Dominance of 2:1 clay minerals in soil potassium across a weathering gradient: Insights from XANES speciation and sequential extraction

Potassium (K) is essential in the soil–plant–human continuum, yet its chemical speciation in soils with different weathering stages and in their clay fractions remains poorly understood. Herein, we investigated K speciation and fractionation in 15 tropical soils (Luvisols, Acrisols, and Ferralsols) and 8 clay fractions. Potassium K-edge X-ray absorption near edge structure (XANES) spectra of soils and clays revealed the dominance of 2:1 clay minerals at 70%, particularly beidellite, illite–smectite, montmorillonite, and illite. Kaolinite was minor at 10%, whereas primary silicates remained detectable at 18%. Potassium associated with organic matter contributed to only a small fraction at 2%. Less-weathered soils contained beidellite together with illite, illite–smectite, and montmorillonite, whereas highly weathered soils had kaolinite and montmorillonite. However, illite–smectite and montmorillonite cannot unequivocally be distinguished by XANES spectroscopy. This spectroscopic evidence indicates that the XANES underlines the primary contribution of 2:1 phyllosilicates as the host phase for K in the soils, particularly in less weathered soils. Operationally-defined sequential extraction of soils further revealed little water-soluble and exchangeable fractions, with no trend between the soils across different weathering stages. Nonetheless, clay-rich soils possessed a larger fraction of exchangeable K, while sandy soils contained little. Considering relative fractions, however, sandy soils exhibited higher values of water-soluble and exchangeable pools, though these corresponded to small fraction. This finding highlights the importance of assessing both absolute and relative K pools when evaluating soil health. Our results highlight the dominance of 2:1 clay minerals as primary K sources and sinks in soils across a weathering sequence in tropics, while primary silicates also contribute to long-term K cycling. Combining spectroscopic and sequential extraction provides insights into K speciation and fractionation in soils that can be used to assess soil health and sustain K availability in soil systems.