Effect of wheel traffic on the physical properties of a Luvisol

The effects of machine traffic were assessed on a Luvisol in a temperate climate area in Belgium. Soil samples were taken from topsoil (0.07-0.25 m) and subsoil (0.35-0.50 m), on plots under long-term reduced tillage (RT) and conventional tillage (CT). Cone index (CI), bulk density (BD) and precompression stress (Pc) were chosen as indicators of mechanical strength. Mercury intrusion porosimetry was used to characterize the soil microporosity structure. It was presented in two forms: (i) cumulative pore volume vs. equivalent pore radius r, from which four classes of porosity were defined: r < 0.2 μm, 0.2 ≤ r < 9 µm, 9 ≤ r < 73 µm and r ≥ 73 μm; (ii) pore-size distribution (PSDs). In the reference situation where there had been no recent passage of machines, the voids with 0.2 ≤ r < 9 µm were the most important class in RT topsoil. The voids with r ≥ 73 µm represented the main porosity class in the topsoil of CT. In the subsoil, for both tillage systems, the porosity was almost equally distributed between voids with 0.2 ≤ r < 9 µm and voids with r greater than 9 µm.
Machine traffic was carried out when the soil water content was close to the optimum Proctor. Although unfavourable, these wet conditions often occur during the beet harvesting period in Belgium. The highest modifications in soil structure (increase in BD and Pc, reduction of macroporosity r ≥ 73 μm) were observed in the topsoil of CT. More limited modifications were noticed in the soil structure of RT topsoil and subsoil layers but these latter are problematic in that the soil would no longer be loosened by subsequent tillage. These modifications could lead to soil consolidation as a result of wheel traffic year after year.