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Legumes are gaining importance in Manitoba agriculture. One reason is the ability of a well-nodulated legume to produce nitrogen, eliminating the need for commercial nitrogen fertilizer.
Legumes form a unique relationship with rhizobia, a soil bacteria through which nitrogen is made available to the plant. In a process called nitrogen fixation, atmospheric nitrogen is taken from air in soil spaces and converted to a usable form. Since gaseous nitrogen is the most abundant element in the atmosphere and is continually replenished, the supply remains constant.
Nitrogen fixation occurs in the root nodule. Rhizobium bacteria from the soil enter the plant roots and multiply, eventually causing a swelling which forms the nodule. The shape and size of nodule varies with legume species.
Rhizobium requires specific host plants for nitrogen fixation to occur. Bacteria must be able to infect the root and effectively "fix" the nitrogen. Sometimes bacteria infect the root but are not able to fix nitrogen. In other situations, bacteria fix nitrogen but cannot enter the root, so nodulation does not occur.
Inoculation refers to the addition of effective rhizobia to legume seed prior to planting.
All legumes should be inoculated with rhizobia, even if the legume has previously been grown in the soil. Inoculation provides the most effective strains of bacteria for the type of legume.
Commercial inoculum is available for specific legumes:
Inoculation with the proper strain ensures early and effective nodulation and makes nitrogen available to the plant in the early growth stages.
There are three common methods to inoculate seed. The seed should be sown immediately after inoculation.
This method is a great time saver because seed is treated or coated with rhizobia by a seed processor or distributor and can be directly seeded. Successful nodulation, however, depends on proper application and storage techniques.
Once seed is ready to be inoculated, certain management practices may influence the effectiveness of the bacteria.
Commercial inoculum is most effective when used before the expiry date indicated on the label. Inoculum should be stored in a refrigerator or other cool, dark place to maintain its viability.
Inoculated seed should be sown as soon as possible. If this cannot be done the same day, place the seed in plastic bags and store in a cool, dark place for no more than two days. If seeding is delayed longer, re-inoculate seed.
Rhizobia bacteria will not survive in acid soils. Although acid soils are not usually a problem in Manitoba, lime should be applied before seeding where soil pH is low. Some rhizobia strains now available in the alfalfa group can fix nitrogen in acid soils. However, most Manitoba soils require only the regular strains of alfalfa rhizobia.
Inoculated seed should be sown into a moist seedbed as nodule bacteria cannot survive in dry soil. However, too much moisture can also be harmful. Rhizobia need oxygen to survive, and wet soil with reduced soil air levels may cause serious bacterial losses.
Generally, seed treatments used in Manitoba do not seriously harm bacterial cultures, but fungicide-treated seed requires special care. To ensure nodulation occurs, use two or three times the usual amount of inoculum.
Bacterial cultures should not be applied to seed treated with mercury or copper disinfectants, since these chemicals harm the bacteria. Instead, add inoculum to cracked wheat or sawdust and drill into the soil before seeding.
To determine if nitrogen fixation is occurring, dig up a plant and cut a few of the nodules. If nodules are red or pink inside, fixation is taking place.
Proper treatment of bacterial cultures before and after inoculation leads to successful nodule formation. But remember, management practices associated with other crops must also be applied to the legume stand - legumes do not grow by nitrogen alone.