Assessing Temporal Changes of the Soil Mechanical Strength of a Volcanic Ash Soil at Different Bulk Densities Under Pasture

This study analyzed the time- and water content-dependent changes in penetration resistance (PR) profiles of an Andosol at different dry bulk densities under pasture. PR was measured from the soil surface to a soil depth of 80 cm in a soil compaction trial (T0, 0.65 Mgm(-3); T1, 0.75 Mgm(-3); and T3, 0.85 Mgm(-3)) established in May 2019 in a Petroduric Silandic Andosol under pasture grazed by sheep. Both the volumetric water content and matric potential were measured with sensors at the soil surface and at depths of 10, 20, and 60 cm. In addition, undisturbed soil samples were collected to determine the precompression stress. Water retention curves and air permeability (6 kPa matric potential) were measured. The soil compaction events induced an increase in bulk density and a decrease in wide coarse pores as well as air permeability, reflecting an increase in precompression stress and penetration resistance. As the soil water content (WC) decreased, the PR increased at all studied soil depths; under the driest conditions (WC: 28 Vol. %), the highest accumulated PR values were assessed between 0 and 10 cm. At field capacity, the highest registered PR values, after compaction events at the soil surface, did not reach the critical value of 2 MPa; however, they represented the soil with the highest bulk density and precompression stress as well as with a lower volume of macropores and air permeability. Accordingly, as the water content decreased, the maximum values of PR were observed near the soil surface in the densest soil. The initial differences in PR values assessed between treatments at the soil surface, disappeared few months later because of soil drying and animal trampling reflecting that the soil physical properties tended to reach an equilibrium with the applied external stresses.