Soil property trends and classification of alluvial floodplains, South Carolina Coastal Plain

Abstract

Alluvial soils derived from sediments of Piedmont origin occur in the Atlantic Coastal Plain of the southeastern U.S. In South Carolina, high-order rivers receiving sediment are Savannah (SV), Congaree (CN), Wateree (WA), Santee (ST), Lynches (LY), and Great Pee Dee (PD). This study investigated distribution of soil properties on natural levee, valley flat, and backswamp landforms within and among the six river floodplains and compared results to the taxonomic classifications used historically during soil survey production. This study also investigated the contribution of sediment sources to soil characteristics. Data were compiled for 65 soil pedons sampled from 16 soil survey areas during a 41-year time period. Pedons were located in three Major Land Resource Areas (MLRAs), and the results should aid in development of both land resource area and ecological site concepts. Data analyses centered on clay and silt contents, mineralogy, base saturation (BS), and cation exchange activity (CEA) classes. Evaluation of data from the six river floodplains revealed common properties among soils on fluvial landscapes across multiple MLRAs. Four soil systems, each derived from a predominant rock type, composed the Piedmont drainage basin of each river. The absolute area of each soil system differed between rivers, but sediment supplied typically did not result in great soil texture variation per landform. The land area proportion of the Carolina Slate Belt soil system had a close relationship to silt contents of the alluvial soils. On the valley flat landform of five floodplains, particle-size control section (PSCS) median clay content ranged only between 302gkg−1 and 405gkg−1. Among all valley flats median PSCS silt content ranged between 361gkg−1 and 511gkg−1. Slight positive downriver trends for clay were observed on SV, WA, and PD and for silt on SV and WA. Although local BS variation could be high between pedons, slight positive downriver BS trends on valley flats were observed on SV, WA, and ST. Negative BS trends were seen on LY and PD. Kaolinite was the dominant clay mineral in soils of all landforms on the six floodplains. Weatherable primary minerals in sand and silt fractions resulted in higher BS except for the LY soils, where resistant minerals composed sand and silt fractions. The semiactive CEA class occurred predominantly on all three landforms. Taxonomic analysis supported soil assignment to the suborders Aquepts or Udepts on all floodplains. Great group level Eutrudepts in South Carolina are first reported here, and they occur extensively enough on SV, CN, WA, and PD to warrant recognition in soil map units. Because most soil properties showed little variability for soils occurring in floodplains for all six rivers, botanical assessments will be needed to determine the magnitude of the role the properties have toward defining ecological sites. Ultimately, the alluvial systems may be aggregated as a single land resource area within the Coastal Plain.