Subsoil Saturated Hydraulic Conductivity in Relation to Soil Properties in the North Carolina Coastal Plain

Abstract

AbstractLaboratory measurements of saturated hydraulic conductivity (Ksat) of soil cores from nine North Carolina Coastal Plain Ultisols demonstrate that Ksat generally decreases with increasing depth, while grade of blocky structure generally becomes stronger with depth. The best single predictor (second‐order polynominal, r2 = 0.81, p < 0.001) of Ksat is the proportion of pores with diameters >30 µm (LP) as calculated from water retention characteristics, followed by the coefficient of linear extensibility measured on soil pastes (COLE paste, r2 = 0.50). Total porosity is correlated negatively with Ksat (r2 = 0.16, p > 0.10). Multiple regression of Ksat on LP and COLE paste decreased correlation slightly (R2 = 0.77, p <0.001). Weighted mean ped diameter (WMPD), which is an indication of structure size and grade, is not significantly (p < 0.10) correlated with Ksat. Inter‐ped planes of weakness are not well developed in these weakly structured soils and do not contribute much to saturated flow. Stratification of pedons by drainage class and clay content had minor effects on regression correlation. Large pores are the dominant conduits for saturated water movement in these soils as measured by the method used in this study. The proportion of large pores, probably mostly of biological origin, decreases with increasing soil depth as biological activity is attenuated. The COLE paste measurement provides an index of the potential for swelling of clay minerals and resultant constriction of water‐conducting channels when soils are saturated.