Soil Acidification from Nitrogen Fertilizer Application in a Dryland Winter Wheat – Summer Fallow System in the Pacific Northwest.

Commercial N fertilizers are widely used in crop production in drylands of Pacific Northwest (PNW). Continuous application of chemical N fertilizer may contribute to soil acidification and influence crop production. A study was established at the Columbia Basin Agricultural Research Center (CBARC) near Pendleton, OR in 1940 to study the effects of tillage and fertilizer N management on soil properties and crop production. The experiment was designed as a randomized block split-plot experiment with three tillage systems (Moldboard plow, MP; disc plow, DP; subsurface sweep, SW) as a main plot factor and five fertilizer rates as a subplot factor (0, 45, 90, 135, 180 kg ha-1 crop-1). We evaluated soil pH and crop yield during 1995-2010. Soil pH was not significantly influenced by tillage system in 0-10 cm soil depth. Soil pH was significantly more under DP than under MP in 10-20 and 20-30 cm depths. Soil pH also decreased with increasing N rate across all tillage systems, mainly in 0-10 and 10-20 cm soil depths. Wheat yield increased significantly with 45 and 90 kg N ha-1 crop-1 application compared with no N treatment but yield was not significantly different between 90, 135 and 180 kg N ha-1 crop-1 treatments. This suggests a constraint in yield gain at higher N rate attributable to soil acidification and associated changes in nutrient availability.