Are Your Soybeans Nodulating?

Non-nodulating soybeans in 2015, in 2016 and again in 2017.

I have received a deluge (pun intended) of questions regarding the overall lack of soybean nodulation and general pale green coloration of the crop. As a doctor—a Ph. D.—I prescribe less rain and sunshine. Call me in two weeks if the problem still exists.

Outside of obvious weather-related issues, here are the four most common questions I have received and my responses:

1. Why is nodulation such a problem this year? Abiotic stress such as low pH (≤ 6.0), saturated or droughty soils and cool soil temperatures can negatively impact nodulation (Valentine, et al. 2011). Duzan, et al. (2004) reported that root hair deformations (a physiological precursor to rhizobia infection and nodulation) was 64 and 82 percent of the control when rhizosphere (root zone) temperatures were 59° F and 63°F when compared with 77° F. Results suggest that cool soil temperatures have likely limited the infection sites available for nodulation to occur. This effect may have been exacerbated in no-till or compacted conditions. In short, fewer nodulation sites on the roots mean a greater likelihood of forming fewer nodules.

2. I double inoculated my soybeans on virgin ground. Why is my nodule count low? First, please refer to #1 above regarding abiotic stress on soybean nodulation. Second, remember to read and follow the application, compatibility and planting timing of inoculants. While reading various inoculant labels, I saw everything from “not tested” to “not compatible to plant within hours to weeks to months of application.”

Finally, remember there is a poor correlation between nodule number and N2 fixation, so don’t get overly concerned about nodule count. Nodule efficiency is what matters and you can’t measure that by counting. In short, read the labels. Make sure everything is compatible and your application and planting window is adequate prior to purchasing the product.

3. How long will soybeans continue to put on new nodules? Larry Purcell, Ph. D., crop physiologist at the University of Arkansas, indicated that they can measure very active N2 fixation almost until the end of seed fill. Given the normal life span of an active nodule is four to five weeks, this would suggest that soybeans will continue to put on new nodules (if the environment is conducive and rhizobia are present) until R6 (late pod fill).

4. Should I apply nitrogen (N) to these poorly nodulating soybeans and, if so, how much? My general answer is no and none. Applying N to soybeans beyond a “starter” rate (≤~30 pounds) will lead to a rapid and dramatic inhibition of N fixation (Sinclair, 2004). Though it does not appear that the applied N is directly damaging to the N fixation machinery (nodules), it will reduce or stop fixation. If the soil NO3 levels drop, then N fixation can resume in about a week (Sinclair, 2004).

Over-application of N will shut down whatever rhizobia are actively working. Furthermore, our 2014 and 2015 data shows that a soybean plant takes up 3.75 pounds of N in above-ground tissue per bushel of grain. So, an 80 bushel-per-acre crop removed 300 pounds of N per acre. This does not account for below-ground uptake or nitrogen loss and efficiency from the applied nitrogen. In short, that is tough math for a positive return on investment.

Literature cited:

• Gaspar A, Laboski C, Naeve S, Conley SP. Dry Matter and Nitrogen Uptake, Partitioning, and Removal across a Wide Range of Soybean Seed Yield Levels. Crop Sci 2017. doi: 10.2135/cropsci2016.05.0322
• Dr. Larry Purcell, Ph.D. (personal communication 7/16/15)
• Duzan HM, Zhou X, Souleimanov A Smith DL. Perception of Bradyrhizobium japonicum Nod factor by soybean [Glycine max (L.) Merr.] root hairs under abiotic stress conditions. 2004.Journal of Experimental Botany 2004;55(408):2641-2646. doi:10.1093/jab/erh26
• Sinclair. Improved Carbon and Nitrogen Assimilation. In: Soybeans: Improvement, Production, and Uses. Third Edition. Agronomy No.16. Edited by Bierman HR, Specht JE. 2004.
• Valentine A, Benedetto V, Kang Y. Legume Nitrogen Fixation and Soil Abiotic Stress: From Physiological to Genomics and Beyond. Annual Plant Reviews. 2011;42:207-248.

Shawn Conley is a Wisconsin State Soybean and Wheat Extension Specialist.