A SIGNIFICANT DEGREE of soil acidity has long been considered a troublesome condition for legumes, particularly those more desirable as forage for livestock. Yet it is highly probable that native legumes once grew well on those same troublesome soils when they were first cleared of virgin timber or plowed out of the prairie sod. Failures of many legume crops to return the cost of their seedings have generated the desire to find some acid-tolerant legume of high feeding value. The hope of finding such a legume has long been a tantalizing problem for many agronomists.
Now that we are moving forward to a better understanding of how the clay, or the dynamic fraction of the soil, becomes acid and sour, and how it is a kind of a jobber that may carry in stock the supply of plant nutrients from which it takes acidity, we have come to realize that legumes may actually tolerate acidity. We now know that this is true, provided they have access to a well-balanced supply of nourishment from the soil, or have ample fertility in the presence of acidity.
Soil Acidity is Natural
An acid condition is perfectly natural in regions where intensive and productive agriculture is located. Such an industry establishes itself by the help of, and on soils formed by, annual rainfalls high enough to have water going down through the soil. It demands moderate temperatures. Such climatic conditions leach the soil with carbonic acid and carry plant nutrients away in solution as we know from the hard waters that make up the ground water. These are the conditions that prevail in the Eastern half of the United States. They also prevail in the temperate zone where annual rainfalls are more than approximately 30 inches. In other words, they prevail where the precipitation exceeds the evaporation. Such combinations of climatic forces of soil development produce a clay, the particular chemical composition and structure of which give it a high capacity to adsorb and exchange, that is, to take up and to give up, many elements of both nutrient and non-nutrient values to crops. It is, in reality, a fortunate natural circumstance, namely, that where the rainfalls are high enough to grow crops abundantly there should also be this large capacity of the clay to hold acidity to a correspondingly high degree. This is good fortune because such high capacity for holding soil acidity is also the same high capacity for holding fertility or crop nourishment.
Since the highest agricultural productivity now exists in those soils recognized as acid, and also in those which were acid to possibly a lesser degree in their virgin state, can the soil acidity itself be the cause of the difficulties in growing legumes? If these virgin soils built themselves originally to high levels of organic matter by taking nitrogen from the only ultimate source, which is the air, surely there must have been some legumes and some nitrogen-fixing bacteria growing in them.
Acidity and Fertility on Clay
In breaking rock down to make soil, it is the carbonic acid in the rain water that is the main responsible chemical reagent. Clay is a residue of that process. Different kinds of clay result according to different combinations of temperature and rainfall. At the same time that these different clays are being produced, they are also being stocked with varying amounts of the different chemical elements, both plant nutrients and non-nutrients. This happens while these separate elements are being broken out of the rock and while moving down through the soil in dilute solution. They are being concentrated on the clay through its action both as a filter and as an adsorber. Some elements are held more firmly by the clay than others. Then too, since the clay of our most productive soils is highly negative in its electrical charge, it necessarily holds chemical elements of the opposite or positive charge. But when hydrogen in the soil is also positive in charge, such as calcium in lime, potassium, or magnesium, we may well expect that the clay should be holding hydrogen or acidity as a non-nutrient along with these other elements which represent the fertility.
When carbonic acid is spending its soluble hydrogen or its acid in breaking down rocks to make their calcium, potassium and other elements soluble and available for plants, it should be no surprise to us that this mixture of positively charged elements, whether nutrient or not, should be caught up by and concentrated on the negatively charged clay. Can it not be readily appreciated then that acidity and fertility are therefore interchangeable and that any root growing into a soil center or spot of fertility is also going into possible acidity?
Here then in these facts is a good foundation for our thinking about and understanding of the acidity-fertility situation in the soil. Acidity helps to develop a fertile soil under moderate rainfalls and stocks it with fertility, but under higher rainfalls this same acidity that puts nutrients on the clay can be excessive enough to take them off. Thus, when the soil has naturally become acid to a high degree, this condition merely means that the fertility has been removed to a dangerously large extent. In reality, it is then the starvation of the plant for nourishment in an acid soil rather than the excessive degree of acidity that makes what we call soil acidity, so troublesome in the growing of crops. It is not the arrival in the soil of the hydrogen or acidity, but rather the departure of the fertility that is the real trouble.
It is this interchangeability of acidity for fertility on the clay that makes possible the very feeding or nourishment of the plant by the soil. Experimental studies using the clay as a medium for growing plant roots have shown that the root gives off carbon dioxide and thereby surrounds itself with acidity from carbonic acid. This acidity then exchanges place on the clay with some of the nutrient elements like calcium, ammonium, magnesium and others, which are then free to move into the plant roots. It is by this trade of acidity to the soil that the plants are fed.
Liming Restocks the Soil
When limestone is put on the soil, it accepts acidity from the clay, just as other minerals do in the rock weathering processes. As a carbonate, it changes the active acid, or hydrogen, into water, of which compound the hydrogen is not such a highly active acid element. Therefore, the limestone corrects or neutralizes the soil acidity.
