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Hydromorphology and Plinthite Characterization of Some Alabama Coastal Plain Soils




Smith, Richard

Type of Degree



Agronomy and Soils


Estimating seasonal high water tables (SHWTs) within soils by the evaluation of redoximorphic features is critical for soil interpretations. Certain redoximorphic features indicate contemporary moisture regimes, while others indicate past moisture regimes (relict). Alabama Coastal Plain soils present challenges in identifying contemporary seasonal saturation due to low carbon and oxide content, perched water tables rich in dissolved oxygen, and the presence of plinthite. The role plinthite plays in identifying contemporary seasonal saturation is unclear, and characterization of its occurrence and properties will provide knowledge as to its role in contemporary hydromorphology assessment. This study, jointly conducted between the Natural Resources Conservation Service (NRCS) and the Alabama Agricultural Experiment Station (AAES), monitored vi the depth and duration of SHWTs (2004 – 2006) of some Alabama Coastal Plain (CP) soils to develop relationships between SHWT metrics and hydromorphic features. Twenty piezometers were installed in eleven CP pedons (Paleudults and Kandiudults, most with plinthite and sandy epipedons of varying thickness) at various depths. Water table data were recorded every six hours and daily rainfall was obtained from proximate weather stations. Soils were described, sampled, characterized and classified according to standard techniques. Plinthite was separated and quantified using a slaking technique. Characterization of plinthite versus whole soil properties was also conducted. Plinthite contained 31% more carbon, 4% more clay, 259% more DCB extractable Fe (Fed), and 1280% more AOX extractable Fe (Feo) than the corresponding whole soil. Active Fe increased with saturation, providing evidence of contemporary Fe reduction in these soils. Rainfall patterns were inconsistent during the study period, and only water table data within years of normal rainfall (as per Soil Taxonomy) were evaluated. Perched water tables were not consistently associated with any pedogenic feature. Horizons containing soft iron (Fe) accumulations were saturated for 11% of the monitoring period, horizons containing plinthite were saturated 20% of the monitoring period, horizons containing chroma = 2 Fe depletions were saturated 25% of the monitoring period, and depleted horizons were saturated 56% of the monitoring period. The presence of plinthite in these soils occurred over a wide range of saturation durations (1 to 47% of the monitoring period). Eight of the eleven sites had water tables consistent with their field drainage class assessment.