WEATHERING OF SOIL CLAYS WITH DILUTE SULFURIC-ACID AS INFLUENCED BY SORBED HUMIC SUBSTANCES

Humic substances extracted from an Alfisol, Mollisol, and Histosol were sorbed to the clay (< 2 mu m) suite extracted from an Alfisol Bt horizon. The clays with sorbed humic substances as well as clays with indigenous organic matter and with Organic matter oxidized with H2O2 were weathered by incubation in 0.05M H2SO4 for 1320 h and in 0.0025M H2SO4 for 1176 h. To evaluate weathering, the quantities of Mg, K, Al, Si, and Fe released and clay dissolved were monitored. In both weathering studies, Mg was preferentially released by the clays. Because Mg occurs mainly in the octahedral sheet of clays, the high release of Mg suggests preferential dissolution of the octahedral sheets. In the 0.05M H2SO4 dissolution study, no differences were found with respect to K, Al, Si, and Fe release from the different clays, but slightly less Mg was released from clays with sorbed humic substances. At the completion of the incubation in 0.05M H2SO4, 21 to 24% of the clays had dissolved. Using a less concentrated acid (0.0025M H2SO4), notable differences were apparent in the degree of weathering among the clays. Approximately, 3 times more Mg was released from clays with organic matter oxidized than from clays with indigenous or sorbed humic substances. Similar quantities of Mg were dissolved from clays with indigenous or sorbed humic substances. K release was similar from all clays. Using 0.0025M H2SO4, a larger portion (5.8%) of the clay with organic matter oxidized was dissolved than of clays with indigenous (2.6%) or sorbed humic substances ( < 1%). Data for Mg release and clay dissolution in 0.0025M H2SO4 both suggest that the weathering of clays was reduced by indigenous organic matter or sorbed humic substances. No weathering differences could be detected between clays with sorbed humic versus fulvic acids or between sources (Mollisol, Alfisol, or Histosol) of humic substances.