Iron and Silica in Water, Acid Ammonium Oxalate, and Dithionite Extracts of Some North Carolina Coastal Plain Soils

Sequential water, oxalate, and dithionite extracts of a Paleaquult and two Paleudults were analyzed for iron and silica. The ratio of oxalate iron to dithionite iron (active iron ratio) suggests that there is little accumulation of amorphous iron compounds in the Paleudults. It also suggests goethite may be the main iron oxide mineral. The active iron ration in the gray matrix of the Paleaquult shows that the common fine yellowish or brownish flecks and mottles contain a high proportion of amorphous iron compounds that have not aged sufficiently to become crystalline. Water table studies suggest these may be associated with the seasonal variations of water table depth. The silica extracted, especially by the dithionite, probably comes directly from the quartz sand and kaolinitic clay rather than from dissolution of iron compounds.