THE ROLE OF ORGANIC-ACIDS IN THE SOIL SOLUTION CHEMISTRY OF A PODZOLIZED SOIL

The soil solution chemistry of a podzolized soil in the north of Sweden was monitored for four years using percolation lysimeters. Weak organic acids were a major constituent of the soil solution and are important because of their ability to form complexes with aluminium. Dissolved organics leached from the mor layer enhance the weathering rate in the eluvial horizon by forming complexes with aluminium, especially during the autumn when the leaching of dissolved organics was greatest. The weak organic acids were titrated and their pK(a) values were evaluated. Aluminium was speciated with an ion-exchange method and by applying equilibrium calculations. Formation constants for the organic aluminium complexes were calculated to be log K(Alorg) = 5.42 +/- 0.32 M-1 (n = 13) in spring and summer and log K(Alorg) = 4.87 +/- 0.14 m-1 (n = 6) in autumn. Equilibria of Al3+ with solid phases were also examined using solubility constants. Percolation lysimeters below undisturbed and cut-off mor layers were compared.