Chapter II: Some General Principles Which Apply to the Growing of Clovers (2)

=Clovers as Soil Improvers.=--All things considered, no class of plants grown upon the farm are so beneficent in the influence which they exert upon the land as clovers. They improve it by enriching it; they improve it mechanically; and they aid plant growth by gathering and assimilating, as it were, food for other plants.

All clovers have the power of drawing nitrogen from the air and depositing the same in the tubercles formed on the roots of the plants. These tubercles are small, warty-like substances, which appear during the growing season. They are more commonly formed on the roots within the cultivable area, and therefore are easily accessible to the roots of the plants which immediately follow. Clovers are not equally capable of thus drawing nitrogen from the air, nor are the same varieties equally capable of doing this under varying conditions. The relative capabilities of varieties to thus deposit nitrogen in the soil is by no means equal, but up to the present time it would seem correct to say that relative capability in all of these has not yet been definitely ascertained. With reference to the whole question much has yet to be learned, but it is now certain that in all, or nearly all, instances in which clovers are grown on land, they leave it much richer in nitrogen than it was when they were sown upon the same.

They also add to the fertility of the surface soil by gathering plant food in the subsoil below where many plants feed. They have much power to do this, because they are deep rooted and they are strong feeders; that is, they have much power to take up food in the soil or subsoil. Part of the food thus gathered in the subsoil helps to form roots in the cultivable area and part aids in forming top growth for pasture or for hay. If grazed down or if made into hay and fed so that the manure goes back upon the land the fertility of the same is increased in all leading essentials. This increase is partly made at the expense of the fertility in the subsoil. But the stores of fertility in the subsoil are such usually as to admit of thus being drawn upon indefinitely.

Clovers improve soils mechanically by rendering them more friable, by giving them increased power to hold moisture, and by improving drainage in the subsoil. Of course, they have not the power to do this equally, but they all have this power in degree and in all the ways that have been named.

Clovers send down a tap root into the soil and subsoil as they grow. From the tap roots branch off lateral roots in an outward and downward direction. From these laterals many rootlets penetrate through the soil. When the plants are numerous, these roots and rootlets fill the soil. When it is broken up, therefore, particles of soil are so separated that they tend to fall apart, hence the soil is always made more or less friable, even when it consists of the stiffest clays. The shade furnished by the clover also furthers friability. This friability makes the land easier to work, and it is also more easily penetrated by the roots of plants. The influence on aeration is also marked. The air can more readily penetrate through the interstices in the soil, and, in consequence, chemical changes in the soil favorable to plant growth are facilitated.

The roots of clovers are usually so numerous that they literally fill the soil with vegetable matter. This matter, in process of decay, greatly increases the power of the soil to hold moisture, whether it falls from the clouds or ascends from the subsoil through capillary attraction. The moisture thus held is greatly beneficial to the plants that immediately follow, especially in a dry season and in open soils, and the influence thus exerted frequently goes on, though with decreasing potency, for two, three or four seasons.

Reference has already been made to the tap root which clover sends down into the soil and subsoil. In the strong varieties this tap root goes down deeply. When the crop is plowed up, the roots decay, and when they do, for a time at least, they furnish channels down which the surface water percolates, if present in excess. Thus it is that clover aids in draining lands under the conditions named. The channels thus opened do not close immediately with the decay of the clover roots, hence the downward movement of water in the soil is facilitated for some time subsequently.

It has been stated that clovers have more power than some other plants to gather plant food in the soil. In some instances they literally fill the soil with their roots. When other plants are sown after the clover has been broken up they feed richly on the decaying roots of the clover. Thus it is that clover gathers food for other plants which they would not be so well able to gather for themselves, and puts it in a form in which it can be easily appropriated by these. The nitrogen in clover is yielded up more gradually and continuously as nitrates than it could be obtained from any form of top dressings that can be given to the land. In this fact is found one important reason why cereal grains thrive so well after clover.

Since the roots of clovers act so beneficently on soils, it is highly important that they be increased to the greatest extent practicable. Owing to the relation between the growth of the roots of plants and the parts produced above ground, development in root growth is promoted much more when the clover is cut for hay than when it is fed off by grazing. Experiments have also demonstrated that the development of root growth is much enhanced in medium red clover by taking a second cutting for hay or seed. They have also demonstrated that more nitrogen is left in the soil by clover roots after a seed crop than after a crop of hay.

From what has been said, it will be apparent that the extent to which clovers enrich the soil will depend upon the strength of the growth of the plants and certain other conditions. It will not be possible to reduce to figures the additions in plant food which clovers add to the soil other than in a comparative way. Dr. Voelker has stated that there is fully three times as much nitrogen in a crop of clover as in the average produce of the grain and straw of wheat per acre. Dr. Kedzie is on record as having said that in the hay or sod furnished by a good crop of clover, there is enough nitrogen for more than four average crops of wheat, enough phosphoric acid for more than two average crops and enough of potash for more than six average crops. He has said, moreover, that the roots and stubble contain fully as much of these elements as hay.

It will also be apparent that where clover grows in good form no cheaper or better way can be adopted in manuring land, and that in certain areas the judicious use of land plaster on the clover hastens the renovating process. It is thought that in some instances the mere loading and spreading of barnyard manure costs more than the clover and plaster. Especially will this be true of fields distant from the farm steading. It is specially important, therefore, that in enriching these, clover will be utilized to the fullest extent practicable.

