TRACTIVE DEVICE EFFECTS ON SOIL PHYSICAL-PROPERTIES

Soil compaction can alter the root environment positively or negatively, depending on soil physical properties and weather conditions. Track-type tractors have the potential for causing less soil compaction because the tracks usually have a greater surface area than wheels for tractors with equivalent power ratings. The objective of this study was to determine the effect of tracked and wheeled tractors on soil physical properties related to the plant rooting environment. Plots were established on a Chequest silty clay loam (Typic Haplaquoll) in Lee County, IA in 1984. Plots were moldboard plowed to a depth of 200 mm and disked to a depth of 125 mm prior to trafficking, and field cultivated to a depth of 100 mm after trafficking. Soil cores (75 mm in diameter) were taken in August 1987 at 50-125 mm and 125-200 mm depths from untrafficked areas and areas trafficked by a steel-tracked crawler, rubber-tracked crawler, two wheel drive tractor, and four wheel drive tractor. Measurements included bulk density, saturated conductivity, air-filled porosity, and pore-size distribution. The untrafficked soil cores had lower bulk densities than trafficked cores at both depths, but lower bulk density was associated with lower ground pressure only for the 50-125 mm depth. Air-filled porosity at field capacity ( 1.0 m tension) followed the same trends as bulk density. The greater porosity of the untrafficked cores, compared with the trafficked cores, at both depths was owing to the presence of pores with greater than 30-mu-m equivalent pore diameters. Among trafficked treatments, cores from the upper depth of the steel-tracked crawler plots had greater porosity for 60-300-mu-m pore diameters compared with cores from the wheeled tractor plots. Generally, the wheeled tractors (with ground pressures of about 125 kPa) created a more compacted condition in the soil than the track-type tractors (with ground pressures of 30-40 kPa) at the upper depth, but physical property differences in trafficked cores below the 125 mm depth were minimal.