Soybean plants have a high demand for nitrogen as soybean grain contains a large amount of protein. An 80-bushel per acre soybean crop requires approximately 302 pounds of N per acre. As soybean yield increases, many farmers question if nitrogen supplied through fixation and the soil is adequate to maximize yield.
With funding from the Ohio Soybean Council, the soybean and small grain production lab at The Ohio State University evaluated nitrogen fertilizer application to soybean in eight Ohio counties in 20 separate trials. Various nitrogen sources (urea, slow-release nitrogen, and foliar nitrogen) and application timings (pre-plant, at planting, and R3 soybean growth stage) were evaluated. Overall, four out of 20 trials resulted in a soybean yield increase with nitrogen application. At today’s soybean price, nitrogen application to soybean had a positive economic return at one location out of 20.
Recently, these results from Ohio State were included in a synthesis analysis conducted by researchers at the University of Wisconsin-Madison to determine soybean response to nitrogen application across the United States. The analysis included 5,991 plot-specific yield datapoints across 16 states spanning from North Dakota to Louisiana and Nebraska to Maryland. The effect of nitrogen application on soybean yield was relatively small. A single nitrogen application was 0.9 bushel per acre greater yielding than the control (no nitrogen applied) while a split nitrogen application resulted in a 1.6 bushel per acre yield increase compared to the control. At nitrogen application rates of 100 pounds N per acre or less, soybean yield response to nitrogen application was highly variable. Fewer datapoints were available for nitrogen applications greater than 100 pounds of N per acre, but projected a nitrogen rate of 300 pounds N per acre to maximize soybean yield which is unlikely to produce a positive economic return.
In fields with first-time soybean production, Rhizobia inoculant and nitrogen fertilizer may be necessary. However, in Ohio under most circumstances, application of nitrogen fertilizer is not needed in fields with a history of soybean production.