With early agriculture in both the Old and New Worlds demonstrating the value of leguminous plants, it is not surprising that the foundations of some modern agronomic practices were established long before Hellriegel and Wilfarth, only 50 years ago, proved that the bacterial root nodules of leguminous plants make it possible for these plants to use atmospheric nitrogen and that members of the Leguminosae differ in this respect from the grasses.

Nodule-forming micro-organisms are not restricted to legumes, however, for nodules have been found on the roots of a number of other plants, including the cycads and shrubs of the genera Alms, Ceanothus, Myrica, Shepherdia, and Elaeagnus. Nodules are known to occur on the leaves of some tropical plants.

Nor do all leguminous plants fix nitrogen, for some are without nodules, notably species of Cassia, the redbud, honeylocust, and Kentucky coffeetree. In fact, the four genera Cassia, Cercis, Gleditsia, and Gymnocludas are generally recognized as exceptions to the habit of nodule formation among legumes. Further, it should be remembered that nitrogen is also added to the soil by the activities of free-living micro-organisms and that some nitrogen is brought down in rain water.

Legumes owe their agricultural importance to two qualities: Their high feeding value, due largely to their protein content, vitamins, and mineral salts, especially calcium and phosphorus, and the benefit they extend to other plants growing in rotation or in association with them by elaborating nitrogen compoundsin their root-nodules and subsequently releasing them in the soil. An annual average of about 100 pounds of nitrogen per acre is taken from the air by a number of the leguminous crops under field conditions, and a considerable part of this nitrogen is left in the soil in the form of nitrogen compounds, sometimes as much as 50 pounds per acre. It should be remembered, however, that neither leguminous plants nor root-nodule bacteria alone can fix nitrogen; this is accomplished only when the two function symbiotically.

Because certain legumes, such as lespedezas and vetches, can grow in acid soils, the belief has arisen that an "acid agriculture" can be developed around such plants. Most legumes, on the contrary, benefit from a. neutral soil, and under extremely acid conditions even lespedezas are decidedly benefited by the addition of lime. This may be due in part to the fact that calcium seems essential to the nutrition of root-nodule bacteria.

There has been considerable debate as to whether nitrogen is excreted by leguminous plants and absorbed by associated nonlegumes. Virtanen and his associates claim that such excretion and absorption take place, but others have held an opposite view. There is some evidence to indicate that excretion and absorption may take place when certain environmental conditions are present, as when the plants are shaded. It is of interest to note that Garman and Merkle have recently found higher quantities of available calcium, magnesium, potassium, and nitrates as well as a higher pH value in soils under black locust litter than in other locations. Enough is not yet known about this general phenomenon to permit drawing reliable conclusions.

Legumes do not, however, enrich the soil under all conditions. The nitrogen content of a virgin soil is probably constant, and accumulation of nitrogen seems possible only if the original amount has been reduced by erosion or cultivation. Constant cropping of legumes may even deplete the, supply of nitrogen in the soil. It should be kept in mind that, although badly eroded land may recover its natural fertility after a long period in permanent vegetation, depleted soils are not rehabilitated solely by the growth of annual leguminous crops. Fergus has pertinently asked whether we shall adapt crops to soils or soils to crops. He states that the phenomenal increase in the use of annual lespedezas as well as certain grasses, such as orchard grass and redtop, only emphasizes the fact that such plants are adapted to depleted soils, and that they in fact tend further to impoverish the land after it has become too poor for more productive crops. The yield of crops grown on depleted soils is low and of poor quality, for these soils are usually lacking in phosphorus and calcium. Cattle that feed on plants deficient in such essential elements are weakened, and people dependent on the products of such land for their nutrition are likewise undernourished. Legumes are useful, but they cannot be expected to work miracles.

Since good growth of legumes depends on the nitrogen made available by their associated root organisms, it is often necessary to supply them with appropriate bacteria when they are planted in soil where they have not grown before. This is known as inoculation and may be accomplished by spreading soil from fields where the same kind of legume has grown or by applying bacterial cultures developed in the laboratory. Although all nodule bacteria have the same general function, certain kinds seem able to produce nodules on only certain kinds of legumes. If a bacterium from one kind of legume will form nodules on another, and vice versa, they are said to cross-inoculate. Cross-inoculation tests have shown relationships among the nodule bacteria of many legumes, especially crop plants, and these have been classified into cross-inoculation groups according to the species of Rhizobium that inoculates the members of the group.

Jason Ladock

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