Changes in soil structure, bulk density and organic carbon content, caused by tillage systems, affect shear strength parameters. The present study was proposed with the objective of evaluating the effects of increased in soil organic carbon (SOC) content on the shear strength of an Hapludult (Argissolo Vermelho Amarelo arênico, Brazil System). Soil surface samples (0-0.02 m), with SOC content varying from 6.5 to 18.8g kg-1 and soil moisture equilibrated at tensions of 6, 100 and 500kPa, were submitted to the direct shear tests with normal tensions varying from 13.6 to 443.0kPa. Cohesion and angle of internal friction values were determined. Besides SOC content, the water content, the saturation degree and the soil bulk density of the samples were determined. The angle of internal friction was of 32.5, 34.9 and 35.4 degrees, while the cohesion was of 0.00, 1.06 and 7.95kPa, respectively for the samples with moisture at 6, 100 and 500 kPa tension. In the driest samples (500kPa), the angle of internal attrition decreased with the increase of the SOC content. The cohesion determined in the direct shear tests was independent of the SOC content. However, the cohesion decreased with the increase of the soil water content.
soil shearing; soil organic matter; soil structure; soil mechanics
