Agronomic consequences of tractor wheel compaction on a clay soil

In southern New South Wales, Australia, farming operations using tractors often occur when the soils are moist and prone to soil compaction. However, the extent of soil compaction and its relative impact on crop yield have not been quantified in the region. In this experiment, re-compaction due to tractor wheel traffic in a sodic brown clay (Vertisol) was monitored under simulated controlled traffic conditions after removal of a pre-existing subsoil pan by deep tillage. Soil physical properties under wheel tracks were compared to those between wheel tracks in terms of bulk density, penetrometer resistance, water content, airfilled porosities and changes in ‘‘least limiting water range’’. Differences in the growth and yield of canola (Brassica napus) and wheat (Triticum aestivum) in the two areas were also measured. Although deep ripping increased canola yield by 20% (from 2.0 to 2.4 t ha 1 ), reformation of a compaction pan under the wheel tracks was already detected in the first season of cropping. In the second cropping year, soil in the 0.05‐0.10 m layer under wheel tracks had significantly higher penetrometer resistance (>2000 kPa) and bulk density (1.5‐1.58 Mg m 3 ) and lower air-filled porosity (0.07‐0.09 m 3 m 3 ) compared to that measured between wheel tracks (