Fig. 23.—Bur clover.—Medicago denticulata.

Fig. 24.—Garden pea.—Pisum sativum.

Varieties of Forms.—While these bacilli seem to be normally present in most soils, it seems to be necessary that they should adapt themselves for this symbiosis[59] with each of several groups of the legumes in order to exert their most beneficial effects. In many soils there appears to exist a “neutral form”, which requires about a season’s time or more to adapt itself specially to the several leguminous groups so that a great advantage is gained by infecting either the seeds or the soil with the forms already adapted, when no similar plant has lately occupied the same ground. Thus the bacillus of the clover root is of little or no benefit to beans, peas or alfalfa, and the root-bacilli of each of the latter are relatively ineffectual when used to infect either of the other groups. The same is true of the bacilli of lupins and of acacias, as applied to leguminous plants of any other groups.[60]

Mode of Infection.—The infection is especially effectual when applied to the seeds before sowing; and for that purpose there may be used either the turbid water made by stirring up in it some earth of a properly infected field, or else water charged with a pure culture of the appropriate kind, commercially known under the name of nitragin, now manufactured for the purpose. Or else, the field to be sown may be infected by spreading on it broadcast, and promptly harrowing in, a wagon-load of earth per acre from a properly infected field. Such earth must not be allowed to dry, or to be long exposed to light.

Specially effective (“virulent”) and hardy forms of such bacteria have been produced under artificial culture by Dr. Geo. T. Moore of the U.S. Department of Agriculture. These cultures can be sent by mail on cotton imbued with them, for the infection of seeds.

It is very important that the bacillus should be present in the earliest stages of the growth of the seedlings; otherwise the latter will undergo a longer or shorter period of starvation, unless the soil contains, or is furnished with, a sufficiency of available nitrogen to supply their immediate wants. When such a supply is very abundant, the legume crop will sometimes develop no nodules at all; but the best crops appear to be the result of a thorough infection, and abundant formation of the excrescences.

Cultural Results.—The marked results obtained in certain soils by inoculation with the legume-root bacillus are exemplified in the following table, showing results of experiments by J. F. Duggar, at the Alabama Experiment station.[61]

TABLE SHOWING INCREASE OF PRODUCTION
BY SOIL INOCULATION.
PER ACRE. TOPS. ROOTS. NITROGEN.
lbs.lbs.lbs.Value.
Hairyvetch,not inoculated1943877 $ 1.05
““inoculated30451452106 15.90
Crimson clovernot inoculated1062664.3.65
““inoculated48401452143.721.25