Successful soybean nodulation without rhizobia
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).