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 mM NaCl at 8 degrees 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 approximate to 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.