It has, however, been shown that this neutralizing effect from the liming operation is not so much the particular benefit derived by the crop, because compounds of calcium that do not neutralize the acidity, like calcium chloride, calcium sulfate or gypsum, and even ordinary cement for example, can improve the legume crop as well as calcium carbonate. Liming the soil puts calcium (or both calcium and magnesium if dolomitic limestone is used) on the clay, and thereby makes up this shortage on the list of nourishment of the crop. It feeds the plant this one nutrient which the better forage legumes need so badly for their good growth and which is so readily removed from soils under higher rainfalls. It is the calcium put in, more than the acidity put out, that comes as the beneficial effect from liming the soil.
Shortage of Fertility
Experimental studies that provide accurate control of soil fertility, have demonstrated that the better legumes require large amounts of fertility in addition to nitrogen. Though they can take nitrogen from the free gaseous supply in the air, they do not carry on this special activity of cooperating with their nodule-producing bacteria unless the soil generously provides calcium, potassium, phosphorus, magnesium, and all the other strictly soil-borne or mineral nutrients. In spite of the fact that a legume may take much nitrogen from the air, it is equally true that usually more than half of the nitrogen content in such a legume is taken from the soil. Legumes are apt to be overrated in our belief that they can provide their necessary nitrogen wholly from the supply in the air. They are also perhaps generally overrated in our speaking of them so commonly as soil-builders. They are, however, better feeds because all the other elements of fertility besides nitrogen contribute much to make them so. These extras must be supplied through the soil.
It may seem unusual, yet very good yields and fine appearing crops of soybeans have been grown experimentally only to have less nitrogen, and thereby less protein, in the final crop (including both roots and tops) than was in the seeds originally planted. The reason for this was that the calcium supply in the soil was low. Nitrogen had gone from the crop to the soil rather than in the opposite direction, as is commonly expected. Deficient calcium in the soil was also the occasion for deficient phosphorus in the crop. Then, too, unless potassium was amply provided in the soil, this element, too, like the nitrogen and the phosphorus, was wandering in the wrong direction while the crop was growing. That the bulk of the crop should be increasing and the plants seemingly growing well while the final crop was containing less nitrogen, or less phosphorus, or less potassium than was originally in the seed, may seem impossible, but such is what feeding results with animals seem to suggest.
Experiments at the Missouri Station, using different degrees of acidity, controlled in the soil, for soybeans, have shown more nitrogen taken from the air by this legume crop when the soil was acid and fertile, than when nearly neutral and of exactly the same fertility. Soybeans started in a soil at a high degree of acidity measured as pH 5.7, made that soil much more acid when they brought it to the pH 4.2 during their growth. And yet in spite of that increase in acidity, they were taking nitrogen from the air amounting to as much as 25 per cent of the total in the crop. The comparison crop that started on nearly neutral soil of the same fertility also made its soil more acid, but it did not fix so much nitrogen from the air. Here, then, in these trials the legumes fixed more nitrogen, or would have been considered better soil builders, where the soil was left more acid but was given the fertility by calcium, phosphate, potassium, and other nutrient elements. These legumes were tolerating acidity and functioned as nitrogen-fixers when given the necessary fertility.
Non-legume crops fit into the same category as legumes. They, too, have their troubles in growing on soils where there is increased acidity that means less fertility. They are not taking the mineral nutrients from the soil, nor are they synthesizing their complex compounds of such high food values, even though they may be producing bulk or tonnages per acre. For example, spinach, a non-legume, so often considered for the minerals it might put into the diet, contained less calcium and magnesium when grown on a neutral soil with ample fertility, than when the equivalent fertility was provided for the crop and the same soil left acid. In addition, the crop on the neutral soil contained more than enough oxalic acid to make all the calcium and magnesium it contained insoluble, and therefore indigestible. On the acid soil the crop contained more of these mineral nutrients than its oxalic acid contents would take out of digestive use.
Acidity not Detrimental
Our troubles in growing legume crops have not been so much with the soil acidity itself. They have been rather with our measurements of the acidity, and our emphasis on it when we had no convenient and accurate enough means in the laboratory for observing and measuring the fertility. Now we have been shown that soil acidity is simply the indicator of the degree to which the clay must keep holding hydrogen, when there are not significant amounts of the elements of fertility to take its place. It is the acidity, or hydrogen, that has the highest activity in the soil of all the chemical elements. Without acidity there is less trading, or less business going on, in the soil. When all plants, both legumes and non-legumes, produce acid by which to feed themselves, shall we condemn this character by which they barter? It therefore behooves us to think about liming more as a provider of calcium for plant nourishment, than as a carbonate to remove acidity. We should also provide other elements of fertility as well as calcium. With legume and non-legume crops, our acid soil trouble is not a question of their tolerating the acidity, but of our failure to appreciate that no plant can tolerate starvation. Yes, legumes can tolerate acidity if given fertility.