Document Type

Article

Publication Date

8-2020

Keywords

initial Mehlich-3 K availability in soil; variable rate technology

Abstract

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.

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