Understanding Commercial Inoculants

Three basic forms of commercial inocula are solid, liquid and freeze-dried. The most commonly used are solid, peat-based inoculants that can be purchased for seed or direct soil application, according to Colorado State University. Liquid inoculants are available in broth culture or as frozen concentrate. Broth or frozen concentrates usually are mixed with water and sprayed into the seed furrow at planting. Because liquid inoculants must be kept frozen or refrigerated during shipment and storage, their availability through normal distribution channels is limited.

Seed-applied inoculants exist as planter box additives, preinoculated seed and custom inoculants. The planter box additive, where inoculant is mixed with seed in the planter box, is most common. This can be accomplished by applying dry inoculum or a slurry directly to seed. The dry method is least desirable because of uneven distribution and poor adhesion of inoculum to seed. The slurry is prepared by mixing the inoculum with water for better adherence to the seed coat. Seed also may be prewetted before mixing it with dry inoculants. Do not leave dry inoculum in the planter box overnight, or let it get wet from rain or dew.

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Many small-seeded legumes, such as alfalfa, are preinoculated by seed conditioners, distributors and dealers. After conditioning, they apply a sticking agent to the seed, followed by the dry inoculum, or they incorporate the inoculum into a seed coating. Keep preinoculated seed in a cool environment during shipping and storage. Use the seed within one year of inoculation, or reinoculate it prior to planting. Rhizobia cells are living bacteria that must be kept viable until planting.

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Custom inoculation usually is done on the farm or by the seed distributor. This involves application of a nutrient-rich adhesive formulation, followed by a peat-based inoculum. This method assures viable inoculum if seed is stored properly following application and used within one year.

Do not confuse seed inoculation with chemical seed treatment. Most seed disinfectants, including fungicides, can be toxic to rhizobia bacteria. Do not apply inoculum to seeds that are treated with a bactericide, such as streptomycin, unless you use a resistant strain of the rhizobia. Although some rhizobia species are slightly tolerant to certain chemical compounds, inoculating chemically treated legume seed requires special precautions. Check with the inoculum manufacturer regarding combining products. Avoid inoculum products premixed with pesticides or other toxic chemicals.

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