Sequential Leaching of Volcanic Soil to Determine Plant-Available Cations and the Provence of Soil Minerals Using Sr Isotopes

Sequential leaching experiments using H₂O, H₂O₂, NH₄Cl, and HCl were performed on surface soils on an andesite substrate at four sites in the Kawakami mountainous area, central Japan. The solutions extracted from the dehydrated soil by H₂O, H₂O₂, and NH₄Cl have relatively constant ratios with respect to Ca, Mg, and Sr, while they have variable ⁸⁷Sr/⁸⁶Sr ratios depending on the site. The elemental ratios and Sr isotopes in the extracted solution are different from those of the soil minerals but identical to those of the soil solution and the plants. Sr isotopic data indicate that the residues after extraction from fine-grained minerals by NH₄Cl and HCl are a mixture of acid-resistant minerals derived from bedrock and from arid areas in China. We suggest that there is a vital exchange of Sr and other cations between plants and the soil pool of exchangeable cations through the soil solution, while soil minerals, except chlorite, do not participate in the exchange reaction.