Profit‐maximizing potassium fertilizer recommendations for soybean

Potassium (K) fertilizer has important yield and cost ramifications in soybean [ Glycine max (L.) Merr.] production. Rate recommendations are often based on expected yield response as predicted by a soil test. To that end, soybean response to K application rate studies were analyzed using 86 site‐years from 2004 to 2019. We estimated a generic yield response curve across soybean cultivar and soil texture to allow calculation of profit‐maximizing K rates for producers in the mid‐southern United States that also consider crop value and fertilizer cost. Further, we compared profit‐maximizing fertilizer‐K rates with those currently recommended. Using a spreadsheet‐based decision aid, soybean prices and yields, fertilizer‐K cost, and a range of initial soil‐test K (STK) values, as observed over the last 10 yr, we find that current uniform fertilizer‐K rate recommendations were greater than the predicted profit‐maximizing rates. Profit‐maximizing rates added profit ranging from US$2.32 ha −1 at initial Mehlich‐3 K availability values of 110 mg K kg −1 to US$29.35 ha −1 at 60 mg K kg −1 on average. The corresponding fertilizer‐K rate reductions were 11 and 48 kg K ha −1 , respectively, resulting in attendant yield penalties of only 28 and 52 kg ha −1 . Furthermore, K fertilization was not economically justifiable beyond STK of 128 mg K kg −1 on average. Hence, performing soil tests and calculating profit‐maximizing fertilizer‐K rates showed promising returns to producers at lesser fertilizer‐K use by sacrificing a minimal amount of yield. Also, variable‐rate applied K fertilizer in comparison to uniform rate application was rarely cost effective within the tested assumptions.