When Should You Use a Soybean Inoculant Product?
A soybean inoculant product should be used when planting soil conditions are such that you believe the soil populations of Bradyrhizobia are low, or you have reservations about adequate levels of native rhizobia to provide sufficient numbers to nodulate the soybean plant.
Legume inoculants should be used if one or more of the following soil conditions exist:
- Soil that has not hosted the specific legume
- Soil that has not hosted the specific legume for more than 3 years (CRP land, for example)
- Soil pH of less than 5.8 (The pH should be adjusted by liming prior to inoculation)
- Soil pH of more than 8.5
- Soil organic matter of less than 1%
- Drought or flooding
- Topsoil conditions exceeding 80ºF
- Soil erosion
- Use of soil treatments and chemicals injurious to soil bacteria and inoculants
As technology and genetic selection of Bradyrhizobia strains improve, newer and more efficient strains of Bradyrhizobia should be used to replace native strains which may be residing in the soil. It has been suggested that Bradyrhizobia become less effective as they reside in the soil for extended periods. Thus, there is the potential benefit of adding high levels of newly selected strains of Bradyrhizobia each time a legume is planted by using commercial inoculant
products. It is important to note that the presence of a nodule does not assure nitrogen fixation. Native rhizobia become ineffective over time, but will still nodulate the legume. You can evaluate nodules by examining the color at different times during the growing season. Active nitrogen fixation is indicated by a pink to beef-steak red color in the nodule tissue.
It is important to remember that inoculants are LIVE product and fragile:
- Store product under 77ºF
- Do not allow product to freeze
- Do not expose treated seed to direct sunlight – use tarps
- Store product out of sunlight and direct heat
- Do apply product with an atomization applicator (rupture of cells)
- Read and follow the labels
Environmental factors can cause poor nodulation:
- Non-optimal soil pH
- Floods and droughts
- Increasing number of years since soybeans were last planted
- Deep planting
Floods and droughts
Fields flooded for more than one week decrease nitrogen fixation and survival of bacteria because bacteria need oxygen. Sandy soils and droughts in years when soybeans are not being grown also can also lower populations. Flooded ground is dependent on various factors: CEC, OM, Tile, Rotation, Amount of rainfall during season. Flooded ground can displace rhizobia from the zone where seed is placed in the soil by leaching, making the upper levels in the soil devoid of populations of rhizobia.
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