AL-ORGANIC MATTER ASSOCIATIONS AS CEMENTING SUBSTANCES OF OCHREOUS BROWN SOIL AGGREGATES

Abstract

The resistance of soil structural units to breakdown in water, Na resin, or H resin suspension was determined kinetically for a very water-stable organic soil (ochreous brown). Aggregate stability to Na or H resin was mainly a function of the more or less hydrophobic nature of Al-organic matter associations. Soil organic matter dissolution was a good tracer of the wetting process, and our study shows that two kinds of Al-organic matter associations act as cementing substances in such very water-stable, temperate, organic soils. The first association was organic matter similar to fulvic acids but richer in polysaccharides and peptides, weakly associated with exchangeable Al. The second association was organic matter also similar to fulvic acids but much richer in oxygenated functional groups and in nitrogenous compounds, probably strongly associated with poorly-ordered hydrous Al oxides. They were disrupted by Na and H resin, respectively. In both kinetic treatments, correlations among soil disaggregation, exchanged or/and dissolved Al, and dissolved organic matter were observed and interpreted as follows: breakup of soil aggregates into smaller units (slaking) could have been caused by the pressure of entrapped air which increased as a function of the soil wettability state from water treatment to Na resin treatment and finally to H resin treatment.