Do soybeans need supplemental nitrogen? Will they ever respond if you apply fertilizer nitrogen?

To apply or not apply nitrogen on soybeans is a question being asked by agronomists and farmers alike. So farmers and their teams—if they are agronomists—are doing their own testing to satisfy their curiosity.

Roundtables and conferences on this topic have concluded a couple things:

1. Soybeans require a lot of nitrogen, 4 to 5 lbs. per bushel of yield
2. Yield responses are inconsistent and unpredictable
3. Applying nitrogen is an environmental risk

Researchers continue to explore the practice and try to understand when soybeans need nitrogen, when and how much to apply and what form of nitrogen may give the best response.

Emerson Nafziger reviewed his work on supplemental nitrogen on soybeans during the ILSoyAdvisor webinar he presented on December 1. He acknowledged that the response to supplemental nitrogen was greater in 2015 than normal. These following points summarize his 2015 work:

Response when using a high N rate: 100 lbs. of N for each application time: I was recently in a meeting when we discussed nitrogen responses and it seems that it takes 100 lbs. of N to more consistently generate a response. At 5 lbs. per bushel, that 100 lbs. should support another 20 bushels of soybeans. I’m not sure what is magical about the 100 lbs., but at that rate, an application won’t be economical if it doesn’t bump yield significantly.
Large response in a low organic matter, sandy soil with irrigation: Two factors are at play here, soils with low organic matter or light, sandy soil have little residual or mineralizable nitrogen, yet there is a large yield potential with irrigation. You could predict a possible response in this scenario.
Little response on average to most application times: Nafziger’s results suggest that application timing didn’t make a difference—that is what his results show in 2015. In most cases nitrogen is applied at R3, at pod set and before pod fill. Agronomically that timing makes most sense. But another possible time to apply is before or at planting, because soybeans can respond to some additional nitrogen before fixation truly kicks in, sometime in mid-June.
Planting time and 4X applications produced some increases and some decreases: In irrigated plots at Chillicothe, the check plot produced 63.9 bpa an acre. However, an application at planting produced 86.3 bpa an acre, while four applications at planting, R1, R3 and R5 produced 91.1 bpa. In this trial 100 lbs. at planting gave a 23 bushel gain and was profitable, but a 4x application only produced 5 more bushels at an economic loss.
Inconsistencies: Still no clear proof that high-yield soybeans benefit more from N application. That is the question that looms over us. We predict that 80 bushels of soybeans and higher will require supplemental nitrogen because soil reserves and biological nitrogen fixation probably won’t meet demand.
Soil Resources: Soil nitrogen reserves may be part of the key to inconsistent responses. Nafziger sampled soils and plants at several sites to see if they can find clues to responsiveness.

Nafziger reported his responses in the webinar for sites in Monmouth, Chillicothe, Urbana, Brownstown and DeKalb. When you look over his data you immediately realize inconsistencies apply to the data across sites and timing. But you also realize that sometimes there is a response to the application of extra nitrogen—but it’s not predictable. It’s this unpredictability that makes applying supplemental nitrogen so challenging.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or leave a comment below.