The fact has long been known, even as long ago
as the days of Pliny, and probably much before those days, that clover,
when grown in the rotation, had the power to bring fertility to the
soil. This fact was generally recognized in
modern agriculture and to the extent, in some instances, of giving it a place even in the short rotations. But until recent decades, it was only partially known how clover accomplished such fertilization. It was thought it thus gathered fertility by feeding deeply in the subsoil, and through the plant food thus gathered, the root system of the plants were so strengthened in the cultivated surface section of soil as to account for the increased production in the plants that followed clover. According to this view, the stems and leaves of the plants were thus equally benefited and, consequently, when these were plowed under where they had grown these also added plant food to the cultivated portion of the soil, in addition to what it possessed when the clover seed which produced the plants was sown upon it. In brief, this theory claimed that fertility was added by the clover plants gathering fertility in the subsoil and depositing it so near the surface that it became easily accessible to the roots of other plants sown after the clover and which had not the same power of feeding so deeply. This theory was true in part. The three important elements of plant food, nitrogen, phosphoric acid and potash, were and are thus increased in the soil, but this does not account for the source from which the greater portion of the nitrogen thus deposited in the soil was drawn, as will be shown below. It was also noticed that when the seed of any variety of clover was sown on certain soils, the plants would grow with more or less vigor for a time and then they would fail to make progress, and in some instances would perish. It was further noticed that if farmyard manure was applied freely to such land, the growth made was more vigorous. Yet, again, it was noticed that by sowing clover at short intervals on such soils, the improvement in the growth of the plants was constant. But it was not understood why clover plants behaved thus under the conditions named. It is now known that ill success at the first was owing to the lack of certain micro-organisms, more commonly termed bacteria, in the soil, the presence of which are essential to enable clover plants to secure additional nitrogen to that found in the soil and subsoil on which to feed. When manure was applied, as stated above, the clover plants secured much or all of their nitrogen from the manure. Bacteria were introduced in very limited numbers at first, it may be through the medium of the seed or in some other way, and because of an inherent power which they possess to increase rapidly in connection with continued sowing of clover at short intervals, they came at length to be so numerous in the soil as to make possible the growth of good crops of clover where these could not be thus grown a few years previously. Careful observers had noticed that certain warty-like substances were found attached to the roots of clover plants, and that the more vigorously the plants grew, the larger and more numerous were these substances, as a rule. It was thought by many that these warty substances, now spoken of as nodules, were caused by worms biting the roots or because of some unfavorable climatic influence or abnormal condition of soil. It is now known that they are owing to the presence of bacteria, whose special function is the assimilation of free nitrogen obtained in the air found in the interstices; that is, the air spaces between the particles of soil. This they store up in the nodules for the use of the clover plants and also the crops that shall follow them. The nodules in clover plants vary in size, from a pin head to that of a pea, and they are frequently present in large numbers. Bacteria are present within them in countless myriads. They gain an entrance into the plant through the root hairs. The exact way in which benefit thus comes to the clover plants is not fully understood, but it is now quite generally conceded that the nitrogen taken in by these minute forms of life is converted into soluble compounds, which are stored in the tissues of the roots, stems and leaves of the plants, thus furnishing an explanation to the increased vigor. It cannot be definitely ascertained at present, if, indeed, ever, what proportion of the nitrogen in clover is taken from the air and from the soil, respectively, since it will vary with conditions, but when these are normal, it is almost certain that by far the larger proportion comes from the air. But it has been noticed that when soil is freely supplied with nitrogen, as in liberal applications of farmyard manure, the plants do not form nodules so freely as when nitrogen is less plentiful in the soil. The inference would, therefore, seem to be correct, that when plants are well supplied with nitrogen in the soil they are less diligent, so to speak, in gathering it from the air. In other words, clover plants will take more nitrogen from the air when the soil is more or less nitrogen hungry than when nitrogen abounds in the soil. And yet the plants should be able to get some nitrogen from the soil in addition to what the seed furnishes to give them a vigorous start. This power to form tubercles, and thus to store up nitrogen, is by no means confined to clovers. It is possessed by all legumes, as peas, beans and vetches. It is claimed that some of these, as soy beans, cow peas and velvet beans, have even greater power to gather nitrogen from the air and store it in the soil than clover, since the nodules formed on the roots of these are frequently larger. In some instances, on the roots of the velvet bean they grow in clusters as large as an ordinary potato. With reference to all these leguminous plants it has been demonstrated that under proper conditions good crops may be grown and removed from the soil and leave it much richer in nitrogen than when the seed was sown. It is thus possible by sowing these crops at suitable intervals to keep the soil sufficiently supplied with nitrogen to grow good crops other than legumes, adapted to the locality, without the necessity for purchasing the nitrogen of commerce in any of its forms. They may be made to more than maintain the supply of nitrogen, notwithstanding the constant loss of the same by leeching down into the subsoil in the form of nitrates, and through the more or less constant escape of the same into the air in the form of ammonia, during those portions of the year when the ground is not frozen. They will do this in addition to the food supplies which they furnish, hence they may be made to supply this most important element of fertility, and by far the most costly when purchased in the market, virtually without cost. The favorable influences which these plants thus exert upon crop production is invaluable to the farmer. They make it possible for him to be almost entirely independent of the nitrogen of commerce, which, at the rate of consumption during recent years, will soon be so far reduced as to be a comparatively insignificant factor in its relation to crop production. It is possible, however, and not altogether improbable, that