Soybeans grown on soils without a rhizobia population will use available soil nitrogen, and if soil nitrogen levels are low because of soil type, soil erosion, etc., the symptoms of nitrogen deficiency may occur. Field studies in Northern Michigan showed 45 percent and 23 percent soybean yield increases in 2004 and 2005 where inoculant was used and fields had not seen a previous soybean crop. Soybean plant coloration differences were obvious in this study, as inoculated plots appeared much greener. On other Michigan sites with a history of soybean cropping, soybean seed inoculation increased yield by an average of 1.3 bushels, with significant increases at 6 of 14 sites from 2003 to 2005.
Because most commercially available inoculant products are relatively inexpensive, the 1.3 bushel per acre expected average yield increase will generally cover application costs. Therefore, it is recommended to apply Rhizobium inoculant to soybean seed each time you plant soybeans. This is especially true on sandy soils, where it is more difficult for the Rhizobia to survive from one season to the next.
Also, it is important to inoculate on any soil if soybeans are to be grown for the first time.
Two types of inoculants can be used: seed- applied and soil-applied. The seed-applied inoculate is more effective when mixed with water to form a slurry, which is used to coat the seed. This should be done as close to planting time as possible, preferably within several hours. The soil-applied inoculate may be easier to apply than the seed-applied inoculate, but it is more expensive on a per acre basis. Thus, seed applied inoculant is generally the method of choice for most growers.
In addition to the recommended short time period between seed applied inoculant application and planting, there are other potential challenges associated with proper soybean seed inoculation. Care should be taken in applying inoculant to fungicide or insecticide treated seed. Fungicide and insecticide labeling should be checked to ensure that the particular pesticide product is compatible with Rhizobium inoculant.