LEGUME Inoculation and Fixation of Air Nitrogen

Nearly 2,000 years ago historians recorded the observation that better crops of grain were obtained when the seed was planted in soil which had just previously borne a crop of legumes. Here began our written knowledge of crop rotation. Although at this time and for many years afterwards nothing was known about bacteria or nitrogen gas, the beneficial effect of nitrogen fixation of legumes in a rotation (vetches, beans, lupines, clovers, etc.) was early recognized. It is only within the last century that the agents concerned with legume plants in the utilization of atmospheric nitrogen have been discovered and are now known to be minute soil-inhabiting plants which are called bacteria. When these bacteria come in contact with suitable legume roots in the soil they enter and cause the plant to form growths in the course of their multiplication which are commonly referred to as nodules or tubercles. These growths are very characteristic in outward appearance, varying mainly on account of the plant on which they occur, although their external appearance is sometimes modified by soil conditions. Nodules being the evidence of nitrogen- fixation and of the presence of certain kinds of legume bacteria in a soil, are growths with which every farmer should make himself familiar.

Nature has distributed the legumes and their bacteria very liberally to the various soils throughout the world, but since the bacteria which will produce nodules on a particular legume will not necessarily produce them on another species, it does not. always happen that the proper kind of nodule-forming bacteria are in the soil.  Legumes are therefore classified on the basis of their ability to accommodate the same organisms in their roots. Thus, the nodule bacteria of the plants within each of the following four groups of legumes are considered the same for practical purposes: (1) Alfalfa, bur clover, and sweet clover; (2) crimson, red, and alsike clover; (3) cowpea, velvet bean, peanut and Lima bean; (4) vetch, garden pea, and sweet pea. The soy bean requires its own special nodule bacteria. With the modern interchange of legume seed between widely separated parts of the world and the development of conditions detrimental to nodule bacteria it is often necessary to bring them into a soil.

If legume bacteria are lacking in a soil on account of soil conditions, as, for instance, soil acidity, this condition should be corrected before replenishing the supply of bacteria in this soil. The process of introducing nodule bacteria into the soil is commonly called inoculation. The material for this purpose is obtainable from two general sources: (1) artificially prepared: cultures and (2) field soil in which the proper bacteria are known to be present. It is possible by careful laboratory work to take the bacteria from nodules and to propagate them on sterilized artificial food in the absence of other bacteria. Inoculating material prepared in this manner is obtainable from various governmental and commercial agencies.  On the other hand, soil known to contain the proper bacteria and free from diseases and pests, makes an excellent source of inoculating material for the farmer. At times it may be advisable to establish a source of bacteria on the farm by growing seed treated with pure culture on a small plot of soil. With this source at hand, the soil may be transferred to other fields as it is needed. When a soil is once seeded with the proper bacteria and it is maintained favorable to their growth with necessary additions of lime, fertilizers, and organic matter and an occasional growth of the legume on which they function, they should continue to live indefinitely in this soil.

After legume bacteria penetrate the roots they begin to draw nitrogen from the air and so alter it that the plant may absorb and utilize it in the building of tissues. While the bacteria are fixing nitrogen they draw on the plant roots for the carbohydrates, moisture, and minerals necessary for their growth. Through the work of these bacteria greater legume crops are produced, uniformly higher in nitrogen than most nonlegumes or legumes which do not have the benefit of this bacterial association.

The amount of nitrogen fixed by legumes and their bacteria varies with the species, the conditions, and the time of growth.  Under favorable conditions a single crop of legumes may fix as much as 200 pounds nitrogen per acre in a year and a crop put on the soil to fill a gap for a short time may only add from 40 to 60 pounds per acre in a season. In the choice of legumes for planting, those which grow vigorously under the existing conditions and which meet the needs of the type of farming practiced should be considered.

The main part of the nitrogen fixed by legumes stays in the plant until it dies and decays, although in a dormant period of the plant or in extremely dry weather, a small amount of it may pass into the soil by the “sloughing off" of the nodules. The fate of the nitrogen in the legume crop is entirely dependent upon its utilization. The nitrogenous organic matter in the stubble as a rule remains in the soil where it rots and is thereby made available for subsequent non-legume crops. The greater part of the nitrogen in legumes is usually in the part that is cut for hay or seed.