Modelling the pH dependency of dissolved calcium and aluminium in O, A and B horizons of acid forest soils

• We developed a simple model to predict Ca2 + concentration in soil water.
• We combined two aluminium solubility models with the Gaines –Thomas equation.
• The model successfully described LogCa2 + using a limited number of input parameters.
• Model predicts that molar Ca/Al < 1 occurs at levels of Caex smaller than 4%.

The molar ratio of the calcium (Ca) and aluminium (Al) activities (Ca/Al ratio) in soil water is an important indicator for forest health conditions, with values less than 1 indicating negative conditions for forest growth. Here we used and calibrated a simple model, with only modest parameter requirement, to describe the Ca activity in soil water of acid forest soils according to the Gaines–Thomas exchange equation. Next, the model was combined with the Al(OH)3 equilibrium model, applicable at pH > pHthreshold, and the recently developed Al solubility model (based on organic complexation theory, applicable at pH < pHthreshold) proposed by Gruba and Mulder (2008). The pHthreshold is the pH of the soil above which the Al(OH)3 equilibrium model and below which the organic complexation model control the Al solubility.Calibration of the Gaines–Thomas selectivity coefficient for calcium–aluminium exchange resulted in LogKCa-Al(gt) close to zero. A combined model, including the Gaines–Thomas equation and an aluminium solubility model, was tested subsequently for internal consistency, using a set of 32 samples each from A and B horizons of forest soils (Cambisols) located in Southern Poland (pHH2O range 3.3–5.4). The observed relationship between pHH2O and CaX for a large number of O, A and B horizons from Polish lowland soils, as reported in a second data set, was in accordance with the general curvi-linear pattern predicted by the models. The observations in data set II also indicate that at any given CaX, the value of pHH2O increased in the order O < A < B horizon. Probably, this order is explained by the increase in pHKCl and the decrease in Ca activity. Model predictions suggested that molar Ca/Al < 1 occurred at levels of exchangeable Ca smaller than 4%.