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FutureStarrLegume Inoculant OOR
Many studies have been conducted on the application of commercial inoculants into soils that already contain the correct rhizobial bacteria. In some studies, a significant yield increase has been observed. In other studies, no response occurred. Acid soils often have poor survival of the rhizobia. One way to evaluate the response of inoculants is to test several inoculants and an untreated control in fields using replicated strip tests. When in doubt about the rhizobial population in a field, it is a good practice to apply inoculum, especially if the legume has never or not recently been grown in that field.Commercial inoculants are composed of rhizobial strains selected for maximum fixation potential. However, even when more efficient strains are introduced into the soil, there is no guarantee these strains will compete well with native strains for entry into plant roots.
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. 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.
Inoculation is recommended when the field has no past history of growth of your particular legume, or when you have a high value crop for which you want to ensure successful growth. Often, inoculant rhizobia can remain viable in the soil without the presence of a legume for years, and then be ready to form nodules when its host plant is sown. Field history that includes a legume can increase the soil rhizobia population and result in improved nodulation (Mothapo et al., 2011). Specifically, inoculation is recommended if the field has been out of host plant production for 3–5 years, or never planted to the host. Further, inoculation can help increase rhizobia populations in fields with unfavorable environmental conditions for the bacteria's long-term survival, such as pH below 6.0, extremely sandy soils, or periodically-flooded conditions. Past history that includes a diversity of legume species—common in organic systems—has been shown to increase the diversity of rhizobia types present in the field (Grossman et al., 2011). (Source: eorganic.org)