| SHORT ANSWER: Yes, alfalfa seed should always be inoculated with
the proper Rhizobium bacteria. Inoculation is easy and inexpensive and provides
good assurance that plants will fix nitrogen. EXPLANATION:
Alfalfa seed should always be inoculated with live rhizobium bacteria (Rhizobium
meliloti), specific for alfalfa. The only time producers do not need to inoculate is
when planting preinoculated seed before its expiration date.
In some fields with recent histories of alfalfa or sweetclover, alfalfa
may not benefit from inoculation, but it is difficult to identify those fields before
planting. After emergence, inoculation is risky, difficult, and expensive.
Many strains of rhizobia are present in soil, and some may form nodules
on alfalfa roots, but not all nodules fix nitrogen. Effective N-fixing nodules on alfalfa
are generally pink to deep red on the inside and can fix several hundred pounds of
nitrogen per acre each season.
Specific rhizobium bacteria for alfalfa assure effective nitrogen
fixation by new stands. Together they convert atmospheric nitrogen into a form available
to plants if effective nodules are present. Nodules are the result of an infection by an
effective strain of bacteria (Rhizobium meliloti).
Inoculating alfalfa seed: Inoculation is nothing more than
mixing bacteria with the seed before planting. Dry inoculant can be mixed with seed, but
it is difficult to assure good mixing and to avoid much of the inoculant from falling on
the ground some distance from the seed. Certain brands of alfalfa seed are pre-inoculated
(with or without lime coating) and do not need additional inoculant.
Stickers help nodule forming bacteria adhere to seed. Excellent
commercial preparations of stickers and rhizobium are available from inoculant
manufacturers. For best results, follow the manufacturer's instructions.
An alternative to commercial stickers is to use milk or a water-based
solution containing 10 to 20 % table syrup or sugar. One pint of the sticky mixture
provides sufficient sticker for a bushel of seed. Moisten all seeds (in a concrete mixer,
if possible), then add the inoculant. If the mixture is too moist, add more inoculant,
finely ground limestone, or powdery dry soil.
Shelf life of inoculants: Expiration dates appear on inoculant
packets and pre-inoculated seed bags. The date indicates when most of the bacteria will
have died under normal storage conditions. Properly inoculated seeds have thousands of
bacteria per seed; however, only one bacterium is needed to infest a seedling's root.
Bacteria on seed in hot soil die, a few at a time. Nevertheless, even after 2 or 3 weeks,
there are usually enough live bacteria remaining to be effective.
Heat, direct sunlight, and drying are all detrimental to the survival
of rhizobia. For this reason, it is important to store inoculant packets in a cool place
(refrigerate if possible). Even with the large amounts of rhizobia initially applied, many
bacteria may die during prolonged storage.
If there are questions about the viability of bacteria on inoculated
seed due to the length of storage or storage conditions, then seed should be reinoculated.
Do not use water to moisten lime-coated seed. Fresh inoculant can be applied with mineral
oil as a sticker. About 1/2 ounce of mineral oil per pound of seed is
sufficient.
Cost/benefit of inoculation: Inoculant is normally included with
the higher priced seed. Cost for preinoculated seed is 5 to 10¢/lb more than comparable
raw seed. Inoculant costs about $1/A, if applied by the producer.
All legumes (including alfalfa) can establish and survive without
nitrogen-fixing bacteria, but plants cannot fix nitrogen. This means that without added
nitrogen, plants will have a yellowish appearance and yields will be low. With high rates
of nitrogen fertilizer application, uninoculated stands can be productive.
If nitrogen fertilizer is applied to alfalfa for normal growth and
quality, 200 - 400 lb. actual N/A would be required, costing $50 - $200/A. Since
inoculation is easy and cheap, it is probably the best insurance farmers can buy. |
