Critical shear stress variability in claypan soils with depth

Abstract

Soil erosion from land management activities reduces agricultural productivity and contaminates waterways. Understanding erosion processes within agricultural fields is critical to developing alternative management scenarios to better manage soil resources. Claypan soils comprise approximately 5% of the agronomic area in the US Midwest; however, little is understood about the erosion characteristics within the claypan soil profile. Claypan soils are defined by a dense, impermeable layer that is more resistant to erosion. In this study, we used geotechnical methods to examine claypan soils in agricultural fields in southeast Kansas that showed a rapid transition from high clay to low clay content on the soil surface. Laboratory erosion measurements with an erosion function apparatus (EFA) demonstrated a two-layer soil system at both locations. At Site 1, we found a high plasticity clay layer at 25 cm depth in the soil profile, with a hydraulic conductivity 100-fold less than the surface soil, an unconsolidated undrained triaxial strength more than double that at the surface, and a critical shear stress that was on average five times higher than that measured in the surface layer. This high plasticity clay layer dissipated, with lower elevation locations showing similarities in soil strength and critical shear stress at the surface and 25 cm in the soil profile. At Site 2, laboratory experiments showed a similar two-layer soil structure, though the clay layer did not dissipate but instead remained at a lower position in the soil profile. In situ erosion measurements with a field jet erosion test (JET) apparatus showed a higher critical shear stress and lower erosion rate in the soils above the claypan. Soils not in the claypan area showed greater similarity in critical shear stress and erosion rate with depth in the profile. Calculating erodibility coefficient as a function of critical shear stress using the JET test results identified a cluster of measurements with very high critical shear stress and low erodibility. This cluster of soils were located on the claypan area as they were collected 25 cm down. These results reveal some of the sources of variability found in claypan soils and indicate the need for more careful planning to manage the soil that will result from the compositional changes. Management practices to reduce erosion will also require alternative approaches to accommodate the inherent spatial variability of soils and changes within the soil profile.