Soil pH, Soil Organic Matter, and Crop Yields in Winter Wheat-Summer Fallow Systems

Soil acidification has become a major environmental challenge for crop production in the inland Pacific Northwest (iPNW). We evaluated the eff ects of tillage and N-fertilizer management on soil pH, soil organic carbon (SOC), soil N, and crop yields from 1995 through 2010 in an ongoing long-term experiment in eastern Oregon. Tillage systems included moldboard plow (MP), disk plow (DP), and subsurface sweep (SW) and N-fertilizer rates were 0, 45, 90, 135, 180 kg N ha(-1) crop(-1) in a dryland winter wheat (Triticum aestivum L.)-summer fallow (WW-SF) system. Soil pH, SOC, and N were monitored in 1995 and 2010, and crop yields were monitored every other year. Soil pH was lower in the higher N rate treatments. Long-term N fertilizer application increased soil acidity in 0- to 10-cm depth by 0.3, 0.2, and 0.3 units in MP, DP, and SW, respectively, for every 1000 kg N applied through ammonical N fertilizers. Soil pH was higher in DP than MP in 10- to 20-and 20- to 30-cm depth profiles. Th e SOC and N concentrations in the top 30-cm depth were lower in 2010 than in 1995 across all treatments. Wheat yield increased significantly withincrease in N rates from 0 to 90 kg N ha(-1) crop(-1). Th ere were no further yield increases above 90 kg N ha(-1) crop(-1). Soil acidification, SOC and nutrient dynamics should be carefully monitored in cropping systems using ammonical N fertilizers, particularly under high rate of N application and reduced tillage.