Cool Bean: Soybeans teach us about nodulation

Soybean is unique among major grain crops because it can get much of its nitrogen from the atmosphere through biological nitrogen fixation. That happens through a partnership between soybean roots and rhizobia bacteria. When the relationship is working well, soybean plants form nodules on their roots. Those nodules convert atmospheric nitrogen into a form the plant can use.

But biological nitrogen fixation doesn’t happen at the same level in every field or every year. Soil nitrogen, weather, yield potential, crop growth and stress conditions can all affect how much soybean relies on soil nitrogen compared with fixed nitrogen. That makes soybean nitrogen management a challenge, especially as growers continue pushing for increased yields.

We planted in 2025 a paired set of non-nodulating and nodulating soybean isolines from the University of Nebraska-Lincoln across 10 Wisconsin locations as part of a broader soybean-bioinoculant trial. The larger trial was designed to evaluate whether Bradyrhizobium bioinoculants could increase soybean nodulation, biological nitrogen fixation and yield. For this article, however, we focused on the non-nodulating and nodulating isolines as a tool for understanding how much yield could be supported by soil nitrogen alone.