Should you be applying nitrogen (N) on soybeans? The quickest answer is no—soybeans are a legume and have the ability to fix their own nitrogen, but everyone knows that!
The more complicated answer is yes—if you are going for top yields. Everyone knows that soybeans have the ability to feed themselves, but what you may not know is that nitrogen production and nitrogen demand in soybeans don’t exactly match up. Nitrogen fixation occurs rapidly from V3 up until right after R1. This is when the vegetative growth begins to decelerate. However, pod growth at this point is on a steep uphill climb. The nitrogen demand for pod growth is at peak between R4 and R5. The plant’s nitrogen fixation at the time of peak pod demand is at a very minimum. This is the reason that you may want to consider adding nitrogen to high-yield soybeans.
A nitrogen application in the R3 to R5 window will ensure the pod growth demand is met. One way this can be accomplished is through a foliar nitrogen application. In my high-yield soybean trials dating from 2008 through 2014, adding foliar nitrogen has paid off on average. Soybeans not treated with foliar N have yielded 60.3 bushels per acre and treated soybeans yielded 65.3. Five bushels of soybeans was about three times the cost of the nitrogen, so it has paid in the long run. The other nice thing that we have found using foliar nitrogen is that it is compatible with late glyphosate or fungicide applications.
Another possible method of adding nitrogen to a high-yield soybean system is with preplant incorporated UAN. I have no personal field experience with this method yet; however, I have trials in the field this year and I am anxiously awaiting results. We applied forty units of nitrogen with a pre-emergence residual herbicide, just like we would do in corn. This was mechanically incorporated, and then the beans planted. Untreated soybeans of the same varieties were planted alongside. So far there have been no visual difference between the treated and untreated trials. Once the soybeans get through pod set, we will be out doing some pod counts. If this method works as well as the foliar method, it could be used instead, and would save you that late trip across the field. Results of these trials will be shared with you in the future.