Assess soybean root nodulation each year in every field, regardless of the field history or inoculant strategy. The ideal time to assess nodulation is at the R1 (beginning bloom) stage.
Assessment at this stage will give a clear picture of whether or not a crop has sufficient ability to produce its own nitrogen (N) via biological N fixation.
The critical period for adequate N supply to soybeans is R4 (full pod) to R5 (beginning seed) for maximized yield.
- Assess root nodules on a 5-10 plants from 2-3 different areas that are representative of the field.
- Use a shovel to gently dig out each root system. Soak the roots in a pail of water to remove soil, as nodules can easily be stripped from the lateral roots during extraction.
- Count the number of nodules per plant. If there is a wide variation between nodule counts, select more plants for an accurate estimate across the field.
- Cut a few nodules open to assess the colour. If the nodules are pinkish-red inside, they are actively fixing N. Leghemoglobin in the nodules, like hemoglobin in blood, is produced by rhizobia bacteria during N-fixation and changes colour when exposed to oxygen.
What is considered adequate nodulation?
Results from a recent MPSG-led study evaluating various inoculant products, rates and formulations found that soybean plants need at least 10 nodules per plant to reach 90% of maximum yield (Figure 1) (MPSG 2018).
Rescue Nitrogen Treatment
If the crop appears yellowish-green in colour and nodules are either absent or not actively fixing N, a rescue application of N may be necessary at R2 (full flower) to R3 (early pod). Broadcast granular N or direct liquid N below the crop canopy at 50 lbs N/ac. Note that liquid N contact with leaf material can burn the foliage and reduce yield.