Knowing the susceptibility of a soil to compaction is important to adjust soil management, tillage practices, and traffic to cause minimal detrimental effects on soil properties. The objective of this study was to determine the preconsolidation pressure (σ'p) and the compression index (Cc) at two depths of two soils under no-tillage and conventional tillage, including a wide variation of the state of compaction. Undisturbed soil samples were collected with rings (5.4cm wide and 2.0cm high), at 0 to 2 and 10 to 12cm depths, from a Paleudalf (89gkg-1 clay at the surface) locate in Santa Maria, RS, Brazil (29°45' South latitude and 53°42'longitude West) and a Haplortox (467gkg-1 clay) locate in Ibirubá (RS), Brazil (28°30' South latitude and 53°30' longitude West). For each soil type and tillage condition, undisturbed soil samples were taken at several places and different periods to obtain natural variation of bulk density, i.e, different initial states of compactness. The uniaxial compression test was done using successive static loads of 12.5, 25, 50, 100, 200, 400 and 800kPa, during five minutes. Field bulk density values were separated into four classes (1.31 to 1.45; 1.46 to 1.60; 1.61 to 1.75 and 1.76 to 1.80Mgm-3) for Paleudalf soil and into three classes (1.15 to 1.30, 1.31 to 1.45 and 1.46 to 1.60Mgm-3) for Haplortox soil. In the surface layer of the Paleudalf soil, preconsolidation pressure values for conventional tillage were about four times smaller than those for no-tillage, up to bulk density of 1.60Mgm-3, and about two times for bulk density greater than 1.61Mgm-3. The Haplortox soil did not show any influence of tillage on preconsolidation pressure values. The Haplortox soil had larger values of compression index. For each soil, compression index values were significantly correlated with bulk density.
soil compaction; no-tillage; compression index; preconsolidation pressure