Soybeans, like other legumes, have the unique ability to form a symbiotic relationship with nitrogen-fixing bacteria, primarily Bradyrhizobium japonicum, colonize the plant's roots and help form nodules that turn atmospheric nitrogen (N₂) into ammonium (NH₄⁺), a form plants can use to grow and thrive.
This adding of Bradyrhizobium inoculant is a natural nitrogen-boosting process which is especially important in nitrogen-poor soils like those found in Western Canada where native Bradyrhizobium levels are low or even non-existent.
Inoculant Types
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Peat-based or liquid inoculants: Applied on the seed or in furrow, making early contact with the main root.
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Granular inoculants: Placed in the furrow at planting. These place rhizobia within the planting furrow and encourage nodules to form on both main and lateral roots.
Why Adding Rhizobia Matters Year Over Year
Our soils do not contain native Bradyrhizobium and thus soybeans rely on the introduction of commercial inoculants in order for the plants to fix nitrogen. Studies have shown that introduced populations decline over time and have a rapid rate of genetic change in the soil.
Bottom line is that the commercial inoculant that was researched to have the best Bradyrhizobium strain will no longer be the same in subsequent years. Nodules may form but they may not be effective or efficient at fixing nitrogen. This is why it is recommended to use a commercial inoculant every year with your soybean crop.
Benefits of Successful Nodulation
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Better nitrogen uptake means stronger growth and more pods.
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Higher yield potential and improved protein levels.
When to Double Inoculate and Why
Using two inoculant formulation types boosts the odds of good nodulation, especially when conditions are tough (dry weather, sandy soils, or if you’re planting soybeans for the first time). More rhizobia and more root contact points mean a better shot at success.
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New soybean fields: No natural rhizobia to help with nodulation.
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Previous inoculation didn’t work: Maybe the last attempt wasn’t effective, or weather got in the way.
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Long breaks between soybean crops (4+ years).
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After flood or drought: Harsh conditions hurt soil microbe survival.
Yield Advantage
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Fields planted to soybeans for the first time can see a +24% yield bump (about 7.4 bu/ac) when double inoculated¹.
Best Practices
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Match the inoculant type and application method to your soil and cropping history.
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Granular products tend to do better in dry or stressed soils when compared to peat and liquid formulations under the same conditions.
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Work with your agronomist to make the right call for your fields.
1Indian Head Agricultural Research Foundation (IHARF). Inoculant Options for Pulse Crops. Available at: https://saskpulse.com/resources/inoculant-options-for-pulse-crops/