ALUMINUM 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 coverin g a pH range from 3.8 to 5.1. Solid phases were characterized by extra ctions with acid oxalate and pyrophosphate (pH 10). The solubility of Al was studied in a batch experiment in which samples were equilibrate d with 1 mM NaCl at 8 degrees C for 5 days. We also monitored the diss olution kinetics of Al and Si, in some samples. The oxalate and pyroph osphate extractions suggested that secondary Al was mainly organically bound in most soils, and imogolite-type materials seemed to constitut e much of inorganic secondary Al. No single gibbsite or imogolite equi librium could explain Al3+ activities. In all samples Al solubility, d efined as log{Al3+} + 1.65pH, was closely related to the molar ratio o f aluminium to carbon in the pyrophosphate extracts (Al-p/C-p). Solubi lity increased with the Al-p/C-p ratio until the latter reached approx imate to 0.1. This indicated that solubility was controlled by organic complexation, at least when Al-p/C-p was small. Silica dissolved slow ly in most soils used in the kinetic experiments. We conclude that imo golite-type materials in the upper B horizon dissolved slowly because of coating with humic substances or ageing or both.