Nutrient Availability in Surface- and Sub-Soils Along the Precipitation Gradient of Eastern WA.

Current university fertility guidelines for dryland winter wheat production in Eastern WA provide recommendations only for nitrogen (N), phosphorus (P), potassium (K), sulfur (S), and chloride (Cl). However, identified knowledge gaps include micronutrient fertility research, and changes in subsoil nutrient availability across Eastern WA’s precipitation gradient. In 2016, we assessed nutrient availability within the rooting zone at 16 locations in the Washington State University (WSU) Cereal Variety Testing Trial. Soil samples were collected at 0-15, 15-30, 30-60, 60-90, and 90-120 cm increments and analyzed for exchangeable macro- and micro-nutrients, available soil water, pH, bulk density, and soil organic matter (SOM). Annual precipitation was the major driver of winter wheat productivity, which explained 74% of the variability in winter wheat yields across all varieties, confirming the recognition of water as the major limitating factor for productivity. We also observed differences in nutrient availability along the precipitation gradient in both the surface soil (0-15 cm) and subsoil (60-120 cm). In the surface soil, bulk density, SOM, P, S, and copper increased with increasing precipitation. In the subsoil, available soil water, exchangeable calcium, and manganese increased with precipitation, while pH and boron decreased. In both the surface and subsoil, manganese, iron, and base saturation changed along the precipitation gradient; whereas K, zinc, chloride, sodium, and residual nitrate were unrelated to precipitation. Presumably, SOM was the major driver of nutrient availability differences observed in the surface soil, and pH for micronutrient availability in the subsoil. These data will help direct future fertility research needs, particularly for micronutrients, in Eastern WA.