Why are some inoculants prepared using a single
rhizobial strain, while others use multiple strains?
Inoculants sold in the
U.S. and Canada may vary in the number and the type of rhizobia
they contain, and may even include other beneficial soil organisms.
Rhizobial inoculants are to be considered distinct from a number
of "soil conditioners" that are detailed on the web. Many
of these are said to contain rhizobia, but don't specify which is
the target legume.
Most of the inoculants manufactured for sale in Canada and Australia
contain a single inoculant strain
and are intended for use with one specific group of legumes, each
nodulated by, and effective
with, that strain. By contrast,
comparable inoculants produced for the U.S. market will often, but
not always, contain several strains. The Canadian and Australian
philosophy is that should a mutation
occur in the single strain, causing it to lose its nodulating- or
nitrogen-fixing
ability, this will be quickly recognized and can be corrected. That
single strain, selected on the basis of the points listed previously,
will maximize nodulation and nitrogen fixation. The U.S. approach
is that on average, one or more of the strains will work, assuring
at least a reasonable level of nodulation and nitrogen fixation.
Leaving aside the question of mutation, which is infrequent, there
have not been many studies of which approach is likely to give the
better result. Rennie and Dubetz
(1984), with soybean, concluded that a single highly efficient strain
gave better nitrogen fixation than a combination of strains, but
this could vary with the particular strain or strains used. One
additional disadvantage of multiple strain inoculants is in quality
control, where it may be more difficult to determine which strains
are present, and how their numbers vary over time.
A number of producers also sell some inoculants that contain a
combination of strains each for a different legume. These are usually
intended for use in a common setting (for example, a home garden)
where several different legumes may be sown in a relatively restricted
area, or for mixed pasture situations. Such packaging can be convenient
to the home grower or market gardener. Finally, strains of rhizobia
may be mixed with other specific biocontrol or phosphate-solubilizing
organisms. Products of this type are likely to increase in the future,
with the move toward a more sustainable
agriculture. They present no problem provided a) the second
organism added is compatible with the rhizobia present, and b) inoculation
rates are adjusted for any difference in numbers of rhizobia present.
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