Subsoil Blocky Structure Formation in Some North Carolina Paleudults and Paleaquults
- R. J. Southard and
- S. W. Buol
The relationships among subsoil blocky structure, soil drainage class and landscape position were studied on nine Ultisol pedons from the Middle Coastal Plain of North Carolina. The pedons represented fine-loamy, fine-silty, and clayey families of Typic and Aquic Paleudults and Typic Paleaquults. The grade of blocky structure was strongest in poorly drained soils and in moderately well-drained soils below the seasonal high water table. Soils of fine-silty families had the strongest blocky structure. Weighted mean ped diameter (WMPD), which was interpreted as a combination of ped size plus grade of structure, was highly and positively correlated with percent silt; WMPD decreased with increasing clay or sand content. Within a given particle-size family, WMPD increased in more poorly drained soils. There was no consistent trend of WMPD with depth. Stronger grades of structure observed in the field were associated with the presence of thicker and more abundant clay films on peds and with thick oriented pore argillans in thin section. Plotting the location of the pedons relative to the edge of the Coastal Plain surface and to the location of the water table revealed that the subsoil zone with the strongest grade of structure coincided with the region of the subsoil that is saturated about 75% of the time. Poorly drained soils become better drained as the Coastal Plain is dissected by stream downcutting and saturation-desiccation cycles occur less frequently. Blocky peds that formed when the soils were poorly drained are degraded by other soil forming processes such as eluviation and bioturbation.
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