Ammonia-forming Bacteria.—A large number of different bacteria appear to be concerned in the formation of ammonia from compounds of the albuminoid group, (and probably from humus). Among these is one of the most common in soils (Bacillus mycoides, root bacillus), which while forming ammonia carbonate in solutions of albumen, is also capable of reducing nitrates to nitrites and ammonia in presence of a nutritive solution of sugar.
The “hay bacillus” (B. subtilis), so abundantly developed in hay infusions, and one of the most abundant in cultivated soils, has together with B. ellenbachensis, B. megatherium, B. mycoides, and others, by some been credited with important action in favoring vegetation; so that a fairly pure culture of B. ellenbachensis has been brought out commercially in Germany under the name of “Alinit.” Rigorous culture experiments made by Stutzer and others have, however, failed to show any general benefit from the use of alinit in infecting either land or seeds. But there is no doubt of the Effects of Bacterial Life on Physical Soil Conditions.—It is apparent that all conditions favoring the life of aerobic (air-needing) bacteria tend also to produce the loose, porous state (tilth) of the surface soil so conducive to the welfare of culture plants, designated by German agriculturists as “Bodengare.” Whether or not this condition is directly due to bacterial processes, as is thought by Stutzer (Landw. Presse, 1904, No. 11) it is assuredly a highly important point to be gained, and is essentially connected with the presence of humus in adequate amounts, which is also a favoring condition of abundant bacterial life. It seems that the preference given to the shallow putting-in, or even surface application of stable manure, existing in Europe, is largely based upon the marked effect upon the looseness of the surface soil, generally credited to the physical effect of the manure substance itself, but apparently largely due to the intensity of bacterial action thus brought about.
Fig. 18.—Bacillus subtilis.
(Wollny, after Brefeld.)
Fig. 19.—Bacteria producing ammoniacal fermentation:
A, C. mycoides: B, B. stutzeri.
(From Conn, Agr. Bacteriology.)
Fig. 20.—Bacillus megaterium.
(From Migula.)
Root-bacteria or rhizobia of legumes.—Among the most important bacteria, agriculturally, is that which enables plants of the leguminous order—(peas, beans, vetches, clovers, lupins, etc.),—to obtain their supply of nitrogen from the air independently of those contained in the soil. The source of nitrogen to plants was long a disputed question; it was at first supposed (by de Saussure) that it was obtained directly from the soil by the absorption of humus; but this was disproved, and Liebig then contended that it was derived directly from the atmosphere through the ammonia in rain water. This was then shown to be wholly inadequate; and Boussingault proved conclusively that plants do not take up nitrogen gas from the air. This was subsequently denied by Ville; but investigation at the Rothamstead agricultural station by Lawes and Gilbert definitely confirmed Boussingault’s results. At the same time they also proved very definitely that while grass and root crops deplete the soil of nitrogen, clover and other leguminous crops leave in the soil more nitrogen than was previously present, even when the entire, itself highly nitrogenous, leguminous crop is removed from the land. The improvement of lands for wheat production by rotation with clover had long ago become a practical maxim; but the cause was not understood until, in 1888, Hellriegel and Wilfarth announced that the variously-shaped excrescences or tubercles which had long been observed as frequently deforming the roots of legumes, are caused by the attacks of bacilli capable of absorbing the free nitrogen of the air and thus enabling the host-plant to acquire its needed supply by absorbing the richly nitrogenous matter thus accumulated in the excrescences. The minute rod-shaped organism was named Bacillus radicicola by Beyerinck; Rhizobium leguminosarum, by A. Frank, who has published an extensive treatise on the subject.[57]
