Checking the Pulse of Nitrogen Fixation

How much nitrogen is your soybean crop capable of fixing?

Soybeans fix nitrogen. With soybean yields of 40, 50 or even 60 bushels per acre, fixation, residual nitrogen and nitrogen mineralization provide more than enough nitrogen to produce bushels and pounds of protein per acre. But today many scientists and agronomists are beginning to question whether nitrogen is becoming a limiting factor in soybeans as we move into the 70-, 80- and 90-plus bushel range.

Farmers can improve fixation by making sure soil has a near neutral pH, low EC and sodium, and has good structure and aeration so the rhizobia and roots can breathe. They can also inoculate soybean seed every spring. This practice has pluses and minuses and comes with a small cost of $3 to $5 per acre.

Some argue that inoculating seed annually is a good practice and a risk management tool. If you are already planting soybeans in rotation with corn on a regular basis, there is a natural population of rhizobia in the soil to infect roots and form nodules and a symbiotic relationship with soybeans. However, inoculant companies often claim the natural population is lazy and while it can fix nitrogen, levels are low and it can’t withstand stresses such as heat, water, anaerobic conditions, etc. The inoculants you purchase are a fresh, efficient population that can produce more nitrogen. Another reason to inoculate is not every acre in a field will have equal levels of viable populations. Heat, drought and other stress can decimate a population on one acre, but not on another.

Personally, I think that if you want to produce 70- or 80-bushel soybeans or higher, annual seed inoculation is a good practice. University of Nebraska agronomists have voiced their opinion on whether or not to inoculate and recommend growers use a decision process. They suggest following these steps when deciding whether to inoculate soybeans:

• Understand soybean nitrogen (N) needs and the product label
• Place each field into one of four categories to estimate probability of a yield response
  • No previous history of soybeans and inoculation – Probability of a Yield Response: High
  • More than 5 years since soybeans were last planted – Probability of a Yield Response: High
  • Soybeans have been in rotation once in every five years, but there have been poor soil conditions for bacteria survival – Probability of a Yield Response: Moderate to Low
  • Soybeans in rotation once in every five years with good soil conditions for bacteria survival – Probability of a Yield Response: Low
• Use this information to guide your decision

“If you have managed soil pH well, planted soybeans recently, have medium to heavy textured soils, and have not experienced extended droughts or floods, direct your money away ($2-$4 per acre) from inoculating soybeans and move it to other management options that have an improved chance of return.” ¹

To read the full story click here.

^{1 Mueller N, Elmore R, Shapiro C, Rees J, Thompson L. Making Data-Driven Decisions on Soybean
Inoculation. CROPWATCH. University of Nebraska–Lincoln, Institute of Agriculture and Natural Resources. April 6, 2017}

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.