Capitalizing On Residual Nitrate for Winter Wheat Establishment: a Fall Nitrate Test.

In the Mid-Atlantic States, soft red winter wheat (Triticum aestivum, L.) is frequently grown following corn (Zea mays, L.).  Application of fall starter fertilizer nitrogen (N) (0-45 kg ha^-1) to the wheat crop is a recommended practice.  However, where residual N is not considered, the potential for both economic loss from unnecessary fertilizer application and environmental degradation is increased.  Residual N is frequently the result of dry-land corn production in the region.  Volatility in corn and fertilizer prices causes the economic optimum N rate for corn to fluctuate.  Further complicating the fertilizer N rate decision for dry-land corn is variable summer precipitation.  This study was conducted in sixteen site years on the Coastal Plain and Piedmont regions of Maryland, and large variations in growing season monthly precipitation were observed.  In growing seasons where moisture was limited, corn grain yield reductions of up to 60% were observed.  During the study, the N rate for meeting crop requirements varied by 160 kg N ha^-1 on sandy loam sites and by 100 kg N ha^-1 on silt loam sites.  Even with good N management, the potential for highly variable residual nitrate (NO₃-N) was observed across sites and years.  For example, at 13 study sites over 4 years, corn plots that received 135 kg N ha^-1 had residual NO₃-N levels ranging from 9 to 77 kg ha^-1 in the surface 0.3 m of soil.  This project is examining corn performance relative to applied N rate to categorize sites based on residual NO₃-N. Use of the nitrachek quick NO₃-N test produced rapid measurements of residual NO₃-N which were strongly correlated with results from lab analysis.  Fall starter N recommendations based on quick test soil NO₃-N levels and wheat N response trials are being developed.