Soybean plants have the ability to form a symbiotic relationship with nitrogen-fixing Rhizobia bacteria, which cause the small swellings on the root system called nodules. The soybean plants are developing rapidly and nodules will start to form shortly after plant emergence. If the plant is around 6 inches tall with the first or second true leaves (trifoliolate leaves) unfolded, nodules will start to actively fix Nitrogen (N) gas into plant available N. The number of nodules will increase until the R5 (the beginning of the seed formation) stage or slightly beyond. In Figure 1, the average number of soybean nodules per root for the same treatments at different environments is shown, with the seed inoculated at all locations. The nodule counts were different per site indicating variability between various environments. The second count, at the R4 growth stage, had a larger number of nodules compared with the V4 growth stage.
To evaluate if N fixation is taking place, cut open a few of the nodules and inspect the color inside. Nodules actively fixing N2 have a pink or reddish color on the inside while white or brown color indicates no N-fixation is taking place. High residual N content in the soil or application of N fertilizer will result in fewer and smaller nodules.
Mid-June 2017, 75 soybean fields of producer-participants in the North Dakota soybean survey were evaluated for nodulation. In each field, three sample sites were visited and 2-4 plants were gently dug out with a spade and roots were cleaned in a bucket of water. The growth stages of the plants were recorded and nodules were counted. The average number of nodules are indicated in Table 1. As expected the average number of nodules per plant is increasing from VC (Cotyledon) stage through the various growth stages. During the early part of the season, some of the observed plants did not yet have nodules developed. It is important to scout fields and to make sure the soybean plants are properly nodulating.