When Nitrogen Makes Cents

Dan Davidson, Agronomist PhD, CPAg, CCA

When does applying supplemental nitrogen on soybeans make sense? Spring applications of nitrogen on early planted soybeans may be the ticket.

Does it pay to apply supplemental nitrogen (N) on soybeans? And if so, what form, what amount, what time, and how much? There has been a lot of interest in applying N on soybeans and it dates to an article published by Salvagiotti, et al.* and the University of Nebraska back in 2008. Although this article excited everyone about the possibility of applying N to increase yield, a decade the later the reality is still fuzzy.

Here is what we know about this practice:

  • Soybeans require 4 to 5 lbs. N per bushel produced
  • Soybeans will fix only enough N to produce about 50 bushels
  • Soil provides residual N reserves (nitrate and mineralizable N)
  • High yield soybean (80 bushel or greater) may require supplemental N
  • Research studying supplemental N applications are inconsistent (works about 50% of the time)

Reproductive applications – still questionable: Most researchers have focused applying N after flowering, and particularly at R3 (pod set) growth stage, when N fixation wanes and N demand skyrockets. Sometimes an application works and sometimes it doesn’t. What most researchers fail to measure is the amount of N fixed by the plant, and the amount of reserve N in the soil, if the yield was even great enough to yield a N response.

Early application – an adage: Most agronomists were taught and have long believed that applying N at planting will suppress nodulation and N fixation by the plant. The reasoning is: why would a plant expend energy to fix and capture atmospheric N from the soil, when it can get nitrate for free? Recent work by Dr. Fred Below, crop physiologist at the University of Illinois, has started to discredit that belief.

Preplant applications: However, one of the most promising windows is a preplant application with a weed & feed at a rate of 30 to 50 lbs. N per acre when soybeans are planted early. Most soybeans don’t begin to fix N until V2-V3 growth states in early to mid-June as the soil environment becomes more hospitable to supporting microbial life. As we move planting soybeans from a traditional May 15 timeframe to an April 15 starting window, soybeans still won’t fix N until June, so you can have 6 to 8 weeks of growth before fixation kicks. Making a light application of N with herbicide will help overcome the N penalty associated with cornstalk decay, and provide an early season source of N to support vegetative growth. You may have already observed a malaise, off-color appearance in soybean at the end of a wet June where N fixation has been delayed by damp and cool soil conditions. Early planted soybeans may experience that same malaise if there isn’t sufficient residual nitrate available.

Dr. Below agrees: “I think that it is a reasonable idea, especially if one can include it as 28 or 32% N along with a herbicide burndown or pre-emerge; since the UAN would contain some nitrate (which we know is an effective immediately available source) and the cost to apply with the herbicide would be reasonable.”

Mercedes Gearhart, agronomist with AMS manufacturer AdvanSix, says that some growers along the east coast apply 100 to 150 lbs./A of AMS (ammonium sulfate) preplant and get 20-30 lbs. N and 24-26 lbs. sulfate to jumpstart soybean growth when soybeans are no-tilled or planted early.

Below adds, “I tend to think that the preplant N helps to get the plant off to a better start, which ultimately helps it to nodulate better and fix more N, and then to close the canopy faster. Of course, whether it is going to increase yield is going to depend on the total level of residual N in the soil, how much and when N is released from soil organic matter through mineralization, and how rapidly and extensive nodulation occurs. I tend to agree that it should work best with early planting no-till into cornstalks. However, more definitive validation work is needed.”

Fortalis® is an essential crop input

Your soybean yields can also benefit from Fortalis®, an advanced crop enhancement foliar spray that works by mobilizing calcium already existing within plant tissues, resulting in higher pod retention. When applied as a tank mix, Fortalis® with a fungicide or insecticide contributes to better plant health and higher yield potential. It’s an essential crop input that you need to get the most out of your crop.

Dan Davidson is a PhD agronomist, part-time farmer and soybean expert from Nebraska who consults with soybean checkoff and industry on crop production, marketing, and product development projects.

*Growth and nitrogen fixation in high-yielding soybean: Impact of nitrogen fertilization. F. Salvagiotti, J.E. Specht, K.G. Cassman, D.T. Walters, A. Weiss, and A. Dobermann. 2009. Agronomy Journal, 101:958-970.

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