Physical Transformations in a Vertical Soil‐Saprolite Sequence

Abstract

AbstractA continuous vertical soil‐saprolite‐granitic gneiss profile was studied to determine its morphology, micromorphology, and physical properties. A clayey, kaolinitic, thermic Typic Hapludult formed in 4.5 m of saprolite derived from a granitic gneiss was sampled in 14 different levels from the surface to the consolidated rock to determine changes with depth. Morphologically, the Piedmont soil has a gravelly sandy loam surface over a red, clayey, moderately structured subsoil. The saprolite has a 1.15 m transitional sandy clay loam horizon over 3.35 m of a sandy loam saprolitic material with rock controlled structure overlying consolidated rock. Micromorphologically, content of skeletal grains increases with increasing depth in the subsoil and saprolite. Plasma and compound void volume decrease with increasing depth. Bulk density measurements of 0.95 Mg m−3 are found in the soil surface, 2.46 Mg m−3 in the rock, and about 1.3 Mg m−3 in the subsoil and saprolite. Pore size distribution as indicated by soil moisture release measurements shows 40% more macroporosity in the subsoil than the saprolite. Saturated hydraulic conductivity in the subsoil is 2.2 µm s−1—over twice the 1.1 µm s−1 found in the upper saprolitic horizon. Theoretically derived unsaturated hydraulic conductivities show little difference between the subsoil and saprolite. The significant physical changes accompanying the weathering of this saprolite to soil include: a weathering of sand‐sized weatherable minerals to clay‐sized minerals, an increase in structural strength of the Bt horizon, an accompanying increase in macroporosity in the Bt horizon, and higher saturated hydraulic conductivity in the Bt horizon than in the saprolite.