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.