Topics : Soil Fertility for Forage Crops : Establishment
Download this Fertility at Establishment document in Adobe PDF format.
Inoculation of Legumes
Legumes have the ability through a symbiotic relationship with rhizobia bacteria to fix nitrogen from the air in a form that is available to the plant. This process fixes enough nitrogen to completely meet the needs of the legume for nitrogen. For this process to take place, the plant roots of the legume must be infected by rhizobia bacteria that are specific for each species of legumes. In many soils, rhizobia bacteria are present to infect the plant roots, particularly if the same legume species has been grown in the field in the recent past. However, a general recommendation is that all legumes be inoculated with the proper rhizobia species at seeding. Inoculation is very inexpensive and thus provides good insurance that the plant will have adequate nodulation and thus good nitrogen nutrition. The inoculant must be specific for the legume being planted. Since the inoculant contains living bacteria, the inoculant should be kept in a cool dry place. The best storage place is in a refrigerator. The worse place to store inoculant is on the dashboard of a truck because heat and direct sunlight will kill the bacteria.. Finally, all inoculants have an expiration date. After this date the inoculant may not have adequate live bacteria to do an adequate job of inoculation. Always be sure to check this date before using an inoculant.
Inoculant can be applied in several ways. The most common method is to mix the inoculant with the seed just before planting. A sticker may be used to ensure that the seed is well coated with inoculant. Some seed is pre-inoculated when purchased. The same handling precautions hold for pre-inoculated seed as for the inoculant itself. Another way inoculant is commonly applied is by direct soil application. In this method the inoculant is applied in a granular form through an insecticide or fertilizer box on the seeder. Fluid preparations of inoculant can be directly applied by spraying them in the seed row. Fluid seeding, where the seed is suspended in liquid fertilizer and sprayed on the prepared seed bed, has also become popular. It is important with this method of seeding that the seed not be left in contact with the fertilizer solution too long, because the prolonged exposure can reduce the effectiveness of the inoculation. More information about the nitrogen fixation process and legume inoculation is available at other locations on this WWW site.
Soil pH is extremely critical for this symbiotic relationship between the legume and the rhizobia to be successful. Thus establishing a soil pH between 6.5 and 7.0 at seeding is critical. It is sometimes recommended that nitrogen fertilizer be added at seeding time to take care of the needs of the legume until it is adequately nodulated to meet it nitrogen requirements. Generally, this has been found to be unnecessary. In fact, adding nitrogen from fertilizer or manure can reduce nodulation.
For grass forages some nitrogen, 20 to 40 lbs. per acre, should also be applied at seeding. An additional 30 to 50 lbs. of nitrogen can be applied in the late summer of the seeding year if production warrants. In no-till seedings in sod, such as pasture renovations, no nitrogen should be applied at planting. Nitrogen applied in these situations will stimulate the existing grasses and can provide too much competition to the new seeding resulting in a seeding failure.