Inoculating Soybeans​

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.

Figure 1. Nodules on soybean roots.

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:

  • Virgin soybean ground. In certain areas of the Midwest, it is not uncommon to find fields without a soybean history. In these conditions, it is a best practice to double inoculate with two different sources of inoculum.
  • Soybean crop hasn’t been grown on the field within the last three to five years. With time, the bacteria population can decrease.
  • Flooded or droughty fields. Bacteria need oxygen; therefore, flooded conditions for a week or more can impact survival rates. Drought conditions, particularly on sandy or coarse-textured soils, can lead to a decrease in microbial populations — that includes Bradyrhizobium.
  • High or low soil pH. Optimal pH for soybean plants is roughly 6.0 to 6.8. Levels outside of this range can decrease nodulation, N fixation and Bradyrhizobium japonicum survival.

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. 


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