There are a lot of good reasons to assess nodulation in your soybean and field pea crops. Most importantly, we recommend conducting nodulation assessments in fields with no previous history of these crops. It is also important to do assessments in fields where soybeans or field peas have been part of the previous crop rotations.
Nitrogen fixation is essential to the productivity and profitability of these crops. Nodulation can fail or at least be sub-optimal in experienced fields if conditions in past years were inhospitable to inoculant survival. Past years being overly dry or wet, past crops having minimal investment or past crops being widely spaced apart in the rotation can all lead to decreased current nodulation.
So how are assessments done? Let look at it by crop.
SOYBEANS
Assess nodulation in soybeans at R2 or full bloom (Plants have an open flower at one of the two uppermost nodes on the main stem).
Click Here for Soybean Growth Staging Guide!!
Soybean nitrogen requirements peak during pod fill, so assessing nodulation at flowering will provide enough lead time if coming back in with a rescue nitrogen application is needed.
- Dig up 5-10 plants from 2 or 3 different areas of the field.
- If soils are heavy-textured and sticking to the roots, soak roots in a pail of water to loosen up the soil. Nodules can easily be stripped from the roots if tugging out from above ground.
- Count the number of nodules per plant. Research has shown that soybeans need 10 nodules per plant, regardless of size, to reach 90% of maximum yield.
- Cut open a few nodules to check that they are pink-ish red inside, indicating they are actively fixing nitrogen.
- Note the location of the nodules – successful seed-applied inoculant will result in nodules clustered around the main taproot near the crown of the plant, while in-furrow applications may result on nodules further from the crown. Naturalized Bradyrhizobia populations may form nodules further down lateral roots.
If insufficient nodulation is found and the crop appears pale green to yellow, a rescue application may be necessary at R2 or full bloom to R3 or beginning pod (pods are 1/4 inch long at one of the four uppermost nodes on the main stem).
Broadcast granular N or direct liquid N below the crop canopy to avoid leaf burn. Apply at a rate of 50 lbs N/ac.
More Information on Assessing Soybean Nodulation →
PEAS
Assess nodulation in peas at R1 or flower bud (flower buds present at one or more nodes) to R2 or beginning bloom (Flower open at one or more nodes) to capture peak nodulation.
Click Here for Field Pea Growth Staging Guide!!
Peas have indeterminate nodules that branch as they grow, forming globular or cylindrical shapes.
- A scoring card is used to assess nodulation.
- Select 5-10 plants from 2-3 different areas of the field.
- Visually assess above-ground growth and vigour.
- Using a shovel, carefully dig up plant roots. If soils are heavier, roots may need to be soaked before rating to remove the soil.
- Count the number of nodule clusters and note their location (at the crown, near the seed, or on the lateral roots). Break open a few nodules to determine if they are pink inside, indicating active N fixation.
