This spring, North Dakota growers have planted several annual legume crops, including field pea, chickpea, lentil, soybean and dry bean. Legumes form visible nodules on the root hairs of the primary and lateral roots, two to three weeks after plant emergence (see photo).
Legume species-specific bacteria form a symbiotic relationship with the legume roots. The resulting nodules contain the bacteria. These bacteria can biologically fix nitrogen (N) from the air, making it available to the plant.
If there is abundant N already available in the soil, the plant will not nodulate properly. It is important to dig up some roots and check a field to see if nodules are present.
To check roots, use a small spade or gardening tool to dig up the plants. Do not pull the plants out of the ground as this will cause nodules to be ripped off the roots and therefore the nodule observation will be misleading. Bring a small bucket of water into the field and wash off the soil from the roots.
Check roots in several locations within the field and examine roots of a few plants per location.
Healthy and actively N fixing nodules have a pink or reddish inside color. Nodules that are white, brown or green inside do not fix N.
If the legume plant does not have nodules or the nodules are not healthy and discolored inside, the plant may show yellowing due to a lack of N, assuming there was limited residual plant available N. Under normal conditions, N fertilization is not recommended for most legumes, but If the yellowing is indeed due to lack of N, a rescue top-dress application with N may be warranted.
At the time of digging roots, it is also suggested to check the health of the root system and evaluate if there are root diseases present. Diseased roots will have lower nodule numbers. In addition, in saline areas of fields, or in soybean fields with iron deficiency chlorosis (IDC) lower root nodules per plant is expected.
Although seed may have been inoculated with the right bacteria species, satisfactory nodulation is not a guarantee. Environmental conditions such as drought or excess moisture after planting may result in poor nodulation.
In fields where poor nodulation is observed, it is important to inoculate the seed the next time the same type of legume is planted in the same field.