Soybean Seed: To Inoculate or Not To Inoculate?

It’s that time of year again…  Harvest is over. The holidays are upon us, and once more, it’s time to turn our attention to the next crop year!  During this crop planning time, I am frequently asked if I recommend using an inoculant on soybean seed. The answer is more complicated than a simple “yes” or “no.”

Proper nitrogen fixation is absolutely crucial to obtain high soybean yields. For this to happen, nitrogen-fixing bacteria (specifically Bradyrhizobia japonicum) must be present in sufficient quantities in the soil.  This bacteria stimulates plant roots to produce nodules that absorb nitrogen from the air and convert it into ammonia, which is a useable form that the plant can then take up through its root system. This nitrogen is then used to make amino acids, which are the building blocks for proteins, and also chlorophyll.  Chlorophyll is the molecule that converts sunlight, an energy source, into carbohydrates through the process of photosynthesis. Researchers estimate that between 50 and 80% of the nitrogen needed to produce a normal soybean crop can be provided in this way.

Active fixation usually starts at the V2 stage.  From there on, more nodules develop and the amount of nitrogen that is fixed continues to increase.  The highest demand for nitrogen begins when seeds begin to form in the pods and continues through full maturity.

So when should farmers use inoculants?  That’s always a tough question to answer since each crop year is different and there are literally a 1,000 variables that can factor into the decision.  Let’s start with the “no-brainers.” I would use an inoculant when:

  • Soybeans have NEVER been grown in this field.
  • Soybeans have not been in the field for three years.
  • You are planting into soils that have a pH of 6.0 or lower.
  • You are planting into high pH soils. I would use an inoculant at a pH of 8.3 and above.
  • Your soil has a high sand content. Rhizobia bacteria do not survive as well in sandy soils as they do in soils with high organic matter content.

Times when an inoculant might be recommended include if a field was flooded the previous year for more than a week and also in cool, wet soils like no-till fields. The downside of not using an inoculant when one is needed can be as much as a 50% loss in yield! For the cost, inoculants can be a great insurance policy.

Feel free to call me with any questions related to this or other crop production practices.