Abstract

The role of organic matter (OM) on surface reactivity of two Chilean Andisols was studied. The soils were subjected to sequential remotion of fulvic and humic acids (S/HA,FA) and humin (S/OM). Soil and its fractions obtained were analyzed for determining changes in surface reactivity such as number of active sites, area, and pKa values. In addition, aluminum (Al) and iron (Fe) forms in the soils were estimated by selective dissolution treatments and the adsorption capacity was measured by isotherms in the soils and its fractions. The results show an increase 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 for the P reactions. The highest P adsorption capacity in this soils was associated with humin‐clay interactions.