A toposequence has soils with distinct structural and morphological properties that may affect soil susceptibility to structural deformation in short spatial distances. The objective of this study was to evaluate soil load-bearing capacity and susceptibility to compaction, and quantify the effects that loads above and below preconsolidation pressure cause on soil permeability to air in a toposequence of the Central Lowlands of Rio Grande do Sul, Brazil. Undisturbed soil samples were collected from horizons of soil profiles of three Alfisols (Ap, A1, AB, BA, Bt1, Bt2; Ap, A1, A2, AB, BA, Bt1, Bt2; Ap, A1, BA, Bt1, Bt2) and two Aqualfs (A1, A2, Bg; A, Bg, Cg) along a toposequence. These samples underwent the uniaxial compression test, and soil permeability to air was tested before and after quantification of compression. Under the condition of field capacity, Aqualfs are less susceptible to compaction and have higher load-bearing capacity (higher preconsolidation pressure) than Alfisols. Soil clay content is directly correlated with the load-bearing capacity of the soil. Therefore, it is inversely proportional to susceptibility to compaction, except under high soil moisture conditions, when water lubricates the surfaces, increasing susceptibility to compaction of any soil. Loads up to 25 kPa do not result in physical degradation of the soil, but when a 200-kPa load is applied, even under the condition of field capacity, there is a significant reduction in macroporosity and total porosity, arriving at levels lower than the threshold level (0.10 m3 m-3).
physical properties; compressibility; air flow; porosity; soil moisture
