The potential for nitrogen (N) fertilization to increase soybean yield is known to vary with environmental conditions, but the effects of N timing and rate remain unclear in sandy soils. We conducted a two-year irrigated field study on a Plainfield sandy soil (mixed, mesic Typic Udipsamments) in central Wisconsin to evaluate soybean growth and yield under varying N fertilizer treatments. 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. Soybean dry matter and N content were measured in June, July, and August, and yield was recorded at harvest. Fertilizer effects on yield varied by year. In 2019, the split404 treatment increased yield by 8% relative to the control, while in 2020, no fertilizer treatments significantly affected yield. Starter and 10DAE treatments increased August dry matter and N uptake, but only 10DAE avoided early-season reductions seen with starter. Cover cropping had no significant effect on soybean yield, dry matter, or N content. Split404 improved yield in 2019, but the yield gain was insufficient to justify the added fertilizer cost. Overall, we find no evidence that N fertilization improves yield in irrigated soybean grown on sandy soils in Wisconsin.  

Field experiments were conducted during the 2019 and 2020 growing seasons at the University of Wisconsin Hancock Agricultural Research Station (HARS; 44°07'04.6"N 89°32'20.5"W) on overhead irrigated Plainfield sandy soil (mixed, mesic Typic Udipsamments).

The experimental design was a randomized complete split plot design, with cover crop as the whole plot factor and N timing as the split plot factor. The whole plot treatments were with and without a winter rye cover crop; whole plots were 12.1 m wide by 30.5 m long. The winter rye cover crop was sown at 90 kg ha^–1 in October of each year and was chemically terminated in May prior to soybean emergence. The eight split-plot treatments represented different rate and timing practices. Treatment 1 (control) did not receive any N fertilizer and represented the standard agricultural practice. Treatment 2 (starter) received N at planting in the form of 32% urea ammonium nitrate solution at a rate providing 34 kg-N ha^–1 applied as an in-furrow stream on both sides of the furrow. Treatments 3, 4, 5, and 6 received urea fertilizer (46% N) at a rate providing 101 kg-N ha^–1 as a broadcast application at 10 days after emergence (10DAE), 30 (30DAE), 60 (60DAE), and 80 (80DAE) days after emergence, respectively (Table 1). These applications coincided with V1, VT/RI, R4.5 and R6 growth states and occurred on 10 June, 1 July, 29 July, and 19 August in 2019 and 1, 2, 3, and 4 in 2020. Treatment 7 and 8 received urea fertilizer in split broadcast applications (occurring 30, 60, and 80 days after emergence, coinciding with V1, R4.5 and R6 growth states) at target rates of 202 kg N ha^–1 (split2020) and 404 kg N ha^–1 (split404) split evenly among the three applications.

Approximately 3.5±0.5 mg of each pulverized sampled was analyzed for percent N by weight at the UC Davis Stable Isotope Facility in 2019 and at the Cornell Stable Isotope Lab in 2020.