Apparent cation - Exchange equilibria and aluminium solubility in solutions obtained from two acidic forest soils by centrifuge drainage method and suction lysimeters

Apparent cation-exchange equilibria and solubility of aluminium were analys ed in two acidic forest soils: a Cambisol and a Cambic Podzol. Soil solutio n was obtained by a centrifuge drainage method from fresh soil samples and with suction lysimeters. The total positive charge of the measured cations as well as the concentrations of the cations were generally much larger in the centrifugates than in the lysimeter solutions, which implies that total charge of soil solution is larger in small pores than in large pores. Hydr ogen ion in particular was concentrated in some of the centrifugates, the r atio centrifugate:lysimeter solution being over 10. The total positive char ge of the measured cations decreased with increasing depth with both method s. The apparent cation-exchange coefficients KH-Ca, KAl-Ca, and KK-Ca had d ifferent values in the methods, and the variation in the cation exchange co efficients was larger in the lysimeter method than in the centrifuging meth od. The coefficient KMg-Ca had similar values in both methods. The results imply that mobile solution could not have cation-exchange equilibria with b ulk exchangeable cations in the soils, although solution in small pores see med to have equilibria. Solubility of Al did not follow the solubility of a n Al(OH)(3) phase in the centrifugates, and the centrifugates with a H+ act ivity larger than 60 mu mol were undersaturated with respect to the gibbsit e. Solubility of Al was between gibbsite and amorphous Al(OH)(3) in the lys imeter solutions. Differences between the centrifugates and the lysimeter s olutions in the ion concentrations and in the apparent chemical equilibria were similar for both soils studied.