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CHAPTER 13
             NATURAL AND ARTIFICIAL INOCULATION
   "What really leads us forward is a few scientific discoveries and
their application."
                                                                      -PASTEUR
    It is well known that the leguminous plant, when infected with appropriate
rhizobia, is favorably influenced; its total growth is increased and the
soil is
enriched. The source of this material benefit is the nitrogen of the air,
captured
and made available to the plant by the rhizobia in its root nodules. Unfortunately,
these micro6rganisms are not always present in the soil, and under such condi-
tions nodules fail to form. The leguminous plant must then draw its nitrogen
from the soil, like any other higher plant. These essential facts concerning
the
Leguminosae and their nitrogen nutrition had scarcely been reported before
various methods were devised for introducing the nodule-forming bacteria
into
the soil, and thus insuring nodule formation. This process of adding bacteria
is popularly known as "inoculation." The word "inoculation"
is derived from
the Latin "in" plus "oculare." "Oculus", the
noun, means eye or bud; hence to
inoculate literally means to "furnish with eyes". The term has
been used in at
least three ways: first, referring to a nodule-bearing plant, we say it is
well
inoculated; second, we inoculate seed or soil by the process of applying
bacteria;
and third, we call the cultures thus used, inoculation. To the authors, the
first
use of inoculation, i.e. in relation to nodule formation, appears inexact
and
should be abandoned in favor of nodulation of the plant. The use of inoculation
in connection with seed or soil in reality implies a single process, that
of bringing
the nodule-forming bacteria into proximity with the young plant. Whether
the
bacteria are applied to the seed or directly to the soil is only a matter
of technique.
The third use of inoculation, as synonymous for culture, is unnecessary and
may
be misleading. In this book, therefore, inoculation is restricted to mean
the in-
troductioa of microarganisms intio the soil or other substrate for the purpose
of
increasing crop production. How best to accomplish this inoculation has been
the
subject of numerous investigations. At present two general methods are in
use:
(1) the soil transfer method, the scattering on a field of some quantity
of bacteria-
laden soil taken from a field where a previous leguminous crop of the same
"cross-inoculation" group has borne nodules; (2) the pure or artificial
culture
method, the actual application of cultures of the desired bacteria to the
seed or
soil. The applicability of these two methods, their advantages and disadvantages,
will be considered in this chapter.
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