Calcium Speciation and Solubility in Tropical Agricultural Soil Clays

Calcium (Ca) constitutes a plant macronutrient but is often deficient in acidic tropical soils. However, our understanding of Ca speciation and solubility in such soils remains limited. Herein, we scrutinized Ca speciation in soil clays using K-edge X-ray absorption near-edge structure (XANES) spectroscopy and examined Ca fractionation in soils through sequential extraction. The Ca XANES spectra of soil clay samples revealed an almost equal distribution of inorganic (x̄ = 59 %) and organic (x̄ = 41 %) Ca species. The clay spectra were mostly best fitted with Ca adsorbed to illite-smectite (x̄ = 28 %), Ca adsorbed to montmorillonite (x̄ = 27 %), Ca tannate (x̄ = 20 %), and Ca citrate (x̄ =21 %), whereas calcium carbonate or Ca adsorbed to kaolinite did not contribute to the fits. Similar spectral features of Ca adsorbed to illite-smectite, montmorillonite, and beidellite, as well as Ca sorbed to kaolinite and calcium carbonate, could not unequivocally be distinguished from one another by XANES spectroscopy. Sequential extraction of soil Ca indicated that exchangeable Ca, the most labile and available fraction, constituted the largest Ca fraction, accounting for 62 % of total soil Ca. Other Ca fractions related to residues (x̄ = 15 %), organic matter (x̄ = 10 %), carbonates (x̄ = 8 %), and Fe/Mn oxides (x̄ = 3 %) contributed to a lesser extent. Our findings demonstrate that 2:1 clay minerals—rather than kaolinite as traditionally expected—along with Ca-tannate and Ca-citrate can serve as primary sources and sinks for Ca in tropical soil clays. The potential formation of the ternary complexes of Clay-Ca-OM remains unclear and requires further investigation.