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. We investigated K speciation and extractability in tropical soils (Luvisols, Acrisols, Ferralsols) and their corresponding clay fractions. Potassium K- edge X-ray absorption near edge structure (XANES) spectra of soils and clays, combined with reference compounds, revealed the dominance of 2:1 clay minerals, particularly beidellite, illite–smectite, montmorillonite, and illite (∼70%). Kaolinite was minor (∼10%), whereas primary silicates remained detectable (∼18%). Potassium associated with organic matter contributed to only a small fraction (∼2%). Lower-weathering 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 at the lower weathering stages. Sequential extraction of soils further revealed little water-soluble and exchangeable fractions, with no systematic between the soils across different weathering stages. Nonetheless, clay-rich soils possessed a larger fraction of exchangeable K, while sandy soils contained very little. Considering relative fractions, however, sandy soils exhibited higher relative values of water-soluble and exchangeable pools, though these corresponded to small values. This finding highlights the importance of assessing both absolute and relative 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 tropical weathering sequence, while primary silicates also contribute to long-term K cycling. Combining spectroscopic and sequential extraction provides insights into K speciation and operationally defined fractions in soils that can be used to assess soil health and sustain K availability in soil systems. • Potassium K -edge XANES differentiates K species in soils and clay minerals • XANES reveals 2:1 clay minerals as main hosts of K in soils • Primary minerals are found in soils and clays using XANES technique • Organic matter-associated K contributes little to soil K speciation • Clay fraction mainly controls the contents of different K pools in soils