Nine-Year Evaluation of Phosphorus and Potassium Fertilizer Recommendations for Corn and Soybean in Ohio.

Evaluating regional nutrient management recommendations requires an adequate assessment of soil test levels in relation to parameters of crop growth and yield. The objective of this study was to evaluate soil test phosphorus (P) and potassium (K), tissue concentration, and grain yield of corn and soybean in response to fertilizer P and K application in Ohio. Corn and soybean rotations were established and maintained at three sites from 2006 to 2014 and fertilizer P and K were applied at 1) estimated nutrient removal rates and 2) two-fold greater than estimated removal rates for each rotation. Surface soil (0 – 20 cm), leaf tissue during early reproductive growth, and grain yield were measured to evaluate response to fertilizer P and K. Initial soil test P levels were in the recommended maintenance range (15 – 30 mg kg^-1) for all three sites. Initial soil test K levels were in the recommended maintenance range (100 – 155 mg kg^-1) for two sites and one site was 49 mg kg^-1 above the range. Results indicated that after nine years of corn and soybean production soil test P and K were reduced by an average of 16 and 41 mg kg^-1, respectively, without fertilizer P or K application. Overall, corn and soybean grain yield responded positively to P and K fertilization in 14 of 54 site-years. The results from 33 site-years indicated corn grain yield responded positively in 18 and 9% of site-years as a result of P and K fertilization, respectively. While the results from 21 site-years indicated soybean grain yield responded positively in 5 and 19% of site-years as a result of P and K fertilization, respectively. Six of 14 site-years exhibited a positive grain yield response to P and K fertilization during the last three years of the study. The maintenance range soil test levels for P and K provided a sufficient basis for fertilizer P and K application in corn and soybean rotations.