Effects of Potassium Fertilization on Corn and Soybean Yields and Soil Test K.

As corn (Zea mays L.) and soybean (Glycine max) yields increase the demand for macro nutrients like potassium (K) also increase. Recent, volatility in potash fertilizer price and commodity prices have renewed interest in the agronomics and management of K in corn and soybean. The objectives of this study were: to determine the effects of initial soil test K level and fertilizer K rate on yield, K uptake and K removal in corn and soybean; and to monitor changes in soil test K over time. Three experimental sites with distinctly different soil characteristics were used in this Minnesota (USA) study. They included a Nicollet clay loam (Aquic Hapludolls), Hubbard loamy sand (Entic Hapludolls) and Mt. Carroll silt loam (Mollic Hapludalfs). Three rates of potash (0, 56 and 112 kg K ha^-1) were applied annually in the fall at two of three sites. Rates of 0, 37 and 74 kg K ha^-1 were applied to the Mt. Carroll soil because initial soil test K levels were greater at this site. Treatments were arranged in a randomized complete block design with four replicates. Soil samples taken to a 15-cm depth were analyzed for K with ammonium acetate (dry soil) and Mehlich III (moist soil) extractants using standard methods. All sites were cropped to a corn-corn-soybean rotation. Corn and soybean yields were increased in 8 of 9 sites-years by K fertilization. Yield response to K fertilization ranged from only 2% in 2012 on Mt. Carroll soil to 40% in 2014 on Hubbard soil. Potassium uptake and removal generally increased with K fertilization. On Nicollet and Hubbard soils ammonium acetate K increased slightly with the 56 kg rate; whereas, the 112 kg rate increased soil test K 11 and 23 mg kg^-1 per year on the Nicollet and Hubbard soils, respectively.