A comparison of winter wheat yield and quality under uniform versus spatially variable fertilizer management

The Palouse region of eastern Washington, USA is characterized by steep, highly erodible hills with complex patterns in soil fertility and crop productivity. Because of these patterns, there is increasing interest in applying variable, rather than uniform rates of fertilizer across the landscape. This study was conducted to determine the yield and quality of wheat at several different positions on landscapes managed with uniform vs. variable rates of nitrogen and phosphorus fertilizer. A moderately to severely eroded wheat farm in summer fallow management near St. John, Washington was intensively sampled at intervals of 15 m along several transects 655 m long and separated by 122 m. Each transect crosses a wide range of landscape position, and a variety of soil fertility and crop productivity regimes. Maximum slope steepness along the transects reached 45%, although flat portions also existed. Along the south side of each transect, fertilizer was applied at a uniform rate according to the grower's typical practice (73 kg N ha^-1 and 6 kg P ha^-1). Along the north side, the transects were separated into three management zones that received different rates of nitrogen and phosphorus fertilizer applications to match the existing soil fertility and crop productivity patterns. The three variably fertilized management zones received 22 kg N ha^-1 and 6 kg P ha^-1 (Zone 1), 90 kg N ha^-1 and 6 kg P ha^-1 (Zone 2), and 22 kg N ha^-1 and 18 kg P ha^-1 (Zone 3). Yield and quality of wheat were measured in both the uniform and variable management strips at intervals of 15 m. In spite of applying 51 kg N ha^-1 less in management Zones 1 and 3 than in the adjacent uniformly fertilized strip, there were no significant differences in yield of wheat in any management zone between the variably and the uniformly fertilized strips. Grain yield was significantly different between the three fertilizer management zones, with measured yields of 4097 kg ha^-1 (Zone 1), 4501 kg ha^-1 (Zone 2), and 3339 kg ha^-1 (Zone 3). These differences are largely due to differences between each zone in available water content and nitrate nitrogen in the profile. Protein content of wheat was significantly lower in variably fertilized strips than in uniformly treated strips, leading to significant improvements in grain quality. The parts of the landscape with the lowest available water content in the profile had the highest grain protein content due to water stress-nitrogen interactions.