Relationship between Aluminum in Soils and Soil Water in Mineral Horizons of a Range of Acid Forest Soils

Previous studies suggest that the solubility of Al in acid forest soils may be expressed as the log Al 3+ activity = − n pH + log K H‐Al + log Al p /C p concentration ratio, with Al p and C p representing pyrophosphate‐extractable Al and C, respectively, and K H‐Al is the binding constant of Al to protonated complexation sites on soil organic matter (SOM). We show that a slightly modified model, in which Al p is replaced by Al org (= Al p − KCl‐extractable Al), successfully describes the concentration of Al in A and B horizons of a wide range of acid forest soils (Typic Dystochrepts) developed in different parent materials from a large number of spruce [ Picea abies (L.) Karst.] and mixed forest stands in southern Poland. Analysis of the data suggests a reaction stoichiometry of n = 1.27 and log K H‐Al = 1.98 with R 2 = 0.76. Furthermore, statistical analysis of a large set of published data, in addition to our own data from the Polish soils, indicates that log[Al org /C p ] is significantly correlated with soil pH measured in 1 mol L −1 KCl (pH KCl ), suggesting that the Al binding to SOM is less at small pH KCl due to increased competition with H + : log[Al org /C p ] = 0.82 pH KCl − 4.16 ( R 2 = 0.77, n = 276). Combining the two equations gives a simple Al solubility model for mineral horizons in acid forest soils: log Al = −1.27pH + 0.82pH KCl − 2.18 ( R 2 = 0.82, n = 127). In this equation, pH KCl relates to chemical soil properties, whereas the pH of soil water also depends on solution characteristics like ionic strength. The equation may be applied to acid forest soils having a soil solution pH in the range of 3.0 to 4.2. Such conditions are rather common in surface horizons (A, E, and upper B) of acid forest soils of the temperate and boreal zones.