Successful soybean nodulation without rhizobia

Stu Duncan, K-State Research and ExtensionFigure 1. The left part of the field had a previous soybean crop, while the right side of the field had not. Soybean planting season is rapidly approaching and many Kansas producers may be planting into ground that is new to soybean production or that has been out of soybean production for a number of years. The recent increase in soybean acres in Kansas has brought along, in some cases, issues and concerns in achieving effective nodulation on ground that has never previously grown soybeans. Fields that do not attain good nodulation likely will not receive adequate nitrogen for growth and therefore will display a lack of vigor, yellowing, and low yields. Figures 1 and 2 contrast soybeans with good and poor nodulation.  

Stu Duncan, K-State Research and ExtensionFigure 2. Individual plants from each part of the field showing the visual contrast between well nodulated vs. poorly nodulated soybeans. Functioning nodules on well-nodulated soybeans will provide up to 50 to 60 percent or more of the nitrogen needed by the plant. A large percentage of this nitrogen is fixed during the plant’s the reproductive stages, which is the period of greatest nitrogen demand.

Usually land that has been in recent soybean production will have adequate Bradyrhizobium japonicum communities present in the soil to nodulate the soybean roots without applying inoculant. However, when planting into “new” soybean ground or ground that has been out of soybean production for many years, as is the case for much of the CRP ground being converted back into crop production, proper bacterial inoculation is critical.

Other situations where proper inoculation is critical would include conditions where soil pH is less than 5.5 or greater than 8.5, where there was severe drought or flooding since the last soybean crop, where substantial soil erosion has occurred, or when the soil contains less than 1 percent soil organic matter.

The company-recommended rates for inoculants often have proven adequate when planting into new soybean ground. However, this is not always consistent, and several cases of inadequate nodulation have been reported in recent years. Often a 2X rate or combination of different inoculant products is used as added insurance for achieving a good bacterial population near the seedling roots for root colonization. Even then, it may take more than one properly inoculated soybean crop before soybean plants nodulate adequately.