AGGREGATION AND AGGREGATE SIZE EFFECTS ON EXTRACTABLE IRON AND ALUMINUM IN 2 HAPLUDOXS

Oxisols contain stable aggregates, and sorption and desorption kinetic s 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 a ggregates were separated from two Oxisols that have similar total Pe c ontents but different goethite/hematite and kaolinite/gibbsite ratios. To assess the effect of aggregation on extractable Fe and Al, subsamp les were disaggregated by sonication. Disaggregated samples and intact aggregates were then reacted with oxalate and dithionite-citrate-bica rbonate (DCB). In the larger aggregates, oxalate and DCB extracted 10 to 20% less Fe and Al from intact aggregates than from disaggregated s amples. 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 differe nces between large and small aggregates. In A horizon samples, large a ggregates contained more clay and had correspondingly greater extracta ble Fe and Al than did small aggregates, although the extractable Fe a nd Al in the clays themselves did not differ between large and small a ggregates. Goethite/hematite ratios were 10 to 20% greater in clay fro m large than small aggregates. Thus, the rate and extent of sorption-d esorption reactions may differ between large and small aggregates not only because of physical differences, but also because of slight compo sitional differences.