Factors related to Bh-horizon depth for artificial and natural E–Bh sequences

Aquods occur extensively in the coastal plain of the SE USA, where podzolization is mainly restricted to fluctuating water table conditions. Depths to upper boundaries of Spodic (Bh) horizons vary from a few cm to ≥ 75 cm even under similar drainage conditions. Hence, seasonal water table depth parameters do not explain the wide variation in boundary depth. One objective of this study was to experimentally test the following hypotheses: (H1) Depth to the E–Bh boundary relates inversely to concentration of metal oxides, silt, and clay content of initial (“parent”) material for similar hydrologic conditions and organic acid activity; (H2) depth to that boundary is greater under higher organic activity for a given soil material and hydrologic condition. Another objective was to statistically evaluate how the composition of > 270 natural Spodic horizons collected during the Florida Cooperative Soil Survey Program related to upper boundary depth. Thirteen sandy coastal plain soil materials of varying noncrystalline-metal-oxide- (Al and Fe), silt-, and clay contents were packed in columns and subjected to daily cycles of fluctuating water tables and dilute organic acid applications. Artificial E–Bh boundaries of variable depths were generated in the columns. Boundaries tended to be abrupt but topographically irregular. Clay- and extractable-noncrystalline Al and Fe contents were inversely related to final depth of artificial Bh horizons for the same water-table conditions. Higher oxalic acid concentrations produced deeper boundaries in the same materials; hence, local biogeochemical factors related to organic acid production are also likely to influence the ultimate Aquod morphology on the landscape. Silt-, clay-, and metal-oxide contents were significantly higher in shallower than in deeper natural Florida Spodic horizons.

► Artificial Bh horizons formed under specific hydrology and organic acid elution.
► Horizon depth related to composition of original material and acid concentration.
► Natural Bh horizons also showed compositional trends with depth.