in understanding the nitrogen in soil fertility. It calls for biochemical measures of its functions in the plant’s processes that are building our foods. Nitrogen as a variable is demonstrating too many differing effects and features for us to be contented with an understanding that is no more elaborate and detailed than at present.
Even insects are discriminating to a degree suggesting that they are measuring the effects by nitrogen to a more refined degree. Recently some spinach under study by Dr. Wittwer was supplied with nitrogen at four levels of 5, 10, 20 and 40 M.E. per plant under ten replications. The crop was initially attacked by the insect known as the “thrips” only in the case of the plants given nitrogen at the two lower levels. Not a single plant at the higher amounts of nitrogen supplied was attacked by these insects. As more calcium was combined with the lesser applications of nitrogen these fertility combinations were more effective in reducing the leaf destruction by the thrips. If the insects are so carefully drawing the line on the different physiological behaviors in spinach of variable nitrogen in association with variable calcium offered in the soil, then there is the suggestion that we need to think about nitrogen within the plants in ways other than mere percentages of the dry weights of it in their tissue.
Hogs More Discriminating
Much more discriminating than the choices by the thrips were the choices by hogs of the corn grain in relation to the nitrogen that was a variable according to the green manure providing it. Plots with increasing mineral soil treatments were grown to sweet clover in one plot series and similarly to red clover in the adjoining one. These two legumes served as green manures to be turned under ahead of the corn planting. The corn grains from the separate plots were put into separate compartments of the self-feeder for the hogs. The protein supplement for them was provided separately.
Quite strange as it may seem, the hogs showed decreasing preference for the grain as more soil treatments were used to grow more sweet clover turned under to produce more corn. But as these same extra soil treatments grew more red clover to be turned under, these animals showed increasing preference for the corn resulting from it.
While we speak of the nitrogen of all the different legumes as green manure for corn in a single category, the hogs, and the thrips, if they would use our language, certainly would speak of it under several categories. Do not such animals and insect demonstrations pose the need for research to help us be more discriminating in the nitrogen of our own food as we are going away from the highly elaborated meat proteins and relying more on vegetable proteins? If the hog is so discriminating in what we are prone to say is due to just a difference in the amount of nitrogen turned under, there is an odious reflection on our contentment with our own ignorance of the relations between the fertility of the soil, the food grown on it and our own nutrition by means of it. The press of food that is now taking on an international scope, and the declining soil fertility so long neglected in our passing the responsibility for it to the farm owner, are making us fully conscious of the fact that the nitrogen is still a big problem in agricultural production on which national thinking needs to be focused more carefully than ever.
References Cited:
- Wittwer, S. H.: “Vegetable Crops in Relation to Soil Fertility IV, Nutritional Values of New Zealand Spinach,” Jour. of Nutrition, 31 (1946), 56-65.