Soil K‐bearing minerals respond differently to various potassium extraction methods targeting phytoavailability

Non‐exchangeable potassium (Nex‐K) released slowly from minerals is a key component of long‐term K fertility of agricultural soils. Two extraction methods are widely used to determine soil Nex‐K: the boiling nitric acid (HNO 3 ) method and the tetraphenylborate (TPB) method. However, it is unclear from which minerals the methods extract the Nex‐K. The Nex‐K was determined for 322 agricultural soils sampled across Japan using both methods. The major soil minerals, including three K‐bearing minerals—trioctahedral micas (Micas[Tri]), dioctahedral micas (Micas[Di]), and potassium feldspar (K‐feldspar)—were determined by X‐ray powder diffraction (XRPD). The multiple regression analysis revealed that the boiling HNO 3 method extracts Nex‐K mainly from Micas(Tri) and partly from K‐feldspar, whereas the TPB method extracts Nex‐K mainly from Micas(Di) and partly from Micas(Tri) and K‐feldspar. The cluster analysis, based on the XRPD‐based mineral content, revealed that the soils had at least three geological parent materials. The soils with granitic mineralogical characteristics had high Micas(Tri) and Nex‐K contents, as determined by the boiling HNO 3 method. In conclusion, the mineralogy of the parent material influences the types and quantities of K‐bearing minerals present in soil, thereby controlling the amount of Nex‐K and its extractability by different methods. This understanding allows for the estimation of long‐term K fertility of agricultural soils.