Rice yield response to potassium: An economic analysis

Potassium fertilizer represents a non‐trivial input cost in rice ( Oryza sativa L.) production and its rate recommendation is often based on yield and K deficiency observations alone. However, profit‐maximizing fertilizer‐K rate not only hinges on the yield response to both initial available soil K and applied fertilizer K, but also the crop value and fertilizer cost. To that end, K application rate studies for rice, performed across 91 site‐years from 2001 to 2018, allowed estimation of a generic yield response curve to calculate profit‐maximizing K rates for producers in the mid‐southern United States. To determine whether those calculation efforts are justified, we compared profit‐maximizing fertilizer‐K rates to those currently recommended. Using rice prices and yields, fertilizer‐K cost, and a range of initial soil‐test K values, as observed over the last 10 yr, we find that current fertilizer‐K rate recommendations are too high. Profit‐maximizing rates added from US$0.88 ha −1 at initial Mehlich‐3 K availability values of 75 mg K kg −1 to $28.19 ha −1 at 105 mg K kg −1 on average. The corresponding fertilizer‐K reductions were 0.35 and 56 kg K ha −1 , respectively, resulting in attendant yield penalties of only 4 kg ha −1 and 105 kg ha −1 . Hence, performing soil tests and using decision support software to obtain profit‐maximizing fertilizer‐K rates is expected to enhance producer profit at rice yield penalties that are smaller than fertilizer cost savings. While profit‐maximizing rate recommendations do vary in a field with varying available soil K, using the mid‐range estimate rather than variable‐rate technology was deemed most feasible.