Soybean Nitrogen Fixation: How Rhizobia Power Soybean Nutrition
Soybeans team up with rhizobium bacteria to meet their nitrogen needs, eliminating the need for applied nitrogen. But the process hinges on timing, soil conditions, and environmental stress.
Why don't farmers apply nitrogen to soybeans? The answer lies in a remarkable biological partnership between the legume and rhizobium bacteria - and it's visible to the trained eye. A field photo comparing early- and late-planted soybeans reveals the difference in nitrogen access through nodulation.
In Greensburg, Ind., Steve Gauck, regional agronomy manager at Beck's, explains how the early-planted edge rows in one field show a deeper green hue and more vigorous growth compared to soybeans planted weeks later. "The dark green, taller soybeans have been in the ground for about six weeks and are actively receiving nitrogen courtesy of nodules on their roots," Gauck says.
These nodules are the result of a symbiotic relationship that forms early in the season when specific rhizobium bacteria infect soybean roots. Once established, nodules draw nitrogen from the air and convert it into ammonium, a form soybeans can use for growth. "That's why soybeans typically don't need applied nitrogen like corn," Gauck adds.
But timing is critical. Soybeans need time to establish the rhizobium relationship and begin fixing nitrogen. "Younger soybeans just don't look as green yet because the process is still getting started," Gauck notes. This delay is especially common during cool, wet springs, which slow down nodule development and nitrogen fixation.
The outside rows of this field (left) were planted about three weeks before the interior. Rain forced the delay. The younger soybeans on the right haven't yet benefited from nitrogen from nodules
Environmental factors matter
Shawn Conley, Extension soybean specialist at the University of Wisconsin, highlights that active nitrogen fixation continues until nearly the end of seed fill (R6). Since active nodules have a life span of just four to five weeks, soybeans must continually produce new nodules throughout the growing season.
That requires both a steady supply of rhizobia in the soil and favorable environmental conditions. Heat and drought stress can hinder late-season nodule formation and function. Gauck recommends planting soybeans slightly deeper than some guidelines suggest, to help nodules remain in cooler, more stable soil layers.
Understanding how soybeans naturally meet their nitrogen needs - and what factors may help or hinder that process - can support more sustainable nutrient management strategies and potentially inform decisions about seed treatment, soil inoculants, and planting practices.
