Soybean response to nitrogen fertilizer in different soils

Although yield responses of soybeans [Glycine max (L.) Merrill] to nitrogen (N) fertilizer are rare, occasional yield increases, especially in high-yielding soybeans, have encouraged some producers to apply N. We conducted nine field experiments between 2014 and 2017 over a range of soil types and environments to evaluate soybean yield response to N (as urea) applied at planting, R1, R3, R5, and at all four timings. Our results showed that a single N application at R1 did not increase soybean yield in any location, while applying N at R3 or R5 increased grain yield in only one of nine locations. At a location with irrigated loam soils, N at planting increased grain yield by 22.4 bu ac⁻¹ (35%) in 2015 and 19.7 bu ac⁻¹ (38%) in 2016 but did not affect yield in 2017. Applying N four times did not increase yield more than the application at planting at this location in 2015 and 2016, but it increased yield in 2017. Four applications of N increased yield in three of the other six locations by an average of 5.0 bu ac⁻¹ (6%). Applying N four times or at R5 increased soil inorganic N at R6 at five of nine locations but did not consistently increase yield. Grain yield was positively correlated to Normalized Difference Vegetation Index (NDVI) taken at stage R6 at seven of the nine locations. Except for the yield increases from planting-time N at two locations, yield responses were insufficient to cover the cost of fertilizer N. While in-season application of fertilizer N to soybeans in productive Corn Belt soils in the United States is unlikely to be consistently profitable, N at planting that stimulates early growth and N uptake, especially in lighter-textured soils, may sometimes increase yield substantially.