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DIVISION S-2-SOIL CHEMISTRY

Phosphate-Induced Clay Dispersion as Related to Aggregate Size and Composition in Hapludoxs

^a Soil Science Dep., Federal University of Lavras, Minas Gerais, Brazil, 37200-000
^b Earth Systems Sci. and Policy, CSUMB, 100 Campus Ctr., Seaside, CA 93955-8001 USA

jmlima{at}ufla.br

Although phosphate sorption is a well-understood phenomenon in soils, less is known of its effect on electrophoretic mobility (EM), isoelectric point (IEP), and clay particle dispersion in Oxisols. High phosphate sorption and high stability of aggregates are characteristic of Oxisols. Phosphate sorbed as inner-sphere complexes brings negative charge to the surface of particles, affecting EM, IEP, and clay particle dispersion. The objectives of this research were to determine the effect of residual sorbed P (after one sorption–desorption cycle in 0.015 M NaCl) on EM, IEP, and clay particle dispersion in aggregates of Oxisols with different organic matter contents and hematite/goethite ratios. Aggregates of 1 to 2 and 0.1 to 0.2 mm were fractionated from samples of A and B horizons of two Oxisols, both with 165 g kg^-1 Fe₂O₃, that differ in their organic matter and hematite and goethite contents. Phosphate sorption decreased EM and IEP of B horizon aggregates. It also decreased the amount of dispersed clay, as the IEP decreased to values closer to the pH of the soil suspension, decreasing net positive charge. Then, P sorption increased dispersed clay as the IEP became lower than the pH of suspension. The effect was slightly higher on aggregates with higher hematite/goethite ratio. The changes on those parameters were mostly noted for B horizon samples, where phosphate sorption had a major effect on charge balance because of their lower organic matter content. Small aggregates had less dispersed clay than large aggregates.

Abbreviations: DR, Dark-Red Latosol • EM, electrophoretic mobility • IEP, isoelectric point • UL, Una Latosol