Field pea root nodules can form as early as 14 days after emergence. However, proper nodulation assessment should take place from the 6th to 9th node stages or at R1 (beginning bloom) to capture peak nodulation.
Pea nodules are more oval-shaped than soybean nodules and tend to branch out as they develop. This branching makes pea nodules more difficult to count than other types of root nodules. Therefore, a scoring system is used instead:
- Select 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.
- Assess overall plant growth and vigour, count the number of nodule clusters per plant and note the position of nodule clusters on the plant, according to the worksheet below (adapted from 20/20 Seed Labs Inc.).
- Cut a few nodules open to assess the colour. If the nodules are pinkish-red inside, they are actively fixing N. Brown, white or green nodules are considered ineffective.
- Conduct nodule assessments on additional plants if there is a wide variation across locations and plants.
Source: 20/20 Seed Labs Inc.
What is considered adequate nodulation?
Peas have the ability to fix approximately 55% of their N requirement if they nodulate adequately. Following the previous nodulation scoring system:
- A score of 11-13 means effective nitrogen fixation and action is required.
- A score of 7-10 means the pea crop is fixing N, but not enough to secure 55% of the total N requirement.
- A score of 1-6 means rescue N treatment is required. Re-evaluate the inoculation strategy used, including products and placement, and any potential environmental impact on nodulation efficacy.
Rescue Nitrogen Treatment
In the event of nodulation failure, rescue N can be applied at the 9th to 12th node stages. 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.