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.