Core Ideas Soil texture and precipitation largely impact split fertilizer application efficacy.Split fertilizer application consistently increased grain yield in irrigated sands.In fine‐texture and well distributed rain, split application should be done by V8.In fine‐texture and poor distributed rain, split application made no yield difference.Corn response to split application was greater in coarse‐textured than fine‐textured soils. In‐season N fertilization is increasingly being used as a management strategy to reduce risk of N loss to the environment. This study evaluated the optimal timing for a split N fertilizer application in corn (Zea mays L.) across different environments and soil textural classes in Minnesota. Treatments consisted of pre‐plant (PP) urea applied at 0 to 270 or 315 kg N ha−1 on increments of 45 kg N ha−1 and five split applications (SA) of 45 kg N ha−1 urea ammonium nitrate as starter fertilizer and 90 kg N ha−1 of urea with an urease inhibitor applied at the V2, V4, V6, V8, or V12 stage of corn phenological development. Site‐years were grouped according to grain yield response to fertilizer timing. Irrigated coarse‐textured soils produced 1.5‐ to 1.9‐fold greater grain yield when fertilizer was split applied from V4 to V12 due to improved synchrony of N availability to crop demand and reduced potential for NO3–N leaching. Rainfed, fine‐textured soils had mixed results. Site‐years receiving well‐distributed precipitation produced greater grain yield when fertilizer was split applied from V2 to V8, but early season N deficiency reduced yield for the V12 application. Site‐years with limited precipitation during the late vegetative through grain filling stages of corn had no improvement in grain yield or N use efficiencies for SA because dry soil conditions likely interfered with root development and made N fertilizer positionally unavailable to the crop. This study highlights that the success of SA is largely dictated by soil texture and precipitation.

Optical canopy sensing tools may improve corn (Zea mays L.) nitrogen (N) management, but their usefulness in far northern latitudes remains unclear. For this reason, the utility of SPAD, GreenSeeker normalized difference vegetation index (GS‐NDVI), RapidSCAN normalized difference vegetation index (RS‐NDVI), and RapidSCAN normalized difference red edge (RS‐NDRE) were evaluated to predict corn grain yield, plant N accumulation, and plant N deficiency in 12 site‐years throughout Minnesota. Six to seven N rates (35−45 kg urea‐N ha−1 increment) were pre‐plant applied. Canopy sensing measurements and aboveground plant N accumulation were obtained at V4, V8, V12, and R1 stages. Regardless of the tool, low predictive power of grain yield, plant N accumulation, and N deficiency occurred at V4, likely because of low crop N demand and sufficient N supply. At V8, sensors provided good estimations of grain yield (R2 = .75−.85) but underestimated the agronomic optimum nitrogen rate (AONR) by 33, 94, 102, and 46 kg N ha−1 with the SPAD, GS‐NDVI, RS‐NDVI, and RS‐NDRE, respectively. At V12 RS‐NDRE measurements provided the most accurate estimations of grain yield (R2 = .92) and AONR [R2 = .84 and N rate differential from agronomic optimum nitrogen rate (dAONR) at −2 kg N ha−1]. At R1 SPAD also provided good estimations of grain yield and N deficiency. The mismatch between the best timings for predicting N fertilizer requirements (V12 and R1) and the best timings for sidedressing (V4−V8) highlight that sensing tools may have limited utility to improve the standard maximum return to N approach in the Upper Midwest.

Ammonia (NH 3 ) emissions are an economically and environmentally significant loss pathway of fertilizer and soil-derived N. Chambers are a commonly used method to quantify NH 3 emissions in plot-scale agricultural research. Although this method is widely used, its accuracy may be influenced by the overall design of the chamber, its components, and its interaction with the environment. Four NH 3 chamber designs, including open, open + polytetrafluoroethylene (PTFE), semi-open, and closed, were deployed over a dilute NH 3 solution for 6 h on four dates to determine the effect of chamber design on NH 3 capture efficiency. The solution volume and concentration were measured before and after acid trap deployment, and total volatile NH 3 emission was assumed to be equal to the mass N loss. The NH 3 capture efficiency relative to the estimated total emissions was greatest for the open design (12.9%), whereas the semi-open chamber was the least efficient (3.5%). The closed chamber reduced NH 3 emissions relative to the open and semi-open designs by inhibiting convective gas transport beneath the chamber footprint.

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