Effects of Soil Phosphorus and Potassium Levels on Corn Yield Response to Nitrogen Fertilization, Nitrogen Use Efficiency, and Profitability

Annual investment in nitrogen (N) fertilizer for corn production represents a significant portion of annual input costs. Yield response to N fertilization is affected by soil N supply, crop N demand, and interacting factors that affect crop N use, such as phosphorus (P) and potassium (K) supply. To investigate the effects of soil-test P and K levels on corn yield response to N fertilizer, a four-year study was conducted at two southern Wisconsin sites. Soil-test P and K were maintained at low, optimum, and high levels corresponding with currently used interpretation class ranges for fertilizer guidelines in Wisconsin. Ranges of low, optimum, and high soil-test (Bray-1 P) levels for P were 6 to 17, 16 to 27, and 31 to 51 ppm P, respectively, across both sites. Ranges of low, optimum, and high soil-test (Mehlich-3 K) levels for K were 50 to 104, 120 to 173, and 164 to 262 ppm K, respectively, across both sites. Six N rates (0, 40, 80, 120, 160, and 200 lb. N/a) were applied to each corn crop in a corn-soybean rotation. Agronomic optimum N rate (AONR), economic return to N (RTN), economic optimum N rate (EONR), and partial factor productivity nitrogen use efficiency (NUE) were identified using grain yield response to N and multiple N:grain price ratios ($ lb N and $ bu corn grain). Corn yield response to N fertilization varied by soil-test P and K level. In optimum and high soil-test P and K soils, corn grain yield increased to a plateau with increasing N rates and an EONR (0.1 ratio) of 130 lb N/a was observed across all site-years, with no difference in AONR or yield at the AONR (240 to 242 bu/a) between optimum and high levels. Low soil-test P and K led to inconsistent yield responses to N and reduced profitability regardless of N rate. Results suggest that optimum ranges of soil-test P and K, confirmed with identification of critical soil-test concentrations in this study, of 16 to 23 ppm Bray-1 P and 138 to 182 ppm Mehlich-3 K resulted in maximized corn grain yield and profitability response to N fertilization.