Nitrogen Bacteria and Legumes / With special reference to red clover, cowpeas, soy beans, alfalfa, and sweet clover, on Illinois soils

by Cyril G. Hopkins

Plate 5. Alfalfa: Effect of Bacteria from Alfalfa and from Sweet Clover.

Pot 1.—No bacteria.

Pots 2 and 3.—Bacteria from alfalfa.

Pots 4 and 5.—Bacteria from sweet clover.

The four series of photographs were taken five, six, seven and eight weeks from time of planting, respectively.

From these investigations we thus have conclusive evidence that infected sweet clover soil can be used for the inoculation of alfalfa fields, the bacteria of the two plants acting the same. The infected soil may be obtained from any place where the sweet clover is found growing with abundance of tubercles on its roots. The soil may be collected to a depth of three or four inches and scattered over the alfalfa field at the rate of 100 pounds or more to the acre. It is well to scatter the infected soil at about the time the alfalfa is seeded, and harrow it in with the alfalfa seed, although it may be applied some days or even some weeks before seeding time, and probably it would be all right to apply the infected soil the fall before, for it is known that the bacteria will live in soil for several months, even though the soil be placed in sacks and allowed to become quite dry.

Investigations have shown that 100 pounds of thoroughly infected soil to the acre is sufficient to produce a very satisfactory inoculation within one year from the time it is applied. Of course, somewhat heavier applications may well be made if it can be done at small expense. The infected soil need not be applied with any high degree of uniformity, but special care should be taken that the higher places and watersheds are not missed in scattering it over the field. If a few square yards, or even square rods, should be missed on the slopes or lower land, it would make but little difference, as the bacteria will be washed over such places from the higher land.

After the soil becomes somewhat dry it is easily scattered by hand from the wagon or from a sack which one can carry. Sometimes it is applied by means of an end gate seeder or a fertilizer drill, or it could be spread by a manure spreader with an application of manure.

The question naturally arises whether there is not danger of getting some sweet clover seed with the infected sweet clover soil, and thus of getting sweet clover mixed with the alfalfa in the field.