Compaction Susceptibility of Select Alabama Piedmont and Upper Coastal Plain Ultisols

Abstract

The U.N. categorizes soil compaction as the most important type of physical soil degradation because of soil productivity and quality loss. Subsoil compaction causes great concern because it is difficult to remediate, particularly in forested settings. Soils vary in their reaction to applied dynamic and static forces and susceptibility to compaction based on physical, chemical, biological, and morphological properties. Soil mechanical approaches have developed indices and parameters that describe a soil’s behavior upon compaction for largely engineering applications, while little work has related these to soil morphological and pedological measures. Soil surveys and on-site pedological investigations describe soil morphological properties (e.g. horizonation, structure, consistence) that we hypothesize to be related to soil compaction susceptibility. In order to more fully understand relationships between soil properties and response to applied static and dynamic forces, a two-fold approach utilizing field-based trafficking and laboratory-based consolidometer experiments was used. In the first study, recently developed sensors were used to measure stress transferred through soil with depth during trafficking events for nine Alabama Piedmont sites where soils were Typic, Oxyaquic, and Rhodic Kanhapludults. Soils were described, sampled, and several near surface (0-40 cm) properties were measured. Sites were trafficked with five passes using a CAT 535D forestry skidder and soil morphological and physical properties were related to the resulting compactive forces. Sensors placed in subsurface (~12.5 cm) and subsoil (~25 cm) horizons (measured stress) responded systematically with depth to trafficking pressures. Bulk density increased (6% and 16% for overall and surface horizons, respectively) following trafficking. Soil texture (e.g. clay content, Atterberg limits) correlated with several other near surface properties (r = 0.21 to 0.77, p<0.10). Using principal components, loading factors indicated that textural properties (USDA texture, Atterberg limits) and volumetric water content described the majority of soil property variability. Stepwise linear and principal component regression indicated that soil physical properties (e.g. texture, volumetric water content, and bulk density) and, to a lesser extent, morphological properties (e.g. argillic depth, stratification ratios, and horizonation) were related to measured dynamic stress. In these select Alabama Kanhapludults, soil texture is highly related to sensor-based stress measurements, but morphological properties also describe a portion of variability. These properties are provided in soil surveys and soil descriptions, illustrating their utility in determining compaction susceptibility. In the second study, a consolidometer (static loads from 10-800 kPa) was used to develop compression indices and relate these to soil morphological and physical properties for surface (A), subsurface (E, BE) and subsoil (Bt) horizons (34) collected from Alabama Upper Coastal Plain and Piedmont Ultisols. Soils were described, sampled, and several near surface (0-40 cm) properties were measured. Surface horizons were more susceptible to bulk density change caused by compression (~35% vs ~20%) and had higher compression indices (CP) (0.24 vs 0.19) than subsurface or subsoil horizons. Uncorrelated factors developed using factor analysis indicated that textural properties (USDA texture, Atterberg limits, and Coefficient of Curvature) and dynamic properties (soil organic carbon (SOC), water content at 0.3 bar) described a significant portion of soil property variability. Stepwise linear and factor regression suggested that dynamic properties (SOC, bulk density), static properties (texture), and morphological properties (structure grade) affected compaction susceptibility. Compression indices were largely affected by SOC and texture (e.g. coefficient of curvature, % coarse fragments). Maximum bulk density (to 800 kPa) and bulk density change (from initial to 800 kPa) increased with greater SOC and decreased with structure grade development. The goal of these studies was to better understand compaction susceptibility of select southeastern U.S. Piedmont and Coastal Plain soils through relationships between near-surface soil properties and applied dynamic (soil stress translation) and static compression (consolidometer) measurements. In general, soil textural attributes describe approximately 15-30%, dynamic properties (SOC, WSA) describe approximately 40%, and morphological properties (structure grade) describe approximately 15% of compaction susceptibility variability. The aggregate results from this study illustrate that soil compaction susceptibility is influenced by static, dynamic, and morphological properties. Combining this understanding with pre-existing tools, such as soil surveys, can provide producers—particularly in the forest industry—with valuable information on which sites are more susceptible to compaction. This can contribute to an overall management plan that seeks to minimize the degradation of a precious, life supporting, finite resource—our soil.