Aggregation in soils with small amounts of swelling clays. II. Chemistry and surface properties of Na resin stable soil aggregates

In part i (see IRRD 818362) both multivariate statistical analysis and physico-chemical approaches were used to describe the macroaggregate stability of a series of selected air-dried soils with small amounts of swelling clays. Two main soil classes were distinguished: (a) unstable soils with low contents of both organic matter (om) (om, less than 6% c) and poorly ordered al and fe hydrous oxides (poho), and (b) aggregated organic soils greater than 6% c) richer in non-crystalline minerals. Disruption of aggregates with na resin after 16 h shaking was one of the methods which clearly differentiated unstable and stable macroaggregates. After a standardized na resin aggregate stability treatment, less than 2, 2-20 and 20-200 mu M particle size fractions were separated, freeze-dried and analysed. The object of the present paper is to report on the nature and distribution of the poho/om associations in these homoionic na aggregates and to assess their influence on the surface properties of the aggregates. Stable aggregates of soils with small amounts of swelling clays were characterized after 16 h shaking with sodium resin. Different kinds of organic material operate at various levels of micro-aggregates size, being more aromatic in the coarser aggregates and combined with poorly-ordered al and fe hydrous oxides. Effects on aggregate surface properties are described. For each size of resin stable aggregate, the mesopore volume increased with both increasing aggregating organic materials and poorly-ordered al and fe hydrous oxides.