The genesis and characteristics of the fragipan of the Beltsville soils

Abstract

Three profiles of the Beltsville soil from Fairfax County, Virginia were studied in relation to the mechanical composition, the mineralogical composition, and some of the chemical and physical properties. Soil samples were selected to represent a profile developed over Coastal Plain deposits, schist, and granite, respectively.

A high kaolinite content, a low quartz and illite content, and the presence of gibbsite in the horizons below the fragipan indicate that this material has undergone more intensive or a different type of weathering than the material in the solum.

Two horizons high in clay were found in each of the profiles studied: one above the fragipan, which is the present illuvial horizon, and the other below the fragipan, which may be a buried illuvial horizon. In two of the profiles the heavy textured horizon below the fragipan had a higher clay content than the present illuvial horizon.

Fragipan formation possibly resulted from the deposition of material eluviated from a fine textured mantle above. The heavy texture of the material below the fragipan was probably effective in initiating the development of the fragipan.