A Preplant Soil Nitrate Test for Site-Specific Starter Nitrogen Management in Humid Region Winter Wheat Production.

Variable post-harvest soil residual nitrate-nitrogen (NO₃–N) regularly occurs in dryland corn (Zea mays L.) production. This variability arises because it is difficult to establish season appropriate corn N rates at side-dress application time when little is known about subsequent summer precipitation. Residual NO₃–N presents a challenge because it is vulnerable to loss during the fall–winter groundwater-recharge season, but it also presents an opportunity to substitute for starter fertilizer N in following winter wheat (Triticum aestivum L.) production. To establish a soil NO₃–N sufficiency level for winter wheat a series of starter fertilizer N response studies were conducted in the Coastal Plain and Piedmont regions of Maryland. A soil exchange frequency index (EFI) was used to screen sites where wheat response to starter N was unlikely due to very high fall precipitation and/or coarse soil texture. 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. This linear-plateau agronomic response  was combined with a break-even economic scenario (fertilizer-N cost = grain-response value) to estimate the preplant NO₃–N sufficiency level 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 can facilitate identification of sites where starter N will likely be sufficient for establishing winter wheat and thereby reduce potential losses of NO₃–N.