Aluminium solubility characteristics of spodic B horizons containing imogolite-type materials

Fifteen Swedish and Finnish soil samples from spodic B horizons containing imogolite-type materials were equilibrated with dilute NaCl and HC1 at 8°C. For the NaCl extracts, apparent equilibrium with respect to an AI(OH)3 phase was reached in one to two weeks. Equilibrium with respect to an imogolite-type phase was slower, especially at large soil:solution ratios. The results show that log *Ks (gibbsite), i.e. the logarithm of the solubility constant for reactive AI(OH)3 at 8°C was ~9.40 (8.29 at 25°C) while log *Ks (imogolite) was variable; for the soil studied in most detail it was 7.66 (6.64 at 25°C) As well-ordered gibbsite rarely forms in spodic B horizons, it is suggested that Al(OH)3 and poorly-ordered allophanic materials may be slowly converted to less soluble and less reactive imogolite-type materials.