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.
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.
Steps can be taken to ensure maximum numbers of viable B. japonicum are present in seed applied bacterial inoculants. These include keeping inoculant in cool, dry storage until seed application, ensuring good seed coverage when inoculating, storing inoculated seed in conditions that will minimize bacterial death (e.g. B. japonicum is sensitive to high temperatures), and planting inoculated seed within the recommended time period for the given inoculant product. The rhizobia are sensitive to lack of moisture. Therefore, dry field conditions may contribute to poor nodulation. Also, high residual nitrate levels in the soil will inhibit nodule formation. It may be a good idea to use a non-legume crop in those situations.
If soybeans are planted into fields where excessive residual nitrate is suspected, monitor the field and be ready to apply additional N during pod fill if nodulation has been severely inhibited and nitrogen deficiency symptoms appear. Past research has produced conflicting results regarding late-season nitrogen applications, but the greatest success has been in high-yield situations (>60 bu/acre) where nitrogen can be applied via an irrigation system.
When planting into a site that has had no previous soybean history, monitor the field for nodule development on the soybean roots to ensure inoculation was effective. Do this by digging up plants at different locations in the field and visually assessing nodulation. Do not pull up plants because nodules likely will be stripped off the roots. Small nodules should be observed on the tap root three to four weeks after planting. The number and size of nodules on the roots will continue to increase until the R5 growth stage.
For adequate nodulation there should be 8-15 functioning nodules per plant by approximately 40 days after emergence. Healthy, functioning nodules will appear pink on the inside when split open. If there is a field that has failed to nodulate, a “rescue” nitrogen fertilizer application will most likely be profitable (see Agronomy e-Update No. 302, June 17, 2011, at: http://www.agronomy.ksu.edu/extension/p.aspx?tabid=58).