Influence of single passes with high wheel load on a structured, unploughed sandy loam soil

In a field experiment, a sandy loam was subjected to single passes with a s ugar beet harvester at two different soil water potentials. Different hoppe r fillings resulted in ground contact pressures of 130 kPa (partial load) a nd 160 kPa (full load) underneath the tyre. Bulk density, macroporosity (eq uivalent pore radius >100 mu m), penetrometer resistance, air permeability and pre-consolidation pressure were measured within and next to the wheel t racks at depths of 0.12-0.17, 0.32-0.37 and 0.52-0.57 m. Furthermore, the s oil structure at two horizons (Ahp 7-24 cm, B(C) 24-38 cm) was visually ass essed and classified. The moist plot responded to a wheel load of 11.23 mg (160 kPa) with an incr ease in bulk density and pre-consolidation pressure as well as with a decre ase in air permeability and macroporosity at a depth of 0.12-0.17 m. With a wheel load of 7.47 mg (130 kPa) on the moist plot and with both wheel load levels on the dry plot, only slight changes of the soil structure were det ected. At a depth of 0.32-0.37 and 0.52-0.57 m, the measurements did not in dicate any compaction. An ANOVA indicates that the factor "soil water poten tial" and the factor "wheel load" significantly influence the bulk density at a depth of 0.12-0.17 m. No interactions occurred between these two facto rs. The wheel traffic on the test plot had no effect on the yield of winter wheat planted after the experimental treatment. Bulk density, macroporosity and pre-consolidation pressure proved to be sen sitive to detect compaction because they varied only slightly and are easy to measure. In contrast, the standard deviation of air permeability is larg e. The soil structure determined visually in the field confirms the values measured in the laboratory. The results of the penetrometer resistance meas urements were not explainable. (C) 1999 Elsevier Science B.V. All rights re served.