POORLY ORDERED HYDROUS FE OXIDES, COLLOIDAL DISPERSION AND SOIL AGGREGATION .2. MODIFICATION OF SILTY SOIL AGGREGATION WITH FE(III) POLYCATIONS AND MODEL HUMIC MACROMOLECULES

Poorly ordered ferrihydrites, more or less associated with synthetic humic macromolecules, were added to a poorly structured temperate silty soil. Manipulation of the net soil charge through these additions was measured by potentiometric titration and cation exchange capacity. The consequences of anchoring positively charged Fe hydrous oxides on negative permanent clay surface charges were studied. Both cementation and aggregate water-stability increased with increase in adsorbed iron. The aggregation process was studied by using micromorphology and ultramicromorphology, water retention curves, mercury porosimetry and a fractal approach. Aggregate water-stability was studied when the soils were air-dried, with some irreversible aggregation, and oven-dried or wetted from -1.5 MPa to - 1 kPa thereafter. Soils with added Fe(III) polycation-humic macromolecule associations were less positively charged, less cemented and less stable than soils amended with Fe(III) polycations alone. The effect of these treatments on slaking is discussed.