Soybean plants have a unique relationship with soil-dwelling bacteria called Bradyrhizobia, which results in the formation of nodules. While growers may be familiar with the “elevator speech” on that topic, soybean nodulation is important enough that it deserves a deeper dive. Let’s spend a little time mapping out how soybean nodules form and the process that occurs within those nodules.

Photo provided by Illinois Soybean Association

Nodules are, in many ways, modified soybean root hairs. Single-celled Bradyrhizobia bacteria lie dormant in the soil profile until a soybean root hair comes near. At that point, the bacteria become active and release chemicals that signal the root hair to change developmentally. The first visible sign of this change is curling of the root hair.

Nitrogen-fixing bacteria then form a channel, or highway, in the root hair. Called an infection thread, this channel allows the bacteria to migrate into the root cortex. The root hair and the bacteria then undergo additional, dramatic changes.

Tissue behind the root hair, and the root hair itself, begins to swell into a callous growth. The vascular system of the root also extends into this callous region. The bacteria enlarge in size. Previously possessing a tail, they shed it and become bacteroids. These bacteroids take up residence in the center of that callous structure, what we call a nodule, and they will begin the work of nitrogen fixation.

The process of nitrogen fixation is an interesting one. The photosynthesizing soybean plant sends a portion of the sugars it produces downward through the vascular system and deposits the carbohydrates in the nodule. Rhizobia bacteroids use these carbohydrate reserves to fuel “the stuff of life.” Rhizobia, in return, contribute something rather important to this symbiotic relationship.

Rhizobia take a portion of the energy received from the soybean plant to feed nitrogen fixation. The bacteroids use an iron- and molybdenum-rich enzyme called nitrogenase to split N2 gas. This gas is drawn in from the atmosphere, torn apart, and reformulated into ammonia. While the plant cannot use N2 gas directly, it can use the generated ammonia for plant growth and development.

Because nitrogenase can be derailed by excess oxygen, a second compound is produced by the bacteria and plant. That compound is called leghemoglobin, and it shields the system from too much oxygen, allowing the important business of nitrogen fixation to continue. It’s an impressive system, intimately tied to soybean productivity and to grower income.

The bacteria responsible for nodule formation are not native.  They were introduced when Midwest growers began incorporating soybeans into their cropping rotation.  If soybeans remain in rotation, the population of those bacteria is maintained.  If soybeans are removed for several years, the population often declines enough that most agronomists recommend reinoculation so that nodules and subsequent nitrogen fixation take place.

If you are interested in the process above but need something a little more visual to map the process, check out the online storyboard produced by clicking here or visit my Twitter/X profile at @matt_montgo.