S pring pre-plant N applications are considered a bestmanagement practice in conventionally tilled, poorly drained, medium-to fine-textured soils throughout the northern Corn Belt (Vetsch and Randall, 2004). Corn (Zea mays L.) yield potential is affected by a multitude of agronomic practices (e.g., fertility management), cultivars, and the environment (Evans and Fischer, 1999). Recent data project more frequent heat waves (i.e., >5°C above climatic normal) and increasing air temperatures (1.5-2.0°C) over the next 30 yr that may affect spring frost dates in the northern hemisphere and increase precipitation intensity during winter and spring months (>50.8 mm in 48 h) (Hayhoe et al., 2007; Karl et al., 2009). Warmer spring temperatures may shift corn planting dates earlier than currently recommended (Lauer et al., 1999). Weather variability may increase risk of early applied N losses (i.e., volatilization, leaching, and denitrification) and require reexamination of N management strategies (Scharf et al., 2002). Optimal soil fertility management includes adjusting strategies to account for the proper placement, time, source, and rate (e.g., 4R) of N adapted to site-or region-specific environments (Roberts, 2007). Rapid corn N uptake does not occur until the V10 growth stage (Bender et al., 2013). Delayed sidedress (SD) N applications (e.g., after V10) may help mitigate the time lapse between N application and uptake but further research is needed to refine current grower practices. Pre-plant incorporation (PPI) is a one-pass N strategy where 100% of the N inputs are applied up to planting time and incorporated. A large percentage of Michigan corn land area is grown on calcareous soils with a soil pH > 7.2, which can increase NH 3 volatilization losses, increase N immobilization, and reduce the efficiency of surface-applied urea containing N fertilizers (Havlin et al., 2014). In Michigan studies, blending polymer-coated urea (PCU) with urea (75:25, PCU/urea blend ratio) has improved efficiency of urea containing fertilizers. For example, when April and May precipitation were above average, PCU/urea broadcast 2 to 4 wk before planting and incorporated improved corn yield up to 1.38 Mg ha-1 relative to 100% urea PPI (Franzen, 2017). In the same studies a PCU/ urea blend extended N activity in dry soils, which increased corn grain yield 1.07 Mg ha-1 relative to a V4 to V6 surface

Core Ideas Daikon radish and forage oat cover crops following winter wheat sequestered residual autumn soil NO3–N levels. Nitrogen availability to the ensuing corn crop may be reduced when preceded by radish or oat cover crops. When using radish or oat cover crops, increased 5×5 starter rates (> 45 kg N ha‐1) may be required if full SD is delayed until V11. Radish and oat cover crops preceding corn did not provide a subsequent N fertilizer replacement value but may still be effectively utilized as soil conservation tools. Cover crops (CC) preceding corn (Zea mays L.) may influence subsequent nitrogen (N) availability, but it is not clear whether N strategies require adjustment. Field studies conducted in 2015 to 2016 evaluated the effects of a daikon radish [Raphanus sativus (L.)], forage oat [Avena sativa (L.)], and no CC following winter wheat [Triticum aestivum (L.)] on soil chemical properties, corn growth, grain yield, and profitability. Nitrogen management strategies were equalized to 179 kg N ha–1 and included pre‐plant incorporated (PPI) N, poultry litter (PL) PPI (61 kg N ha–1) plus sidedress (SD) N V11, starter N (45 kg N ha–1) subsurface banded (5 cm below and 5 cm beside the seed, 5×5) followed by V4, V11, or V4 plus V11 SD, and a zero N control. Cover crops reduced autumn soil NO3–N levels 78 to 84% relative to no CC but occasionally reduced N during critical corn uptake periods. Cover crops did not increase soil base cation availability to the ensuing corn crop. When 5×5 starter was followed by full SD at V11, CCs reduced yield 3.2 to 3.9% and profitability 11.8 to 13.2% compared with no CC indicating reduced efficacy and possibly greater 5×5 requirements to maintain yield potential until SD time. In the zero N control, CCs reduced grain yield 11.8 to 14.2% and increased yield response to N 63 to 79% suggesting reduced N availability. Radish and oat CCs provided a trap crop for autumn residual N, but N management strategies may need to account for reduced N availability.

Early planting into cooler soils, increased nutrient removal by higher yielding hybrids, and reduced atmospheric S depositions suggest reassessing S application strategies for corn (Zea mays L.) in Michigan. In 2012 and 2013, fi eld studies were initiated to evaluate corn response to S and N applications by measuring S and N plant concentrations, uptake, grain yield, and agronomic effi ciency (AE). Th e study was arranged as a split-plot randomized complete block with four replications. Main plots consisted of three S rates (0, 23, and 45 kg S ha -1 ) while subplots consisted of six N rates (0, 56, 112, 169, 225, and 281 kg N ha -1 ). Corn tissue V6 S concentrations were in the suffi ciency range for optimal corn growth without S in 1 of 2 yr. Without N fertilizer, signifi cant yield diff erences were observed among the 0, 23, and 45 kg S rates in 2012 (6.7, 7.7, and 9.5 Mg ha -1 , respectively) and 2013 (3.2, 5.7, and 3.5 Mg ha -1 , respectively). In 2013, signifi cant yield increases to S applications occurred only at ≤56 kg N ha -1 . Data suggest fi ne-textured soils with organic matter ≥ 28 g kg -1 and residual S ≥ 6-8 mg kg -1 are suffi cient for maximum corn yield without S when N application rates exceed 56 kg N ha -1 . Choosing optimal corn N application rates may satisfy physiological S requirements under these fi eld conditions indicating N/S ratio may not have been suffi cient at N rates ≤ 56 kg N ha -1 .

Core Ideas Increased soil test P values above the critical threshold reduces the likelihood of grain yield response to P fertilizer application. Soil test P critical concentrations of 14 ppm indicated no change from Tri‐State Soil Fertility recommendations. Shallower soil sampling (0–2 inches) did not improve relationship between soil test P concentration and relative grain yield.

Crop sensors may help growers identify nitrogen (N) deficiencies and optimize grain yield in corn (Zea mays L.), but limited research is available on using these technologies in the Eastern Corn Belt where sidedress applications are more common. Field studies in Ohio (four site-years) and Michigan (two site-years) were conducted to evaluate the relationship of the dark green color index (DGCI) to soil plant analysis development (SPAD) values at the V4, V6, and R1 to R2 growth stages, as well as the relationship to ear-leaf N concentration and grain yield. Five N rates (0-240 lb N acre -1 ) were evaluated in Ohio, and six N rates (0-250 lb N acre -1 ) were evaluated in Michigan. A linear relationship was observed between relative measurements of DGCI and SPAD in all but one site-year at V4 and V6, and in all site-years at R1 to R2. At V6, N application was detected by SPAD in four site-years and by DGCI in two site-years as compared with no N application. At the R1 to R2 stage, both DGCI and SPAD detected color differences in all six site-years. Across all site-years, both relative DGCI and relative SPAD measurements exhibited a linear relationship (P < 0.01) to relative yield at V6 (r 2 : 0.05-0.23) and R1 to R2 (r 2 = 0.51-0.75), but the r 2 values were 0.18 to 0.24 greater for SPAD than for DGCI. Ear-leaf N concentration exhibited similar yield predictions as both SPAD and DGCI at R1 to R2. Results suggest that detection of N deficiencies may be location dependent, but detection of differences between treatments was more frequent using the relative SPAD measurements as compared with the relative DGCI measurements.