Effects of Parent Material Weathering on Chemical and Mineralogical Properties of Selected Hapludults in the Virginia Piedmont

Abstract

AbstractSaprolite is a parent material prevalent in the Piedmont of the USA. Chemical weathering during saprolite development, but prior to soil formation, may obliterate chemical and mineralogical influences due to parent rock differences. The Cullen series and associated red Hapludults mapped in the Piedmont are an example. Cullen soils are derived from saprolites weathered from rocks of varying mafic and felsic composition. The mafic influence (Fe content) is manifested in the Cullen series as a dark red (1OR or 2.5 YR 3/6) B subhorizon. Soils that lack this feature are presumed to be less productive. Family control section chemical and physical properties were consistent among pedons studied, and did not relate to parent rock type or B horizon color. Lesser amounts of extractable Al in the lower part of the series control section provide a clue as to why Cullen soils are preferred for agricultural use over other Hapludults. Fourteen of the pedons studied were in the mixed mineralogy class, two were oxidic, and none were kaolinitic. In the northern Piedmont kaolinitic soils do not appear to exist, except on mafic parent materials. Soils studied had a high iron oxide content (2.0–15%), which did not correlate with dark red colors or parent material. Conversely, they contain small amounts of gibbsite. This data and discrepancies in the oxidic definition question the validity of the present osidic family requirements as a means for identifying soils dominated by weathered minerals. Soils derived from saprolites weathered from a variety of parent rocks have reached a similar mineralogical and chemical state in the family control section.