Tillage and traffic effects on soil physical properties

Soil compaction can alter the root environment positively or negatively, depending on soil physical properties and weather conditions. Two experiments were performed to assess the effects of agricultural traffic on soil physical properties that affect the plant root environment;Crawler-tracked tractors have the potential for causing less soil compaction because the tracks usually have a greater surface area than wheels of tractors with equivalent power ratings. The objective of the first experiment was to determine the effect of tracked and wheeled tractors on soil physical properties. Compaction plots were established on a Chequest silty clay loam soil (Typic Haplaquoll) in Lee County, IA in 1984. Uhland cores (75 mm diam.) were collected in August 1987 at 50-125 mm and 125-200 mm depths from untrafficked areas and from areas trafficked by steel-tracked, rubber-belted, two-wheel drive, and four-wheel drive tractors. The untrafficked cores had lower bulk densities and higher air-filled porosities than trafficked cores at both depths, but lower bulk density was associated with lower ground pressure only for the 50-125 mm depth, which had been field cultivated after trafficking. The untrafficked cores had greater porosity, compared to the trafficked cores, at both depths due to pores \u3e30 [mu]m equivalent pore diameter. The only difference in pore-size distribution among trafficked cores was a greater porosity for the steel-tracked tractor compared with the wheeled tractors for the 60-300 [mu]m pores at the upper depth;The second experiment examined the effect of tillage with controlled wheel traffic on a Tama silty clay loam (Typic Argiudoll) for a field study in central Iowa. The main plot treatments were tillages: no-till, ridge till, and chisel plow. Uhland cores were sampled on five dates from the 50-125 mm depth in three row positions: trafficked interrow, plant row, and untrafficked interrow. The bulk densities of the trafficked interrow positions were high enough to restrict, but probably not exclude, root growth. Soil strength followed similar trends as bulk density, with the trafficked interrow position having the highest strength. The lowest bulk densities and strengths were found for the ridge-till plant row and chisel plow untrafficked-interrow positions. The trafficked interrows generally had less than 0.10 m[superscript]3m[superscript]-3 air-filled porosity at 1.0 m soil water tension, which is below that required for adequate plant root aeration. Trafficked interrows had substantially less macroporosity (\u3e60 [mu]m pore diameter) than most non-trafficked row positions for both dates. In summary, the largest difference in physical properties was between trafficked and non-trafficked row positions. Trafficked interrows were uniformly compacted and poorly aerated in all tillage systems. This is advantageous to support agricultural vehicles, but the compacted condition may limit root growth