Changes in soil compaction indicators in response to agricultural field traffic

In addition to measurements of soil bulk density, compaction is usually measured by changes in soil porosity and penetration resistance. The effects of agricultural field traffic on changes in soil compaction indicators were investigated, a) to quantify the immediate effects of field traffic on the degree of compactness, penetration resistance, and matric suction and b) to evaluate the behaviour of these variables as indicators of post-traffic compaction on soils with different clay content. Two wheeling experiments were performed on two oxisols containing 200 (sandy clay loam) and 480 g kg−1 [clay] (i.e. clay soil). Compaction occurred after wheeling in both soils, but the degree of compactness was higher for the clay soil. Texture affected the behaviours of penetration resistance and matric suction in detecting compaction. With the increase in clay fraction, friction effects diminished, and variation in penetration resistance was not significant, even with compaction. Reductions in soil porosity due to compaction can cause variation in pore size and in the degree of saturation, altering the water retention energy. Due to the proportionally smaller pore size, this mechanism was detected via matric suction at approximately field capacity only for the clay soil. For the sandy loam soil, it is likely that only a larger reduction in porosity and variation in the degree of saturation impose detectable changes in matric suction.