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Soybean Inoculants
Soybean has the ability to fix nitrogen from the atmosphere if properly nodulated. Nodulation requires the inoculation of the seed prior to planting with certain species of rhizobium bacteria specific for soybean. Or, if the field has a previous history of soybean production, the rhizobium bacteria may already be present in the soil. Seed inoculation is highly recommended for North Dakota soils with no previous history of soybean planting. Inoculation of seed for fields having prior soybean history is suggested if new strains of bacteria are to be introduced.
Several types of inoculum carriers are currently available for soybean seed treatment. One of the oldest and very common are the peat-based materials. Peat is a good carrier for soybean inoculum but does not adhere to the seed as well as some other materials. For best results, peat-based materials should be applied as a slurry and mixed with the seed, particularly when planting into ground without a previous soybean history.
Clay-based materials can be applied dry and will stick to the seed. Clay-based materials become sticky when wet and can plug planters. Planter boxes should be left empty and cleaned each night when clay based inoculation products are used.
Granular inoculation materials can be applied separate from the seed through the granular insecticide hopper. Granular materials are more expensive but are usually very effective and are recommended for new soybean land.
Some liquid materials containing molybdenum plus a fungicide plus soybean inoculum are also being sold. Most fungicides reduce the viability of inoculum and the additive of molybdenum is generally not considered necessary for North Dakota soils. Soybean inoculum is more tolerant to thiram fungicide than to most other fungicides, however the length of time inoculum can survive in contact with thiram is not known. The advantage claimed for the liquid materials is the ease of application with a slurry seed treater. Soybean seed treated with carboxin should not be inoculated more than four hours prior to planting.
Some important factors that affect nodulation are the viability of the inoculum, the amount of rhizobium on the seed, and the soil nitrate level. The viability of the soybean inoculum depends on storage time and storage conditions. Many commercial inoculum containers list the expiration date for the inoculum. Inoculum should be refrigerated or stored in a cool place prior to and after purchase. Inoculum is easily killed by direct sunlight so exposure of inoculated seed to sunlight or excessive heat should be avoided.
If peat based inoculum is used, a slurry should be made with materials such as sugar water or skim milk as a carrier. The slurry inoculum carrier should then be thoroughly mixed with the seed as close to planting time as possible. This method will greatly increase the number of rhizobium sticking to each seed.
Good nodulation usually will not occur in fields under extreme dry conditions or with high levels of residual soil nitrogen. Available soil nitrogen will be used in preference to the formation of nodules. Even so, seed planted in fields high in soil nitrate N should always be inoculated to provide a source of rhizobium for future crops.
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