=Clover as a Weed Destroyer.=--Where clover is much grown, at least in some of its varieties, it becomes an aid in reducing the prevalence of many forms of weed growth. It is thus helpful in some instances, because of the number of the cuttings secured; in others because of its smothering tendencies, and in yet others because of the season of the year when it is sown and harvested or plowed under, as the case may be.

Alfalfa and medium red clover are cut more frequently than the other varieties and, therefore, because of this, render more service than these in checking weed growth. The former is cut so frequently as to make it practically impossible for most forms of annual weed life to mature seed in the crop. The same is true of biennials and also perennials. But there are some forms of perennial weeds which multiply through the medium of their rootstocks that may eventually crowd alfalfa. Medium red clover is usually cut twice a year, hence, in it annuals and biennials cannot mature seed, except in exceptional instances, and because of the short duration of its life, perennials have not time to spread so as to do much harm.

The clovers that are most helpful in smothering weeds are the mammoth, the medium and the alsike varieties. These are thus helpful in the order named. To accomplish such an end they must grow vigorously, and the plants must be numerous on the ground. When grown thus, but few forms of weed life can make any material headway in the clover crop. Even perennials may be greatly weakened, and in some instances virtually smothered by such growth of clover. To insure a sufficient growth of clover it may be advantageous to top dress the crop with farmyard manure sufficiently decayed, and in the case of medium red clover to dress the second cutting with land plaster. If the second growth is plowed under, subsequent cultivation of the surface will further aid in completing the work of destruction.

The crimson variety is sown and also harvested at such a time that the influence on weed eradication is very marked. The ground is usually prepared in the summer and so late that weeds which sprout after the clover has been sown cannot mature the same autumn. In the spring it is harvested before any weeds can ripen. When plowed under, rather than harvested, the result is the same.

When clover is grown in short rotations, its power to destroy weeds is increased. For instance, when the medium red or mammoth varieties are grown in the three years' rotation of corn or some root crop, followed by grain seeded with clover, the effects upon weed eradication are very marked, if the cultivation given to the corn or roots is ample. Under such a system weeds could be virtually prevented from maturing seeds at any time, especially if the medium variety of clover were sown, and if the stubbles were mown some time subsequent to the harvesting of the grain crop. Such a system of rotation faithfully carried out for a number of years should practically eradicate all, or nearly all, the noxious forms of weed life.

=Clover Sickness.=--On certain of the soils of Great Britain and probably on those of other countries in Europe, where clover has been grown quite frequently and for a long period, as good crops cannot be grown as previously, and in some instances the crop is virtually a failure. The plants will start from seed in the early spring and grow with sufficient vigor for a time, after which they will show signs of wilting and finally they die. Various theories were advanced for a time as to the cause before it was ascertained by experiment what produced these results. Some thought they arose from lack of water in the soil, others claimed that they were due to the presence of parasites, which in some way preyed upon the roots, others again attributed them to improper soil conditions. It is now just about certain that they arose from a deficiency of soluble potash in the subsoil. Such, at least, was the conclusion reached by Kutzleb as the result of experiments conducted with a view to ascertain the cause of clover wilt.

The cause being known, the remedy is not difficult. It is to grow clover less frequently on such soils. Sufficient time must be given to enable more of the inert potash in the subsoil to become available. Another way would be to apply potash somewhat freely to these soils, and subsoil them where this may be necessary.

It is thought that clover sickness is as yet unknown in the United States and Canada, although its presence had sometimes been suspected in some sections where clover has been much grown. This does not mean that it may not yet come to this country. Should the symptoms given above appear on soils on which clover has been grown frequently and for a long period, it would be the part of wisdom to take such indications as a hint to grow clover less frequently in the rotation.

=Possible Improvement in Clovers.=--Some close observers have noticed that there is much lack of uniformity in the plants found growing in an ordinary field of clover, especially of the medium red and mammoth varieties. Many of the plants vary in characteristics of stem, leaf, flower and seed; in the size and vigor of the plants; in the rapidity with which they grow; and in earliness or lateness in maturing. So great are these differences that it may be said they run all the way from almost valueless to high excellence. Here, then, is a wide-open door of opportunity for improving clover plants through selection. This question has not been given that attention in the past which its importance demands.

There may be a difference in view as to all the essential features of improvement that are to be sought for, but there will probably be agreement with reference to the following in desirable varieties: 1. They will have the power to grow quickly and continuously under average conditions. This power will render them valuable as pasture plants in proportion as they possess it. 2. They will produce many stems not too coarse in character. This will affect favorably the character of the hay and will also have a bearing on increase in the production of seed. 3. There should be an abundance of leaves. Such production will affect favorably palatability in the pasture and also in the hay. 4. The blossoms should be so short that the honey which they contain may be accessible to the ordinary honey bee. The importance of this characteristic cannot be easily overestimated. It would not only tend to a great increase in seed production through the favorable influence which it would have on fertilization, but it would greatly increase the honey harvest that would be gathered every year, and 5. They should be possessed of much vigor and hardihood; that is, they should have much power to grow under adverse conditions, as of drought and cold. The person who will furnish a variety of red clover possessed of these characteristics will confer a boon on American agriculture.

=Bacteria and Clovers.=--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.