One of the most effective practices in preventing soil compaction is to apply stresses below the bearing capacity of the soil, often estimated by the pre-compression stress (sigma p). To evaluate the effects of cover crops and tractor traffic on ? p and compression index (CI), a field experiment was initiated in 2002 on sandy clay loam Paleudult, in Southern Brazil (Rio Grande do Sul State). Treatments under no-tillage involved winter fallow and two winter cover crops (black oats ( Avena strigosa) and black oats + vetch ( Vicia faba), which was substituted in 2006 by oilseed radish - Raphanus sativus) in the plots and two traffic conditions (with and without wheel-tractor traffic) in the subplots. Undisturbed soil cores were sampled in June and November 2006 to determine soil physical properties as well as sigma p and CI by uniaxial compression tests in an oedometer. Before oedometer tests, soil cores were equilibrated at different water tensions. Regardless of time and traffic conditions, winter fallow showed the highest sigma p value and the lowest CI value at 0.03-0.06 m. As the soil became drier, differences in sigma p between winter fallow and cover crops were reduced. In five years, seven wheel-tractor passages increased sigma p only in the surface layer (0.03-0.06 m), without altering CI. The use of cover crops and absence of traffic reduced the soil bearing capacity and increased soil susceptibility to compaction due to the reduction in soil bulk density and increase of macropores.
