SIGNIFICANCE OF ORGANIC-ACIDS FOR WEATHERING AND THE PODZOLIZATION PROCESS

Weak organic acids were a major constituent of the soil solution in th e elluvial horizon, especially during the autumn when the leaching of organic compounds was greatest. Dissociated organic acids formed compl exes with aluminium. Formation constants for the organic aluminium com plexes were estimated as log K-Alorg = 5.4 +/- 0.3 (n=13) in spring an d summer and log K-Alorg = 4.9 +/- 0.1 (n=6) in autumn. Budget calcula tions showed that the weathering rate was about 3.5 times higher in th e upper soil due to the presence of organic acids, which formed comple xes with aluminium, thereby enhancing the weathering rate. Laboratory batch experiments on feldspar weathering rates, a similar increase in the weathering rate enhancement of 2-3.5 times was found in the presen ce of natural organic solutes. inoculation with microorganisms elimina ted enhanced weathering ability. When citrate was used in the batch di ssolution experiments, an aluminium-silicate phase was formed after in oculation. It was possible to simulate the same podzolization effects by percolating organic acids through a soil column for ten months, but only under nonsterile conditions. A 7-8 cm deep elluvated horizon was obtained and it appeared likely that an aluminium-iron-silicate phase was precipitated in the deeper parts of the column. In sterile column s, elluvation occurred throughout the entire column. Under nonsterile conditions, organic acids, which formed complexes with aluminium and i ron were easily degraded by microorganisms while passing the column. T his decreased the weathering rate and aluminium and iron precipitated in an aluminium-iron-silcate phase in the deeper part of the column. T hese processes are relevant to podzolization processes in soil.