by the aid of electricity a manufactured nitrate of soda or of potash may be put upon the market at a price which will put it within reach of the farmer. The power of legumes to increase the nitrogen content in the soil should allay apprehension with reference to the possible exhaustion of the world's supply of nitrogen, notwithstanding the enormous waste of the same in various ways. The more common sources of loss in nitrogen are, first, through the leeching of nitrates into the drainage water; second, through oxidation; third, through the use of explosives in war; and fourth, through the waste of the sewerage of cities. When plant and animal products are changed into soluble nitrates, they are usually soon lost to the soil, unless taken up by the roots of plants. When vegetable matter on or near the surface of the ground is broken down and decomposed, in the process of oxidation, there is frequently much loss of nitrogen, as in the rapid decomposition of farmyard manure in the absence of some material, as land plaster, to arrest and hold the escaping ammonia. Through explosives used in war there is an enormous vegetable loss of nitrogen, as nitrate salts, which should rather be used to preserve and sustain life than to destroy it. The waste of nitrogen through the loss of sewerage is enormous, nor does there seem to be any practicable way of saving the bulk of it. In many soils the germs which produce nodules are present when clovers are first grown on them. But where they are not present, the clover plants have no more power to gather nitrogen than wheat or other non-leguminous crops. But since in other soils they are almost entirely absent, how shall they be introduced? The process of introducing them is generally referred to as a process of inoculation, and soils when treated successfully are said to be inoculated. Three methods have been adopted. By the first, as previously indicated, the grower perseveres in sowing clover at short intervals in the rotation. He may also add farmyard manure occasionally, and thus, through the inherent power of multiplication in the bacteria, they increase sufficiently to enable the land to grow good crops. By the second method, inoculating is effected through soil which is possessed of the requisite bacteria; and by the third, it is effected through the aid of a prepared product named nitragin. When fields are to be inoculated by using soil it is obtained from areas which have grown clovers successfully quite recently, and which are, therefore, likely to be well filled with the desired bacteria. In some instances the seed is mixed with the soil and these are sown together. To thus mix the seed with the soil and then sow both together broadcast or with a seed drill is usually effective, and it is practicable when minimum quantities of soil well laden with germs are used. In other instances the soil containing germs is scattered broadcast before or soon after the seed is sown. Considerable quantities of earth must needs be applied by this method. It should be remembered that each class of legumes has its own proper bacteria. Because of this, inoculation can only, or at least chiefly, be effected through the use of soils on which that particular class of legumes have grown, or which are possessed of bacteria proper to that particular species. In other words, bacteria necessary to the growth of vetches will not answer for the growth of clovers, and vice versa. Nor will the bacteria requisite to grow medium red clover answer for growing alfalfa. In other words, the bacteria proper to the growth of one member of even a family of plants will not always answer for the growth of another member of the same. But in some instances it is thought that it will answer. The study of this phase of the question has not yet progressed far enough to reflect as much light upon it as could be desired. It is certainly known, however, that alfalfa will grow on soils that grow burr clover (Medicago maculata) and sweet clover (Melilotus alba), hence the inference that soil from fields of either will inoculate for alfalfa. Nitragin is the name given by certain German investigators to a commercial product put upon the market, which claims to be a pure culture of the root tubercle organism. These cultures were sold in the liquid form, and it was customary when using them to treat the seed with them before it was planted. Their use has been largely abandoned, because of the few successes which followed their use compared with the many failures. But it is now believed that these cultures can be prepared and used so as to be generally effective and without excessive cost to the grower. In preparing cultures it has been found that by gradually reducing the amount of nitrogen in the culture of media, it is possible to increase the nitrogen fixing power in these germs from five to ten times as much as usually occurs in nature. It is now known that the bacteria thus grown upon nitrogen free media retain high activity if carefully dried and then revived in liquid media at the end of the varying lengths of time. Some absorbent is used to soak up the tubercle-forming organisms. The cultures are then allowed to dry, and when in that condition they can be safely sent to any part of the country without losing their efficacy. It is necessary to revive the dry germs by immersing them in water. By adding certain nutrient salts the bacteria are greatly increased if allowed to stand for a limited time--as short, in some instances, as 24 hours. The culture thus sent out in a dry form, and no larger than a yeast cake, may thus be made to furnish bacteria sufficient to inoculate not less than an acre of land. It is stated that the amount of inoculating material thus obtained is only limited by the quantity of the nutrient water solution used in increasing the germs, so that the cost of inoculating land by this process is not large. The culture may be applied by simply soaking the seed in it, by spraying the soil, or by first mixing the culture into earth, spreading it over the field and then harrowing it. Inoculations thus tried under the supervision of the United States Department of Agriculture have proved quite successful. Where any legume is extensively grown surrounding soils come to be inoculated through the agency of winds and water. The increase brought to the yield of plants on various soils runs all the way from a slight gain to 1000-fold. And when soil is once inoculated it remains so for a long time, even though the proper legume should not be grown again on the same soil. The amount of nitrogen that may thus be brought to many soils by growing clover and other legumes upon them is only hedged in practically by the nature of the rotation fixed upon. An acre of clover when matured will sometimes add 200 to 300 pounds of nitrogen to the soil under favorable conditions. Where the soil contains the requisite bacteria, the young plants begin to form tubercles when but a few weeks old, and continue to do so while the plant is active until mature. That the plants use much of the nitrogen while growing would seem to be clear, from the fact that toward the close of the growing season the tubercles become more or less broken down and shrunken.
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