Are there different methods of inoculation?
 Inoculant
cultures may be applied to the seed, may be applied directly to
soil, either in the furrow or below the seed, or may be watered
onto the soil at or after planting. Each method of inoculation requires
that the inoculant be packaged and supplied in different ways, and
will supply different numbers of rhizobia.
Brockwell and Bottomley (1995)
noted that seed inoculation supplies 6 -8 x 1010 rhizobia/ha,
while soil and cover inoculation supplies 5 x 10 11 to
2.6 x 1015 rhizobia/ha.
The conventional method of inoculation is to mix inoculant
cultures with a sticker,
such as corn syrup, sugar solution or milk, then roll the seeds
in the sticker until they are completely covered. A finely ground
peat is used and applied at rates to give a minimum of 103
to 106 rhizobia/seed, depending on seed
size. Seeds must then be allowed to dry for several hours
so they will run through the planter. Only as much seed as can
conveniently be planted in a day should be inoculated at one
time. Farmers have expressed concern at the time and inconvenience
involved at a period of maximum activity, and a number simply
dribble the inoculant onto the seed in the planter box. This
is not desirable when nothing is done to ensure sticking of
the inoculant to the seed. A number of companies now produce
seed inoculant preparations with a sticker already added, or
recommend liquid cultures that can also be sprinkled on the
seed in the planter box.
Where planting conditions are not optimum, or the seed to be planted
has been coated with fungicide or bacteriocide, liquid or granular
inoculants can be directed into the seed furrow before it is closed.
Granular inoculants usually are of a larger peat size than for seed
applied peats to ensure they will flow through tubes in the planter
and not plug it up.
Cover inoculation is perhaps most appropriate for revegetation
settings, where seed is often planted toward the end of the
growing season, and any inoculant would have to remain in
the field over winter, with very significant die-off. Watering
in the spring, with the inoculant suspended in the irrigation
water used, should overcome this problem. Plastic-coated granular
inoculants with low internal water availability could also
help spring nodulation.
Preinoculation is where a seed company contracts to provide already-inoculated
seed. While there are new and proprietary formulations, a traditional
method mixes the inoculant in a very strong sticker (40% gum arabic
or 5% methyl ethyl cellulose) then rolls the seed in very finely
ground limestone or rock phosphate until it is evenly covered. With
care in preparation and optimum storage conditions, pelleted seed
can maintain adequate numbers of rhizobia for periods of up to one
month. The difficulty is in being sure that preparation and storage
methods were adequate. One early study found very few surviving
rhizobia per seed. For this reason, preinoculated seed needs to
come from a trusted manufacturer, and with a minimal delay between
production and planting.
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