Aluminium solubility related to secondary solid phases in upper B horizons with spodic characteristics

We examined the aluminium solubility in the upper B horizon of podzols and its relation to the solid phase of the soil in 60 samples covering a pH range from 3.8 to 5.1. Solid phases were characterized by extractions with acid oxalate and pyrophosphate (pH 10). The solubility of Al was studied in a batch experiment in which samples were equilibrated with 1 m m NaCl at 8°C for 5 days. We also monitored the dissolution kinetics of Al and Si, in some samples. The oxalate and pyrophosphate extractions suggested that secondary Al was mainly organically bound in most soils, and imogolite-type materials seemed to constitute much of inorganic secondary Al. No single gibbsite or imogolite equilibrium could explain Al³⁺ activities. In all samples Al solubility, defined as log{Al³⁺} + 1.65pH, was closely related to the molar ratio of aluminium to carbon in the pyrophosphate extracts (Al_p/C_p). Solubility increased with the Al_p/C_p ratio until the latter reached ≈ 0.1. This indicated that solubility was controlled by organic complexation, at least when Al_p/C_p was small. Silica dissolved slowly in most soils used in the kinetic experiments. We conclude that imogolite-type materials in the upper B horizon dissolved slowly because of coating with humic substances or ageing or both.