The benefit from using rhizobium-containing inoculants is not as clear cut as are the benefits from using fungicide and insecticide seed treatments, according to the Mississippi Soybean Promotion Board (MSPB). We all know that these bacteria must be present in the soil in sufficient numbers for proper nodulation on soybean roots to occur. What we don’t know in such a hard-and-fast fashion is just when to expect an economic or agronomic response when inoculants are applied.
Points to consider in the decision of whether or not to use an inoculant are:
- An appropriate inoculant is cheap, generally less than $3/acre. Thus, cost is not a factor in deciding whether or not to inoculate.
- Do not apply inoculants to gain a yield increase. Rather, apply them to ensure that nitrogen fixation will be sufficient for the crop to realize the yield potential from the planted site.
- Inoculants are generally not compatible with fungicide seed treatments, so inoculant application must be made at planting. This will slow the planting operation.
- There is overwhelming evidence that applying inoculants to soils that have recently been cropped to soybeans provides no yield benefit.
- The cheapness of inoculants warrants their application when soybeans have not been grown recently on a site and the risk of insufficient native soil inoculum is high. The importance of this fact is because there is no option after planting but to apply expensive nitrogen fertilizer to overcome the effects of poor nodulation.
- With the change in cropping systems that is occurring in Mississippi, it is a good idea to inoculate when soybeans are planted on a site that has had continuous cotton or corn or if the site has not been cropped to soybeans in the last 4 to 5 years.
- There is a potential advantage from choosing inoculant products that contain more than one strain of bacteria.
- Results from a study that was planted behind the 2011 flood in Mississippi showed no advantage for applying inoculants even though the flood period was several weeks.
For more information, read MSPB’s “Soybean Seed Treatments and Inoculants.”