PRESIDEDRESS SOIL NITRATE TEST EVALUATION IN MARYLAND

Nitrate enrichment of groundwater and the Chesapeake Bay are major concerns in the Mid-Atlantic area. A soil N test could increase N use efficiency and decrease NO3 losses to the environment. This research evaluated the presidedress soil nitrate test (PSNT) for corn (Zea mays L.) in the Piedmont and Atlantic Coastal Plain regions of Maryland. Soil samples were collected to a 30-cm depth when corn was 15- to 30-cm tall from replicated N rate experiments that included seven soil types and five growing seasons. Soil NO3-N and (NO3 + NH4)-N were determined by steam distillation on PSNT evaluation plots. Corn grain yields were measured on PSNT evaluation plots and on corresponding non-N-limited plots to estimate relative yields. Experimental variables included: poultry manure (PM), dairy manure (DM), composted sludge (CS), tillage, and incorporated cover crops that gave 47 treatment-year combinations. The PSNT accurately reflected differences in corn N availability due to prior N inputs from manure, composted sludge, and cover crops. The relationship between relative yield and soil NO3-N was similar for both no-tillage and plow-tillage. Soil NO3-N concentrations > 22 mg kg-1 soil, or (NO3 + NH4)-N concentrations > 27 mg kg-1 soil, were associated with relative yields of 0.95 or higher. The variation in relative yields at soil N concentrations less than the above critical values was too large for predictive use. The relative yield vs. PSNT relationship was better with (NO3 + NH4)-N than with NO3-N alone. The PSNT successfully identified N-sufficient sites across a range of textures, drainage classes, and years. By identifying N-sufficient sites, the PSNT will help farmers conserve fertilizer N and reduce NO3-N losses to the environment.