Aggregation and aggregate size effects on extractable iron and aluminum in two hapludoxs

Oxisols contain stable aggregates, and sorption and desorption kinetics for aggregates typically are slower than for disaggregated samples. The objectives of this research were to determine whether aggregation inhibits Fe and Al extraction more in large than small aggregates, and to determine whether aggregate composition differs between large and small Oxisol aggregates. Water-stable 1- to 2- and 0.125- to 0.25-mm aggregates were separated from two Oxisols that have similar total Fe contents but different goethite/hematite and kaolinite/gibbsite ratios. To assess the effect of aggregation on extractable Fe and Al, subsamples were disaggregated by sonication. Disaggregated samples and intact aggregates were then reacted with oxalate and dithionite-citrate-bicarbonate (DCB). In the larger aggregates, oxalate and DCB extracted 10 to 20% less Fe and Al from intact aggregates than from disaggregated samples. In the smaller aggregates, aggregation only caused a 5 to 10% decrease. Aggregation generally inhibited Fe and Al extraction by DCB more in B than A horizons. There also were small compositional differences between large and small aggregates. In A horizon samples, large aggregates contained more clay and had correspondingly greater extractable Fe and Al than did small aggregates, although the extractable Fe and Al in the clays themselves did not differ between large and small aggregates. Goethite/hematite ratios were 10 to 20% greater in clay from large than small aggregates. Thus, the rate and extent of sorption-desorption reactions may differ between large and small aggregates not only because of physical differences, but also because of slight compositional differences.