Interest in inoculating soybeans has increased in recent years. This is due in part to higher soybean prices, improvements in inoculation products, and increased seed dealer capacity to apply seed treatments. It is also a direct result of the unbiased research conducted at land grant universities and disseminated by the Extension system.
Soybean inoculation research conducted at Michigan State University by Kurt Thelen showed that inoculation increased soybean yields on average by 1.3 bushels per acre in fields having a history of previous inoculation use. Similar results have been documented in long-term soybean inoculation trials conducted in Ohio (two bushels per acre), Indiana (one bushel per acre) and Ontario (1.25 bushels per acre).
Based on this information and the soybean market price, soybean inoculation is a profitable practice. The only remaining questions are why does the response to inoculation vary from site to site and what can producers do to improve the performance of inoculants?
There are several factors that affect nodulation and nitrogen fixation (soil fertility status, soil pH, soil compaction, root health, soil moisture and the viability of Rhizobia bacteria). High levels of soil nitrogen have been shown to reduce nodulation and nitrogen fixation. These conditions are common when soybeans are planted where manure has been applied or a legume such as alfalfa or red clover was the preceding crop. Factors such as soil compaction and root diseases that reduce root and root hair development inhibit nodulation. Conditions that reduce the bacterial populations such as heat, excessive soil moisture and drought can also reduce nodulation and nitrogen fixation.
Recommendations for improving inoculation performance:
- Select high-quality inoculation materials.
- Apply a granular or liquid inoculation material to the soil when inoculating a field for the first time.
- Store, handle and apply the inoculation material according to the manufacturer’s directions.
- Have the inoculation material commercially applied by your seed dealer when possible.
- Maintain optimum soil fertility levels for all nutrients except nitrogen.
- Maintain soil pH between 6.0 and 7.2.
- Reduce soil compaction.
- Consider a fungicide seed treatment, but make sure the product and the application method are compatible with the inoculation material.
- Don’t apply fertilizers directly to the seed at planting.