The Legumes of Many Uses

A GENERAL VIEW OF THE LEGUMINOSAE

Roland McKee

A LEGUME is a plant that bears pods as do beans, peas, soybeans, and cowpeas. In more technical terms, a legume is a superior one-celled, monocarpellary fruit, usually dehiscent into two valves, and having the seed attached along the ventral suture. The fruit is called a pod.

All plants conforming to this definition are grouped together in one large family called the legume family, or Leguminosae. In a listing or classification of flowering plants based on the stage or complexity of development, legumes are intermediate in the list.

Considered in its broadest sense, the legume family contains three divisions, or subfamilies. These are known as Mimosoideae, Caesalpinioideae, and Faboideae. In flower structure these subfamilies differ quite widely and it is this difference that is usually used in classifying or identifying the groups.

In Mimosoideae the flowers are regular and usually in dense heads; in Caesalpinioideae and Faboideae they are irregular and fewer in a cluster or raceme. In Caesalpinioideae the flowers do not have the papilionate or butterfly shape that is characteristic of the Faboideae. It is the Faboideae that is most commonly associated with the term legume. All the economic legumes of agriculture come under this group.

The flowers in the Fabiodeae are distinctive. The petals of an individual flower vary in shape. The largest and most showy petal is known as a banner or standard. It is usually nearly flat and somewhat circular and is the outer one of the petals. The two inner petals, which are folded together, are usually bent or curved and enclose the stamens. These are known as the keel. On either side of the keel are two petals known as wings. When the petals are in normal position and the banner extended, the resulting flower has somewhat the appearance of a butterfly and the flower, accordingly, has been called papilionaceous (papilio, a butterfly, + aceous). Some have called this subfamily the Papilionoideae rather than the Faboideae. Inside the petals, or showy part of the flower, are concealed the stamens and pistil. These are the essential parts of the flower since they are the organs involved in seed production.

Legumes have been known to man from the time of earliest records. Tares, as referred to in the Bible, are thought to be the common vetch. Alfalfa was among the earliest of cultivated crops, as indicated by early historical writings. Its native habitat is presumed to be Persia. In China the soybean dates back to the earliest of preserved writings and probably long antedates written records. Alfalfa can thus be considered the earliest cultivated forage crop and soybeans probably the earliest legume food crop. This early use of these crops suggests that even in the first period of recorded history the superiority of legumes was recognized. Their early use in pastures and crop rotations suggests this conclusion.

The legumes today are universally thought of as having higher feeding value than nonlegumes and this, for the most part, is true. One of the main reasons why they are superior is the fact that in general they contain a higher percentage of protein than non-legumes, and protein is an essential food constituent. The seeds of legumes are particularly high in protein, but the leaves and stems also contain a relatively higher amount than is contained in other plants when they are harvested at a like stage of maturity.

It is true that seeds of some non-legumes have a high protein content and the leaves and stems of some are comparatively high in this constituent, but for the most part, legumes can be considered decidedly superior. Legumes not only have a higher percentage of protein, they also have high-quality protein. This is of prime importance and helps greatly in obtaining high nutritive value in feeds for animals as well as food for human consumption. The quality of the protein of legumes is such as to make them especially valuable as feed to supplement the cereal grains, which do not have the proper protein for a balanced livestock feed.

Legumes are also valuable because they contain a comparatively large amount of calcium and have a fair amount of phosphorus, which is necessary in proper nutrition. Likewise, legumes are recognized as the best source of vitamins A and D for livestock feed and are largely depended upon for supplying these constituents.

Besides being of special value for feed and food, legumes are superior for soil improvement. This is due to the large amount of nitrogen they are able to supply the soil for the use of subsequent crops. It is not, however, merely that they return a large amount of nitrogen to the soil that makes them superior for soil improvement; it is the fact that much of the nitrogen contained in a legume is taken from the air rather than from the soil, as is the case with other plants, and in this way new and additional nitrogen is added to the already existing soil supply. Legumes thus add to the soil nitrogen that it did not previously contain. Non-legumes merely take nitrogen from the soil and return it again.

The total amount of nitrogen that legumes take from the air cannot be known with any exactness since the condition under which they are grown influences the relative amounts taken from the air and soil. In a soil that is low in nitrogen, much more of the element is taken from the air than is the case when there is a large soil supply of nitrogen. It also is probable that the relative amount taken from the air and soil varies with the different legumes. It is generally assumed, however, that at least half the nitrogen in legumes comes from the air.

The total acreage of legumes in the United States, both in cultivated and pastured areas, can only be estimated. Data are available covering acreage cut for hay and seed of the major legume crops but are lacking for acreage used for pasturage and cover crops. From the general information that is available, however, it is estimated, in round numbers, that the acreage of all legumes cut for hay is 40 million acres; cut for seed, 15 million acres; used for cover crop, 5 million acres; and pastured, 40 million acres.

Assuming that this acreage averaged a ton per acre dry weight of tops and roots and that this growth averaged 2 percent nitrogen, the total nitrogen produced would be 2 million-odd tons.

Studies that have been made on roots of legumes indicate that the weight of the roots generally is about one-third that of the tops and that the percentage of nitrogen in the roots is slightly less than that in the tops. The percentage of nitrogen in roots of a number of legumes that have been analyzed ranges from 1.40 percent to 2.30 percent and in tops from 2.10 percent to 2.80 percent.

As previously stated, it is generally assumed that more than half the nitrogen in legumes is taken from the air. This means then that more than a million tons of nitrogen is taken by legumes each year from the air. Of course, all that amount would not be returned to the soil because the hay, pasturage, and seed fed to animals would only be returned in part and mostly as manure. As has been indicated, about one-third of the total nitrogen of most legumes is in the root; this, with the amount returned to the soil would no doubt be well over half of the amount taken from the air.

The nitrogen taken from the soil by legumes, of course, does not add to the total nitrogen of the soil, but such nitrogen, when again returned to the soil, is usually considered to be more readily available for use in plant growth and in this way adds to the nitrogen supply. The total nitrogen in the form of commercial fertilizer used annually in the United States has been less than 500,000 tons. It is thus seen that the nitrogen supplied by the legume crops is greater than the amount used in commercial fertilizers.

Cowpea

Getting nitrogen from the air usually is considered a special function of legumes, although it is known that a few other plants can do so, too. In the case of legumes, taking nitrogen from the air is accomplished through symbiotic bacteria (Rhizobia) that develop in nodules on the roots of the legumes. The bacteria take nitrogen direct from the air as they grow and multiply in the nodule. The nitrogen in turn becomes available to the legume plant and aids in its nourishment and growth. It is this symbiotic association of legumes and Rhizobia that gives legumes a distinct advantage over non-legume plants.

In order to attain the advantage of symbiosis it is necessary to bring the Rhizobia in contact with the young growing rootlets of the legume. In agricultural operations this is a common practice and is referred to as inoculation. Inoculation is accomplished by mixing a liquid or humus culture of the Rhizobia inoculum with the seed just before seeding.