Crop Nutrition

Soybeans can meet most of their nitrogen (N) requirements through biological N fixation, eliminating the need for N fertilizer.

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Fertility

Inoculant

Soybeans can meet most of their nitrogen (N) requirements through biological N fixation, eliminating the need for N fertilizer. Inoculation with products containing compatible rhizobia (Bradyrhizobium japonicum) will facilitate N fixation. Liquid inoculants are placed on-seed and/or in-furrow and granular inoculants are placed in-furrow with the seed. These products can be applied as double or single inoculation. The inoculation strategy will depend on field history, equipment compatibility and cost.

Double Inoculation

Double inoculation is the use of two inoculant formulations or placement techniques. A typical strategy would include liquid on-seed plus granular or liquid in-furrow. Soybean rhizobium is not native to Manitoba soils, so double inoculation is ideal for fields with little to no soybean history. Research in Manitoba has shown an average yield increase of 10 bu/ac and increased protein for inoculant treatments over the untreated control at sites with no history of soybeans.

Single Inoculation

Single inoculation is the use of one inoculant formulation or placement technique. Once introduced to the soil, rhizobia can survive in the soil for many years. Therefore, the amount of applied inoculant can be reduced once soybeans have become established in a crop rotation. If considering single inoculation, follow the checklist below:

From 2013–2021, 48 replicated On-Farm Network field trials in eastern Manitoba compared double vs. single inoculation in soybean fields that had seen at least two previous soybean crops. To date, a statistically significant economic yield response to double inoculation occurred in two out of 48 sites (4%). These findings are consistent with other growing regions and indicates that single inoculation is more economical for fields with a longer soybean history.

No Inoculation

In addition to meeting the single inoculation checklist criteria, single inoculation vs. no inoculation can be tested in soybean fields. Research from other soybean growing regions has pointed to a declining need for inoculant on an annual basis. However, validation is required in Manitoba and is currently being investigated through the On-Farm Network.

From 2016 to 2021, 36 replicated On-Farm Network field trials compared single vs. no inoculation in soybean fields that had seen at least three previous soybean crops, with the most recent grown in the past four years. To date, no significant differences have been found between single vs. no inoculation. This trial is continuing.

Additional Resources

Assessing soybean inoculant strategies – Pulse Beat: Science Edition article, MPSG,  Y. Lawley (U of M), S. Chalmers (WADO), Craig Linde (CMCDC), J. Frey (PCDF) and N. Hari (PESAI)
Single vs. no inoculation – Pulse Beat: Science Edition article, MPSG On-Farm Network
Nutrient uptake and partitioning by soybeans in Manitoba – Proc. Manitoba Agronomists Conference, J. Heard (Manitoba Agriculture), G. Bardella (U of M), K. Podolsky (U of M)
Rescue nitrogen applications on poorly nodulated soybeans in Manitoba – Manitoba Agronomists Conference Poster, John Heard
Nitrogen and soybeans: friends, foes or just wasted fertility? – Proc. Manitoba Agronomists Conference, J. Heard, J. Lee (Agvise) and R. Tone (Tone Ag Consulting)
Phosphorus fertilizer management for soybeans – G. Bardella and D. Flaten (U of M)
Phosphorus balance calculation for a rotation (interactive calculator) – J. Heard and D. Flaten

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