Solubility of aluminum and silica in spodic horizons as affected by drying and freezing

For convenience, soil samples are often dried before storage and experimental use, However, the literature offers examples of drying that results in changes in pH, solubility of organic matter, and dissolution rates of Al. In this study we examined the solubility of Al and Si in fresh soil and in soil that had been dried or deep-frozen. Five Spodosol B horizon soils were subjected to batch titrations, where portions of each soil were equilibrated with solutions with varying concentrations of acid or base added, Extractions with acid oxalate and Na pyrophosphate indicated the presence of imogolite-type materials (ITM) in three of the soils. In the other two soils most secondary solid-phase Al was associated with humic substances, Deep-freezing did not significantly change pH nor the concentration of Al or Si as compared with fresh soil, Drying, on the other hand, yielded pH increases of up to 0.3 units at a given addition of acid or base, whereas Al3+ changed only slightly, implying a higher Al solubility in all of the soils, Furthermore, dissolved silica increased by up to 200% after drying, except in a soil that almost completely larked oxalate-extractable Si. We suggest that drying enhanced the dissolution of ITM by disrupting soil organic matter, thus exposing formerly coated mineral surfaces. In the soil where dissolved Si did not change with drying, it had been demonstrated that Al-humus complexes controlled Al solubility. We suggest that fissures in the organic material caused by drying may have exposed formerly occluded binding sites that had a higher Al saturation than had sites at the surface of humus particles.