The pure-culture method has the advantage of greater case of transportation and freedom from danger of introducing harmful pests upon the farm. Inoculation by pure cultures may be carried out in either of two ways:
(1) A bottle of pure culture of the proper kind of bacteria is opened and the culture mixed with a convenient quantity of water; this diluted culture now is mixed thoroughly with a considerable quantity of soil, preferably from the field where the legume is to be sown; the treated soil is then distributed in the same manner as when inoculation is made by the soil-transfer method.
(2) A pure culture of the proper kind of bacteria should be applied to the seeds in such a way that they will all be moistened. The seed should then be permitted to dry in a shady or dark place and should be planted as soon as possible after it is dry. Drying may be facilitated by adding dry, sifted soil, preferably from the field where the seed is to be sown. Inoculating organisms very often die within a week after the seed is inoculated. It is highly desirable, therefore that the inoculation be made the day the seed is sown. Inoculated seed never should be dried in the sun.
The question is often asked whether it is advisable to inoculate seed with pure-culture method and sow it on honeycombed ground in the spring. No experiments have thus far been completed to determine the advisability of this procedure. Some inoculation would probably result from this practice, because the bacteria on that portion of the seed next to the ground would be protected from the sunlight and would in a short time under ordinary conditions be covered by the freezing and thawing of the soil. While it is hardly possible to obtain as complete inoculation by this practice as by other methods, it is to be preferred to no inoculation.
The manner in which a stand of sweet clover is handled should depend somewhat upon the method and date of seeding and the purpose for which it is sown. Climatic conditions should also be taken into consideration and the handling of the crop governed accordingly.