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Legumes For Cover Crops

( Originally Published Early 1900's )


(Third Prize Address.)

The place of cover crops in soil improvement has long been recognized. The immense importance of the legumes in this connection is of more recent development. The legumes themselves, that is the individual vetch, clover and alfalfa plants are not in themselves so superior to other cover crops. Their great value lies in their ability to serve as hosts for nitrogen fixing bacteria, the influence of which may readily be seen at a glance at these two samples of alfalfa taken from adjoining plots of three square feet each of sandy loam soil, the one inoculated and the other not. A comparison of the two shows the inoculated specimen to be more than twice the height of the uninoculated and more than ten times the quantity. The explanation of this phenomenal difference of growth under identical conditions is found in the presence of nodules of free nitrogen fixing bacteria on the root hairs of the inoculated specimen. This species of bacteria, known as Pseudomonas radicicola, is found only on the roots of legumes and is distinctive in possessing the ability to take nitrogen gas from the air in the soil with which to construct its richly nitrogenous cell protoplasm. The organisms live in the interior of the cells of the root hairs, forming galls or tubercles where they act in a symbiotic relation with the host which furnishes sugars and starches to the bacteria, receiving in return the albuminous bodies of dead bacteria, killed by the plant and changed to bacteroids, which are the richest form of plant food and ready for immediate absorption.

After elaborate experimentation and much careful work with this organism, isolating it from the nodules to artificial culture, it was found that it could be grown in pure culture on many kinds of media. However if it is to preserve its nitrogen fixing power it must be grown on nitrogen-free media, since it will take the required amount of that element for forming its protoplasm from the media, if available, in preference to fixing atmospheric nitrogen. Further work with reinoculating disinfected seeds growing in sterile N-free agar proved conclusively that strains of the organism have developed which require particular species of legumes as hosts. Hence it became necessary to isolate and grown in pure culture organisms from nodules of each of the legumes, the alfalfa organism from nodules on the alfalfa plant, the vetch organism from nodules on the vetch plant and similarly with all.

For commercial use the organism is grown on nitrogen-free agar in small bottles (sample shown). The surface of this media is soon covered with a thick growth of the organism. They are then ready for shipment. Many methods of distributing the organism in the field have been tried out. Inoculating with soil from an adjoining field known to contain the bacteria has the disadvantages of being expensive, uncertain, uneven and of transferring soil diseases. Also it must be remembered in this connection that each of the legumes requires a particular strain of the organism, that used for clover cannot be successfully substituted for that of vetch, for example. The simplest, most efficient and least expensive method is through inoculation of the seed. ,The cultures just described are sufficient for inoculating one bushel, and may be sent through the mail. The bottle is opened just before using and the culture mixed with water, diluting the numbers. This dilution is then thoroughly mixed with the moistened seed which is then allowed to dry sufficiently to permit its passage through a drill.

Upon germination of the seed the organism is present and immediately infects the young root hairs. The resulting nodules act as "fertilizer factories" on the roots of each plant, carrying the host over periods of otherwise plant starvation when the uninoculated would turn 'yellow and die upon exhausting the available food in the soil. Inoculation has been observed to make a difference in the case of alfalfa of from no growth at all to seven tons per acre.

In view of these facts the value of the legumes can scarcely be over-estimated. They alone must be the salvation of great tracts of poor land where sand burs and brush heaps have been the chief factor in keeping the farm at home. With their use the great problem of soil improvement contemporaneous with increased crop production is largely solved.

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