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Soybean can obtain between 50 to 70% of its nitrogen (N) requirements from the air when N-fixing bacteria have established functioning nodules on their roots (Figure 1).1 However for this process to occur, adequate populations of N-fixing bacteria (Bradyrhizobium japonicum, in the genus Rhizobium), must be in the soil or applied to soybean seed.
Figure 1. Soybean nodules (red arrow) at V2 growth stage.
As a legume, soybean have the ability to live in a symbiotic (mutually beneficial) relationship with a specific bacteria. The rhizobia “fix” nitrogen from the air into ammonia, which can be used by the soybean plant. In return the soybean plant provide carbohydrates to the bacteria. B. japonicum is specific to soybean and will not fix nitrogen in other legume species. Likewise, the rhizobial species that fix nitrogen for alfalfa or other legumes will not nodulate and fix nitrogen on soybean.2
Shortly after emergence, nodule formation can be observed on the roots, but active nitrogen fixation does not begin until about the V2 (second-trifoliate) to V3 (third-trifoliate) growth stages. Prior to flowering, there should be 8 to 20 large (about 1/16th to 1/8th inch) and active nodules per plant. The number of nodules per healthy plant (several hundred) and the amount of N fixed is maximized around the R5.5 (beginning seed) growth stage.1 New nodules are formed during most of the growing season, usually ending during the pod-filling growth stages. Active nodules have an internal pink color (Figure 2) and remain active for 6 to 7 weeks before they begin to break down. Nodules that have a black internal color are not functioning and are considered dead.
Figure 2. Cross-section of a healthy soybean nodule.
The application of N fertilizer to a soybean crop is not recommended as it generally does not increase yield potential.4 The greater the supply of N in the soil, the less N fixation occurs by the plant. As the amount of soil applied N increases, the number of nodules decreases and the bacteria become less active. Interestingly, even though soybean plants remove a significant amount of N from the soil, yield does not increase proportionally with increasing N applications.4 Soybean planted in fields with excessive residual nitrate should be closely monitored. If nodulation has been severely inhibited and N deficiency symptoms appear, additional N during pod fill may be helpful. Research has shown conflicting results regarding late-season N applications, with the greatest success occurring when N was applied via irrigation systems in high-yield situations.5
Most universities guidelines suggest planting inoculated soybean seed in fields that have not grown soybean for the previous three to five years.2,4,6
The probability of a yield response to inoculated seed in fields with a recent history of soybean is extremely low. An eight-year study (2000-2008) testing 51 inoculant products in 73 experiments conducted in Wisconsin, Iowa, Indiana, Minnesota, and Nebraska resulted in an average yield response of 0 bu/acre.5 Some highlights of the study are as follows:7
The following conditions are most likely to cause poor nodulation in the field resulting in reduced N fixation:
Sources: 1 2008. Seed inoculation. Integrated Crop Management. Iowa State University. https://crops.extension.iastate.edu/soybean/production_seedinoc.html
2 Conley, S.P. and Christmas, E.P. 2005. Utilizing inoculants in a corn-soybean rotation. Purdue University. https://www.extension.purdue.edu/
3 Pedersen, P. 2003. Soybean seed inoculation. Integrated Crop Management. Iowa State University. https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=2674&context=cropnews
4 Pedersen P. 2009. When to inoculate soybean seed in Iowa. Integrated Crop Management. Iowa State University Extension.
5 Larson, K., Rice, C., and Roozeboom, K. 2012. Ensuring successful soybean nodulation in fields without established rhizobial populations. Agronomy e-Updates, Number 344. Kansas State University Extension. http://www.agronomy.k-state.edu/documents/eupdates/
6 Mueller, N., Elmore, R., and Shapiro, C. 2015. Soybean inoculation: When, where, and why. CropWatch. University of Nebraska-Lincoln.
7 De Bruin, J.L., Pedersen, P., Conley, S.P., Gaska, J.M., Naeve, S.L, Kurle, J.E, Elmore, R.W., Giesler, L.J, and Abendroth, L.J. 2010. Probability of yield response to inoculants in fields with a history of soybean. Crop Science 50:265-272. Crop Science Society of America. https://dl.sciencesocieties.org/publications/cs/pdfs/50/1/265
8 Staton, M. 2014. Identifying and responding to soybean inoculation failures. Michigan State University.
Web sources verified 04/25/18. 140304060139