Saturated hydraulic conductivity (K_s) is one of the more often used properties for evaluating soil suitability for many uses including water relationships for plant growth, potential for pesticide leaching, and wastewater assimilation. Because of the time and expense associated with direct measurement, K_s is often estimated from other properties including texture, clay mineralogy, bulk density, and cementation. Of these properties, texture is most easily estimated in the field and thus, is often given the greatest weight in K_s estimates. Upland soils in the Southern Piedmont of the U.S. have maximum clay contents in upper Bt horizons, and clay content decreases with depth. Because of this clay distribution, upper Bt horizons are commonly estimated to have the minimum K_s in the profile with less clayey lower Bt, BC, and C horizons estimated to have higher K_s. Limited data for soils in the Piedmont, however, suggest that clayey upper Bt horizons have higher K_s than subjacent horizons because of strongly expressed pedogenic structure in upper Bt horizons forming a network of macropores. If the K_s relationships derived from these limited studies hold true for a wide range of soils and landscapes in the region, current estimates of K_s in soil survey databases do not accurately reflect the relative rates of water movement through the horizons. Thus, the objectives of this research were to evaluate saturated hydraulic conductivity for major horizons of soils in the Piedmont of Georgia and to develop relationships between K_s and morphological and/or other properties of these horizons.

               Five hillslopes in the Georgia Piedmont were selected for study. Slopes ranged from 5 to 20%, and all of the sites were in pasture. Soils at all hillslope positions were dominantly fine, kaolinitic, thermic Typic Kanhapludults or Hapludults. At 21 locations on each hillslope, the soil was described from a bucket auger boring, and K_s was measured at three depths with a constant head borehole permeameter. The 21 locations on each hillslope were arranged in three transects that extended from summit to footslope positions. The three depths of measurement represented upper Bt horizons, Bt or BC horizons, and lower BC or C horizons. Based on field measured K_s, three pedons on each hillslope were selected for detailed description and sampling. Laboratory analyses included particle-size distribution, cation exchange capacity, moist and dry bulk density, and citrate-dithionite extractable Fe. In addition, pore size distribution was evaluated from water characteristic curves and image analysis of polished blocks.

               Saturated hydraulic conductivity for all horizons evaluated was significantly different among the sites. Soils developed from saprolite with mineralogical composition that was intermediate between felsic and mafic had lower K_s than those developed from felsic saprolite. Morphological properties were similar for soils at all hillslope positions, and no significant difference was observed for K_s among the hillslope positions.

               Saturated hydraulic conductivity for upper Bt horizons ranged from 9 to 1,600 cm d^-1. Lower Bt and BC horizons (middle measurement depth) had K_s that ranged from 2 to 12 cm d^-1, and BC or C horizons at the lowest measurement depth had K_s that ranged from 3 to 310 cm d^-1. At all sites, the upper Bt horizon had the highest K_s in the profile. These horizons also had the highest clay content in the profile and had the most strongly expressed structure (moderate to strong subangular blocky). The lower Bt and BC horizons had less clay and weaker structure than the upper Bt horizons, and K_s of these horizons was considerably lower than that in the upper Bt horizons. Saturated hydraulic conductivity of BC and C horizons at the lowest measurement depth exhibited two trends. For soils with relatively deep sola, BC horizons at this depth had K_s that was similar to Bt horizons at the middle measurement depth. For soils with thinner sola, C horizons at the deepest measurement depth had K_s that was higher than overlying BC horizons. Clay, bulk density, coefficient of linear extensibility, and the CEC:clay ratio were all weakly correlated with K_s.

               The relationships among K_s, clay content, and other properties indicate that structure has the greatest influence on K_s of the properties evaluated for these soils. Macropores associated with the well expressed structure in upper Bt horizons result in rapid water movement through the horizon even thought clay content is high. Deeper Bt and BC horizons have less clay but also have weaker structure which limits water movement rates. C horizons have insufficient clay to fill packing pores between sand grains and thus, have sufficient large open pores to move water at a relatively rapid rate.