Do High Yielding Soybean Crops Need Additional Nitrogen?

As the envelope on soybean yields has been pushed higher, the topic of supplementing more nitrogen to the soybean crop has been widely discussed.

Soybean Nitrogen Need

Soybean crops need four times more nitrogen per bushel produced than corn.¹ For every soybean bushel produced, 4.8 lb of nitrogen is utilized and over 60% of this need is after R3 growth stage.¹ Therefore, a 50 bushel per acre soybean crop utilizes up to 240 lbs of nitrogen. If we are really pushing yields, a 100 bushel per acre soybean crop needs about 480 lbs of nitrogen.

Soybean Relationship with Nitrogen Fixing Bacteria

So, how does a soybean plant needing so much nitrogen per bushel, yield so well without additional nitrogen applications? Well, that’s because soybean plants are legumes and can capture nitrogen from the atmosphere through a symbiotic relationship with nitrogen fixing bacteria in the soil called Bradyrhizobium japonicum. Through this relationship, the bacteria colonize on the roots of the soybean plant and form nodules. These nodules convert atmospheric nitrogen gas to plant available ammonium.

Soybean plants also absorb plant available nitrogen from the soil. Soils provide additional nitrogen through a breakdown of organic matter through a process called mineralization. Higher organic matter soils have more nitrogen available through mineralization while lower organic soils have less nitrogen available. There may also be additional nitrogen available from the previous crops’ nitrogen applications or residual nitrogen from previous manure applications.

Hidden Hunger

Even though soybean plants can utilize atmospheric nitrogen and uptake plant available nitrogen from the soil, it still may not be enough as we reach new yield levels. In a recent study, it was found that nitrogen supply from soil organic matter mineralization and fixation are not sufficient to fully satisfy soybean nitrogen requirements, especially in high yield environments. This study showed a response to large nitrogen fertilizer amounts was modest and depended on the yield level of the production environment.¹ The response was only 5 bu/acre at a 50 bu/acre yield level but increased to 13 bu/acre at a 90 bu/acre yield level.¹ This study utilized nitrogen rates above economic thresholds to demonstrate the need for more nitrogen to maximize yield, but more work needs to be done to study the best economic rate and timing of nitrogen in high yield environments.

It’s worth noting that great care needs to be taken to not apply nitrogen too early and in excess to encourage nitrogen fixation through nodulation.

James Hasheider
Channel Agronomist

For additional Brady rhizobium information, please visit Do You Know Brady Rhizobium and Factors Influencing Soybean Nodulation.

Source

¹Cafaro La Menza, N., Grassini, P., Specht, J., Brhel, J., Timmerman, A., Whitney, T. and Glewen, K. 2018. Is soybean yield limited by nitrogen supply? CROPWATCH. Nebraska Institute of Agriculture and Natural Resources. University of Nebraska-Lincoln. https://cropwatch.unl.edu/2018/soybean-yield-limited-nitrogen-supply#:~:text=Soybean%20has%20a%20large%20nitrogen,240%20lbs%20N%20per%20acre

Web source verified 10/1/25. 1110_676150