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 Swede n was monitored for four years using percolation lysimeters. Weak orga nic acids were a major constituent of the soil solution and are import ant because of their ability to form complexes with aluminium. Dissolv ed organics leached from the mor layer enhance the weathering rate in the eluvial horizon by forming complexes with aluminium, especially du ring the autumn when the leaching of dissolved organics was greatest. The weak organic acids were titrated and their pK(a) values were evalu ated. Aluminium was speciated with an ion-exchange method and by apply ing equilibrium calculations. Formation constants for the organic alum inium 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 examin ed using solubility constants. Percolation lysimeters below undisturbe d and cut-off mor layers were compared.