Soybeans and Nitrogen Management

Soybeans require a lot of nitrogen, but before applying supplement, recognize yield potential and available soil resources.

We are getting to the point in the season where you may be considering adding nitrogen to your soybeans. My advice – once your yield potential exceeds more than 65 bushels per acre (Bu/A) of soybeans, I would consider adding supplemental nitrogen.

Now, you’re probably thinking to yourself, “Don’t soybeans supply their own nitrogen?” Well, the answer to that question is – it depends. If the yield goal of the soybeans is less than 65 Bu/A, the soybeans typically can produce enough nitrogen to reach the finish line. Wondering what’s so special about this “65 Bu/A” milestone? This is the point where soybeans are no longer able to keep up with the plant’s demand for nitrogen.

Soybean plants can also absorb nitrate from the soil, and there is usually some available nitrogen when a soybean crop follows a corn crop. However, we also need to know the organic matter of the soil. The higher the organic matter in the soil, the more nitrogen the soil can provide to the plant. Generally, depending on rainfall and soil temperature, mineralization per acre can range from 10 to 30 pounds of nitrogen per percent of organic matter.

Once you have decided you will break 65 Bu/A, it’s time to consider a nitrogen application on your beans. Timing is critical, so avoid early season application. If you apply nitrogen too early in the season, the soybean plant will become “lazy” and delay nodulation.

You want to trick the soybean plants into producing nodules which will supply nitrogen to the plant. You could make an application at R3, just as pods are forming. Another option would be a late application at the R5 growth stage. During R5, the nodules are beginning to expire and the plant is not able to keep up with the nitrogen demand.

I would recommend running a tissue test midseason to determine if the plant is nutrient-deficient. Before any application of nitrogen is made, make sure all other nutrients are sufficient, otherwise results will vary.  The example pictured shows us that the plant is indeed nitrogen-deficient, but it also is lacking potassium, magnesium and manganese.

A grower should be shooting for high yields before even considering a nitrogen application. Factors like fertility, variety, and seed treatments are extremely difficult change in-season. Overall, a solid management plan is needed before supplementary nitrogen is considered for soybeans.

Nick Marley is an agronomist and Certified Crop Adviser. He has a B.S. in agronomy and crop science from the University of Illinois and is working on a Master’s in agronomy and crop science at Iowa State University. He is a 2017 Illinois Soybean Association CCA Soy Envoy.