Excretion of nitrogenous compounds from the nodules of leguminous plants, if it is of general occurrence, would be important in farming practice. It has many times been demonstrated that nonlegumes growing together with legumes in such a manner that their root systems are intertwined seem to derive benefit from the association. The growth of the nonlegume is increased and sometimes also its protein content is higher, just as would be the case if additional nitrogen had been made available to it. This effect of associated growth may not be solely or even usually due to excretion of nitrogen from the nodules because the latter has not been proved to be a general occurrence. A. I. Virtanen in Finland, under the environmental conditions there, obtained evidence that led him to think the phenomenon a common one, but workers in places as diverse as Scotland, Australia, Washington, D. C., Wisconsin, and Kentucky have only in a few instances obtained any evidence of excretion. In these instances the amounts concerned have invariably been small, and the experiments have not been reproducible.

The explanation of the beneficial effects of associated growth on the nonlegume may lie instead in loss of nodules from the legume, a process that is apparently normal but which is accentuated by clipping the tops of the legume as would occur in grazing or mowing of pastures. Nodule tissue is exceptionally high in nitrogen (about 6-10 percent), and would decompose rapidly with the liberation of available inorganic nitrogen that might be used by the nonlegume.

Strains of the various species of Rhizobium that differ in effectiveness have long been recognized. Some are almost wholly ineffective .in benefiting the host legume. One of the features of "ineffective" strains now established is that the nodules they produce are smaller and, therefore, contain less active bacterial tissue. Another is that they remain on the roots for a shorter period before deterioration and degeneration occur. They may be no less efficient in fixation per unit mass of nodular tissue, but the shorter size and shorter duration of attachment together account for the smaller contribution made to the nitrogen economy of the legume.

In choosing commercial cultures for inoculation of leguminous crops it is common practice now to select and mix several effective strains isolated from widely planted varieties of the crops in question. There is a tendency, however, to imply that effectiveness or ineffectiveness is wholly a property of the organism. In the efficient operation of the fixation process the plant has its part, and there is some evidence to indicate that ability to be nodulated and to participate in this unique relationship may also be a plant genetic character.

The concept of "cross inoculation" groups of plants that can be nodulated by one species of Rhizobium is not now interpreted as rigidly as at one time was the case. Many individual exceptions in which plants Of one group have been nodulated by an organism isolated from a plant in another group have been observed at various times, but the practical convenience of the grouping-in the preparation of cultures for use by farmers is not impaired. It may, however, be desirable to introduce additional requirements into the selection of strains of rhizobia for incorporation into commercial legume cultures if the newly recognized factor of competition between strains is to be taken into account. It has been shown that effectiveness in nitrogen fixation and dominance in competition between strains are independent. Where two strains are present together in the zone surrounding the root of the legume the strain having the higher initial growth rate may suppress the development of the weaker strain and, therefore, be responsible for almost all the nodules. The object, therefore, will be to select for commercial cultures strains that are effective in fixation in the desired varieties of the crop in question and that are also vigorous in competition with other strains capable of nodulating the same plant.

Commercial legume inoculants are now quite generally dependable. The most common form consists of a heavy suspension of organisms absorbed by peat, packaged in such a manner that moisture losses subsequently are minimized. There is a possibility, however, that lyophilized cultures ( dried under vacuum at low temperature) , that maintain viability for long periods and that would be less bulky, may be developed to the state of practicability. Some changes in procedure followed in seed inoculation may be desirable when seed disinfectants are used. Certain of these compounds are not compatible with rhizobia; others do not reduce nodulation if a heavier rate of inoculation is practiced and if the seed is planted shortly after treatment. An alternative which has been found effective when planting treated pea seed in Washington is to use bulk inoculant which is placed in the furrow through the fertilizer attachment of the grain drill.

Claims have been made at various times by Russian workers that very substantial yield increases in a variety of crops have followed inoculation of the soil with Azotobacter species, and such inoculants have apparently been prepared and used in a large scale. Trials of a similar character in this country in greenhouses and on small plots, where adequately replicated, have yielded predominantly negative results, although in one or two isolated cases significant and otherwise unaccountable yield increases have been obtained.

THE AUTHOR

A. G. Norman was professor of soils at the Iowa State College until the fall of 1946, when he began research work for the Chemical Corps of the Army. He has interested himself particularly in studies of the decomposition of plant materials, and the chemistry of soil organic matter. Before 1937 he was biochemist at the Rothamsted Experimental Station, England.