Soybean plants can capitalize on 50% to 75% of their nitrogen (N) requirements from the air when plants have established, functioning root nodules (Figure 1). Nodules form on the roots of soybean plants and house the nitrogen-fixing bacteria Bradyrhizobium.
For N fixation to occur, Bradyrhizobium japonicum bacteria must be established in the soil. The process works through a symbiotic relationship between soybean plants and the Bradyrhizobium species within the nodules on soybean roots. The plants receive N while providing the bacteria carbohydrates.
The first nodules can form within a week after emergence and become more visible as they become larger. Functioning nodules have a pink to reddish color when cut in half because of the presence of leghemoglobin. Nodules that are black are dead and are no longer fixing N. Nodules can actively fix N in the V2 to V3 growth stages. Soybean N demand peaks during seed development through maturity (R5 to R8 growth stages). Nodules that are white or green internally may not have started to fix N. Typically, a well-nodulated plant has 10 large nodules on the main stem at flowering (R1).
Bradyrhizobium can survive for a few years in the soil; however, certain environmental conditions can impact surviving populations. It is a cheap and solid insurance policy to inoculate, especially in the following conditions:
A soybean crop can require 300+ pounds of N per acre. Ensuring nodulation occurs on your soybean fields should be a priority this planting season, especially with the winter and previous year’s conditions.