Clovers and How to Grow Them

by Thomas Shaw

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.