In a recent study, 21 soybean scholars, including our own Dr. Shawn Conley, reviewed the effect on yield of adding nitrogen. Bottom line: adding nitrogen has a small effect on yield and positive ROI is unlikely.
The study aggregated yield data from replicated field experiments at 105 locations within 16 states between 1996 and 2016. That created a hefty database of 5991 plot specific yield data derived from a total of 207 environments (experiment x year). The study notes that “all individual trials were replicated within their respective environments.”
The study noted that major management decisions such as irrigation and and seeding rate interact with nitrogen response.
In a nutshell, the study, funded in part by the Wisconsin Soybean Marketing Board, concluded nitrogen may bump yield some but not enough to justify the cost.