In order to obtain more absolute knowledge regarding this important subject, a series of pot culture experiments has been carried on under controlled conditions in the pot culture laboratory at the university. Five pots were filled with sterilized sand which was practically devoid of plant food. A supply of phosphorus, potassium, and all other mineral elements necessary for the growth of plants was added to each of the five pots, care being taken to keep the sand practically free of combined nitrogen. Alfalfa seed were then planted in each of the five pots, and at the same time four of the five pots were inoculated as follows:
Pot No. 1.—Not inoculated (check pot).
Pot No. 2.—Inoculated with bacteria obtained from infected alfalfa soil.
Pot No. 3.—Inoculated with bacteria obtained from alfalfa root tubercles.
Pot No. 4.—Inoculated with bacteria obtained from infected sweet clover soil.
Pot No. 5.—Inoculated with bacteria obtained from sweet clover root tubercles.
Plate 5 clearly shows the results obtained and certainly furnishes conclusive proof that the same effect is produced upon the growth of the alfalfa whether the nitrogen-gathering bacteria used for the inoculation are obtained from alfalfa soil, from alfalfa tubercles, from sweet clover soil, or from sweet clover tubercles. It also illustrates the importance of bacteria in growing alfalfa as will be seen by comparing the four inoculated pots with the uninoculated pot, which is No. 1, on the left in each series of views. The upper view was taken when the alfalfa plants were five weeks old; the next series when they were six weeks old; the next, seven weeks old; and the lower series when they were eight weeks old, from the time of seeding.
A duplicate series of pots prepared in exactly the same manner gave similar results.
The infected alfalfa soil was obtained from a field of three-year-old alfalfa, which was inoculated when first seeded, with infected alfalfa soil obtained from an old alfalfa field in Kansas. About one pound of this soil was shaken in a quart of water, the soil allowed to settle, and some of the nearly clear solution used for the inoculation of Pot No. 2. The alfalfa tubercles from which bacteria were obtained were carefully washed in distilled water to free them from adhering soil particles, and then rubbed up in distilled water, a small amount of this water being then used for the inoculation of Pot No. 3. The infected sweet clover soil was obtained from a place by the roadside where sweet clover was growing luxuriantly and well provided with root tubercles. This place was about two miles from the nearest field ever seeded to alfalfa, so far as known. A water extract from this soil was used to inoculate Pot No. 4. The bacteria from sweet clover tubercles were obtained in the same manner as those from alfalfa tubercles, and were used to inoculate Pot No. 5.