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

Abstract

Managing highly variable soil residual nitrate-nitrogen (NO 3 –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 NO 3 –N concentration that was both agronomically sufficient and cost effective for wheat. A subset of studies measured soil NO 3 –N loss (0–60 cm) during the fall–winter season, and additional studies evaluated bromide (Br) to index potential NO 3 –N leaching. A soil exchange frequency index [EFI = rainfall * (available soil water capacity) −1 ] was used to identify where NO 3 –N loss was high and wheat response to starter N was unlikely. A post-planting fall EFI ³ 2.5 was associated with NO 3 –N depletion of >65% (0–60 cm). A significant (P < 0.001) linear-plateau relationship was found between preplant soil NO 3 –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 = grainresponse value) to estimate the fall NO 3 –N sufficiency for wheat, which was 7 mg NO 3 –N kg −1 soil (0–30 cm) and corresponded to 9 mg NO 3 –N kg −1 soil (0–15 cm). These findings show that a preplant soil NO 3 –N test for winter wheat can facilitate identification of sites where starter N will produce economic returns and reduce potential NO 3 –N losses to water resources. Abbreviations: AWHC, available water holding capacity between FC and PWP; Br, bromide; CV, coefficient of variation; EFI, exchange frequency index; FC, soil water content at field capacity; GS, growth stage; IC, ion-chromatography; LPNR, location previous N regimes; NDFF, the percent of plant N derived from fertilizer; NO 3 –N, nitrate-nitrogen; PWP, soil water content at permanent wilting point; UAN, urea-ammonium nitrate; URS, university research stations.