Soybean response to cover crop and nitrogen fertilizer timing on sandy soil

Abstract The potential for nitrogen (N) fertilization to increase soybean Glycine max (L.) Merr. yield is known to vary with environmental conditions, but the effects of N timing and rate remain unclear in sandy soils. A 2‐year field study was conducted in central Wisconsin to determine the effect of a rye ( Secale cereale L.) cover crop and N application rate and timing on soybean yield and in‐season dry matter and N accumulation. Treatments included an unfertilized control; a starter application (34 kg‐N ha − 1 ); single applications of 101 kg‐N ha − 1 at 10, 30, 60, or 80 days after emergence (DAE); and split applications totaling 202 or 404 kg‐N ha − 1 applied at 30, 60, and 80 DAE. All fertilizer treatments were nested within a rye cover crop system, planted in the fall and chemically terminated in May prior to soybean planting. Fertilizer effects on yield varied by year. In 2019, the split404 treatment increased yield by 7% relative to the control, while in 2020 no fertilizer treatments significantly affected yield. Both years, starter and 10 DAE treatments increased August N uptake relative to the control but starter led to a slight reduction in N uptake in June in both years. Cover cropping had no effect on soybean yield, dry matter, or N content in 2020, and poor establishment in 2019 prevented meaningful evaluation of effects. Overall, the yield gain observed in 2019 was insufficient to justify the added fertilizer cost, and the effect was not replicated the following year. Therefore, we find no evidence that N fertilization improves yield in irrigated soybean grown on sandy soils in Wisconsin.