Winter Wheat Starter Nitrogen Management: A Preplant Soil Nitrate Test and Site‐Specific Nitrogen Loss Potential

Managing highly variable soil residual nitrate‐nitrogen (NO3–N) following corn (Zea mays L.) is difficult because it can supply starter N for winter wheat (Triticum aestivum L.) and/or be leached into water resources during the fall–winter water‐recharge season in the humid eastern United States. A series of 65 starter N response studies were conducted in the Coastal Plain and Piedmont regions of Maryland to estimate the soil NO3–N concentration that was both agronomically sufficient and cost effective for wheat. A subset of studies measured soil NO3–N loss (0–60 cm) during the fall–winter season, and additional studies evaluated bromide (Br) to index potential NO3–N leaching. A soil exchange frequency index [EFI = rainfall * (available soil water capacity)−1] was used to identify where NO3–N loss was high and wheat response to starter N was unlikely. A post‐planting fall EFI ≥ 2.5 was associated with NO3–N depletion of >65% (0–60 cm). A significant (P < 0.001) linear‐plateau relationship was found between preplant soil NO3–N concentration (0–30 cm) and wheat grain‐yield response to starter N for locations with EFI < 2.5. This agronomic linear‐plateau relation was combined with a break‐even economic scenario (fertilizer‐N cost = grain‐response value) to estimate the fall NO3–N sufficiency for wheat, which was 7 mg NO3–N kg−1 soil (0–30 cm) and corresponded to 9 mg NO3–N kg−1 soil (0–15 cm). These findings show that a preplant soil NO3–N test for winter wheat can facilitate identification of sites where starter N will produce economic returns and reduce potential NO3–N losses to water resources.