Two North Carolina Coastal Plain Catenas: II. Micromorphology, Composition, and Fragipan Genesis

Abstract A study of two soil catenas of different age in the middle Coastal Plain of North Carolina compares the micromorphology and composition of fragipan and nonfragipan soils. The two catenas differ principally in that one has bisequal sola and fragipans in the less well‐drained members, whereas the other has thinner sola with well‐developed argillic horizons but no fragipans. Presence of papules, as well as high packing density and proximity indices, indicate that collapse of the soil matrix has caused the high bulk density of the fragipan horizons. The relatively low amounts of clay in fragipan horizons is believed to be a result of the greater susceptibility of clay to translocation and possibly destruction during reduction and removal of the iron. Translocation of this clay is confirmed by the marked increase of illuvial clay bodies, primarily illuviation cutans and grain argillians, in B′2t horizons below the fragipan (A′2gx). Relative to their clay content, fragipan horizons contain more fine clay with infrared absorption bands in the 2.9µ H‐bonded OH region than do nonfragipan horizons. This clay lacks definite crystalline form and is thought to be amorphous. Stable at lower temperatures, the infrared absorption band at 2.9µ is lost along with the fragipan brittleness at 300C. Though of low free iron content, this clay has a high pH dependent charge. Under the low pH conditions (4.5–5.0) found in fragipan horizons, this clay may develop strong hydrogen bonds with quartz and other silicates.