Integrated assessment of potassium speciation in Taiwanese soils using chemical extraction and X‐ray absorption spectroscopy

Understanding potassium (K) speciation in soils is essential for evaluating its availability to plants and guiding sustainable nutrient management. In this study, five distinct soils from Taiwan, representing varying management practices and physicochemical properties—including soils with and without long‐term K fertilization, alkaline soil, red soil, and forest soil—were analyzed to determine K speciation. A combination of indirect (sequential chemical extraction) and direct (synchrotron‐based X‐ray absorption spectroscopy) techniques was employed to comprehensively characterize soil K forms. Wet chemical extraction revealed that >95% of total K resides in the residual fraction, while exchangeable, carbonate‐bound, Fe/Mn oxide‐bound, and organic‐bound forms collectively accounted for <5%. X‐ray absorption near‐edge structure and extended X‐ray absorption fine structure analyses provided insights into the local coordination environment of K, revealing a consistent white line feature at ∼3615.2 eV across samples, with intensity trends indicating K availability in the order: alkaline soil > long‐term fertilized soil > forest soil > red soil > unfertilized soil. Linear combination fitting indicated that illite‐smectite is the dominant K‐bearing phase, while soluble and organic‐associated K forms vary with soil type and management. This study demonstrates the advantages of combining wet chemical and synchrotron‐based spectroscopic approaches for an accurate, multiscale understanding of soil K speciation.