Soybean Inoculation Importance

We’ve grown soybeans in EKS long enough the nitrogen fixation process is ‘old hat’ and we give it little thought. It’s an important process, however, and one deserving of a little attention. Two quick thoughts:

1. The nodule forming bacteria (Bradyrhizobium japonicum) on soybean roots that fix atmospheric nitrogen and supply it to plants – like the soybean itself – are non-native to the U.S. Unless they were previously introduced into a soil, they likely are absent – thus the reson we inoculate soybeans in fields where they haven’t been grown before.
2. Soybeans require alot of N. 60 bpa beans require around 300 pounds. While most is supplied via N fixation in root nodules, that process may only provide 40-80 percent of the plant’s N needs for the season with the rest coming mostly from the soil profile. It’s a complex process influenced dramatically by the residual or mineralized N available in the soil profile and by stress (drought/heat) conditions which can inhibit N fixation.

When everything works like it should, we don’t think about it. When it doesn’t, we can see issues.

Take a field with good N levels (applied or residual). If the bacteria don’t have to work for N, they won’t. Excess soil N resulting in reduced nodulation can result in soybeans looking good early yet end up being N deficient during seed fill. Heat/drought stress can also reduce nodulation, limiting what the plant can do. Soils flooded for a period of more than a week can see issues. Sandy soils and low pH soils might be more susceptible to lack of N production from nodulation as well. All can affect the ability of the plant to maximize production based on the N available to it.

How do we guard against N issues?

1. Inoculation at planting as insurance against reduced nodulation has grown in popularity: https://eupdate.agronomy.ksu.edu/article/inoculating-soybeans-is-a-good-insurance-policy-636-3
2. We don’t typically make N applications a part of our fertility program (unless we’re looking at rescue treatments…) but in most cases, up to 40 pounds of N/A can be applied as a starter to help get soybeans started without a detrimental effect on nodulation during the growing season.
3. Understanding when issue may arise is the first step and can help us make good management decisions to maximize production.

The nitrogen fixing nature of legumes is one of the great things about growing them – but it doesn’t mean it’s not a complex process. Even so, soybeans have proven to be an amazingly productive part of our crop rotation. With any luck, nodulation will be the least of our growing season worries.