Mechanisms controlling the activity of free aluminum (A1) in Bw1 horizons of soils developed from volcanic ash deposits in Japan were investigated by means of acid-base titrations and kinetic studies. In a Bw1 horizon, with a high content of acid-oxalate extractable A1, soil solution reached equilibrium with short-range ordered aluminosilicates in the order of days. Relatively fast kinetics of the release and precipitation of A1 and Si indicate a high reactivity of short-range ordered aluminosilicates in the soil. In the Bw1 horizon of an adjacent soil, with a high content of crystalline clay minerals like halloysite and interlayered vermiculite, solution remained well undersaturated with respect to short-range ordered aluminosilicates and aluminum hydroxide. Apparent equilibrium with respect to halloysite occurred after more than 30 days. This halloysite (log K-so(0) = 3.74 +/- 0.02 (25 degrees C)) has a solubility that is less than that reported in the literature (log K-so(0) = 4.36 (25 degrees C)). Our findings suggest that different reactive aluminosilicates may control the activity of free A1 in sub-surface horizons of volcanic ash soils with different mineralogy. (c) 2005 Elsevier Inc. All rights reserved.