Economic considerations of in-season potassium applications to soybean using payoff matrices

Potassium (K) deficiency is a common yield-limiting factor in Arkansas soybean (Glycine max (L.) Merr.) production that can be addressed with innovative supplemental fertilizer application. An established leaf sampling protocol and dynamic critical concentration allow accurate diagnosis of K deficiency with corresponding recommendations for corrective, in-season K fertilizer applications at site-specific rates and times to reach anticipated yield goals. However, the profitability of in-season K fertilizer applications to irrigated soybean remains unclear. Research was conducted in Arkansas from 2021 to 2023 to evaluate multiple rates of in-season applications of muriate of potash to soybean at 15 and 30 days after first flower (DAR1). The economic ramifications of in-season fertilizer applications were quantified by calculating yield averages, partial returns (PRs), and regret when comparing PR to not fertilizing, each assuming 5-year average prices for fertilizer and soybean grain. Significant yield responses to in-season potash fertilizer were found at both 15 and 30 DAR1 times. These yield increases translated to large increases in PR or lower regret when compared to using no fertilizer. Corrective applications of 74–112 kg K ha⁻¹ were often considered optimal, with risk assessments provided to allow informed decisions. Results were summarized by category of leaf-K concentration, and treatment averages were provided to calculate payoff matrices for 15 and 30 DAR1 times. The resulting payoff matrices can be used as a decision support tool with any grain and fertilizer price to facilitate informed management decisions that optimize profitability as soybean and fertilizer prices impact optimal outcomes.