Certain Southeastern U.S.A. soils contain subsurface or subsoil horizons that differ from adjacent horizons by possession of one or more distinctive morphological features. Common terms for these are plowpan or traffic pan, brittle pan, organic pan, and fragipan. Results of micromorphologic studies of these pan types (claypans are excluded from this study) are reported, plus certain compositional characterization data to complement the thin section observations.
Brittle subsurface layers in the Norfolk soils were found to occur in two positions in the profile, immediately below the Ap horizon or plow layer and at or near the base of thick A2 horizons. The former may be induced by implement pressure; the latter may occur at depths of 6 and more inches below the base of the Ap but within the A2 or at A-B contact and seems to be pedogenic. The term “brittle layer” has been used to describe the latter; “arenapan” is suggested as a name for these subsurface, coarse-textured brittle zones.
Organic pans, as they are called locally, are apparent humus B horizons of Ground-Water Podzols. Samples from the Leon soil were employed in this study.
Fragipan development in the Southeastern United States is typified in the Grenada soil, formed from Southern Mississippi Valley loess. The fragipan in this soil occurs at the base of the B horizon, below a zone of slight clay accumulation.
The Leon Bh (“organic pan”) and the brittle layers in the Norfolk A2 horizon show heterogeneous distribution of quartz sand separated by intergranular bridges of fine-grained material. These pan layers appear highly porous in thin section, though many pores appear to be blocked by intergranular bridges. The Norfolk “plow pan” or traffic pan at the base of the Ap horizon exhibits close packing of quartzose sand and clay plus silt, with some humus inclusions. The Grenada fragipan is a close-packed silty matrix containing a few areas of oriented clay and appreciable components of feldspar and mica.
A clay mineral species or complex of 14.7 A. basal spacing, expandable to 17 to 18Å. with ethylene glycol solvation following treatment for removal of interlayer aluminum, appears to reach maximum concentrations in the pan layers of all profiles studied except Grenada. In the Grenada profile, there is a maximum relative concentration of this clay mineral immediately above the fragipan.