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

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.