Soybean Production under Continuous Potassium Fertilization in a Long‐Term No‐Till Oxisol

Potassium (K) is essential for soybean production but over use of K fertilizer negatively impact the environment and farming profit. Therefore, the optimum K recommendation needs to be verified, especially in pedo‐environmental contrasting regions and long‐term cultivated areas. We evaluated the effects of continuous annual K fertilization on plant‐available K by Mehlich‐1 (KM‐1) and ion exchange resin (Kresin), plant uptake (Ktissue) and the yields of two soybean cultivars (Glycine max L.) grown in an Oxisol under long‐term no‐till (NT). The treatments included four K rates (0, 40, 75, and 110 kg ha–1) with four replications. Kresin was highly correlated with KM‐1 (R2 = 0.94, p < 0.0001). Plant‐available K was linearly correlated with Ktissue (trifoliate leaflets) with r of 0.76 and 0.82 (p < 0.0001) for KM‐1 and Kresin, respectively. Maximum KM‐1 and Kresin that expressed the highest KTissue concentrations were in the ranges of 1.72 ± 0.26 to 2.36 ± 0.35 and 2.58 ± 0.22 to 3.11 ± 0.33 mmolc K dm–3, respectively. The Ca+Mg/K ratios in tissues decreased by increasing K application. Soybean yields increased from 1823 to 2611 kg ha–1 as the rates of annual‐K application increased from 0 to 40 kg ha–1. Additional K application did not further improve yields. For yields, the maximum responsive soil test K was 2.16 ± 0.58 mmolc K dm–3 for KM‐1 and 3.42 ± 0.91 mmolc K dm–3 for Kresin; while the maximum responsive Ktissue at R2 stage was 14.5 ± 0.98 to 18.1 ± 2.2 g K kg–1. Our results confirm that soil test for plant‐available K and plant tissue test are important to guide K fertilizer recommendations. Core Ideas Plant available K extraction methods are compared. Soil test K is an important tool to guide annual fertilization for optimum soybean yield. K content in soybean trifoliate leaflets at R2 stage can serve as an indicator of K sufficiency levels.