It is believed that 100 pounds of infected soy bean soil per acre will be sufficient to produce a thorough infection the second year, and it is improbable that one ton of infected soil per acre would produce a thorough inoculation the first season. One ton is only twenty times 100 pounds, while one tubercle which will be produced during a single season from a single bacterium may contain many million bacteria, thus it will be seen that it will be more economical to inoculate rather lightly and allow the bacteria to multiply themselves rather than to inoculate heavily at great expense.
Plate 4. Soy Bean Root Tubercles, Natural Size.
It may be stated that the infected soy bean soil used in these experiments was obtained from Mr A. A. Hinkley of Du Bois, Illinois, who has been growing soy beans on the same land for many years until it has become well infected. Mr. Hinkley has consented to furnish infected soy bean soil so far as he is able to do without serious interference with his regular work, to any one who may desire it, at a price which will cover his expense and loss. This will probably amount to about $1.00 for the first 100 pounds and fifty cents for each additional 100 pounds, in the shipment, including the cost of bags, the purchaser to pay freight from Bois station, which is located in Washington County, Illinois, on the Illinois Central Railroad.
The Alfalfa and Sweet Clover Bacteria.
That soil inoculation with alfalfa bacteria is commonly of very great value in growing alfalfa has been shown very conclusively by the investigations reported in Bulletin No. 76,[6] “Alfalfa on Illinois Soil.” In some places, however, inoculation was found to be unnecessary. A careful and extensive investigation of alfalfa growing in different parts of Illinois revealed the fact, as stated in Bulletin 76, “that the alfalfa bacteria are certainly present in some places in the state while in most other places they are certainly not present in sufficient number to become of appreciable assistance to the alfalfa within three or four years, and the question naturally arises how it happens that some fields are already infected while others are not.” It was suggested in that bulletin that the alfalfa bacteria may “live on some other plants besides alfalfa and that one of these plants is native or has been introduced in certain sections” of the state. It was also suggested “that a few bacteria are always carried with alfalfa seed, and that if the alfalfa is grown continuously or repeatedly in any place the soil will finally become thoroughly infected, and the bacteria will then be carried by flood waters, dust storms, etc., over adjoining fields, and possibly for long distances, especially along river valleys.” This latter suggestion was known to be a fact at the time it was written; and subsequent investigations have furnished conclusive proof that the alfalfa bacteria do live upon another plant; namely, the ordinary sweet clover (melilotus alba). This is a rank-growing leguminous plant, frequently reaching a height of four to six feet. When young it markedly resembles alfalfa, but it can easily be distinguished by its characteristic odor when cut or bruised, as by rubbing between the hands. As the sweet clover approaches maturity it differs very much from alfalfa. The sweet clover grows very tall, and usually branches from a main stem. It has white flowers (there is also a less common yellow variety), and the seeds are borne in small round pods (usually containing only one or two seeds each), arranged on long slender spikes, each spike bearing many pods. The alfalfa commonly grows about two and a half feet high, with many stems growing from the crown of the root, especially after it is two or three years old. It bears purple flowers and peculiar spiral-shaped seed pods. Sweet clover is a biennial plant, dying after reaching maturity, which commonly occurs the second year of its growth. Like many other biennial plants, it probably often lives more than two years if not allowed to produce seed. Alfalfa is a perennial plant, and it is said that there are alfalfa fields which have been cut annually for more than fifty years without reseeding. The similarity of alfalfa and sweet clover when young, and also the similarity of the tubercles formed on the roots of each have long been noticed, and the possibility of the same bacteria living upon both plants has already been suggested in the agricultural press.
During the season of 1903 the writer spent some time in the northern part of Illinois in connection with the general and detail surveys of Illinois soil. Many new fields of alfalfa were observed, and they were carefully examined for root tubercles. In Winnebago County, where sweet clover is very prevalent along roadsides and in waste places, it was noted that the abundance of root tubercles on the alfalfa plants seemed to be closely related to the presence of sweet clover in the vicinity, strongly indicating that the bacteria which live upon sweet clover were also at home upon the alfalfa roots. These indications were strengthened by further investigations in Lake County, especially upon the Fowler farm, near Lake Villa, where a field of alfalfa seeded last spring without artificial inoculation was found to be thoroughly infected with the bacteria, and growing vigorously with a good dark green color. This field had a few sweet clover plants growing in it, and the borders of the field were covered with sweet clover. Other fields of alfalfa seeded in the neighborhood at the same time, but upon soils where sweet clover had not grown near by, were apparently complete failures, many of the plants having died and most of those still living being only a few inches high, very weak, and yellow or pale green in color.[7]