HUMIN-CLAY INTERACTIONS ON SURFACE REACTIVITY IN CHILEAN ANDISOLS

The role of organic matter (OM) on surface reactivity of two Chilean A ndisols was studied. The soils were subjected to sequential remotion o f fulvic and humic acids (S/HA,FA) and humin (S/OM). Soil and its frac tions obtained were analyzed for determining changes in surface reacti vity such as number of active sites, area, and pKa values. In addition , aluminum (Al) and iron (Fe) forms in the soils were estimated by sel ective dissolution treatments and the adsorption capacity was measured by isotherms in the soils and its fractions. The results show an incr ease in surface acidity and the number of active sites when humic and fulvic acids were extracted associated with an increase in phosphate ( P) adsorption capacity of these soils. This fact indicates that humic and fulvic acids produce a blocking effect on active reactant sites fo r the P reactions. The highest P adsorption capacity in this soils was associated with humin-clay interactions.