Soil water content is one of the determining factors in controlling compaction caused by animal trampling or machinery traffic.The objective of this study was to evaluate the effect of water content (pore water saturation) on preconsolidation pressure (σ'p) and compression index (Cc) values, for two soils with contrasting textures, at two depths, cultivated under no tillage and conventional tillage.Undisturbed soil samples (5.35cm of diameter for 2cm of height) were collected in the surface layer (0 to 2cm) and in the 10 to 12cm layer from, during the agricultural year of 1997/1998, for a Paleudalf (89gkg-1 of clay), located in Santa Maria (RS), Brazil (29° 45' latitude South and 53° 42' longitude West), and a Haplortox (467gkg-1 of clay), located in Ibirubá, RS, Brazil (28° 30' latitude South and 53° 30' longitude West).To obtain a wide range of saturation degree for each soil type and management system, undisturbed soil samples were collected at different times to obtain natural variation of saturation degree. Some samples were saturated and equilibrated in the laboratory.For each soil, seven classes of saturation degree were established: <15; 16 to 30; 31 to 45; 46 to 60; 61 to 75; 76 to 90 and 91 to 100%.The uniaxial compression test was done using successive static loads of 12.5, 25, 50, 100, 200, 400 and 800kPa, during five minutes each load.The values of bulk density were also classified in classes, based on sample availability.The values of the σ'p, for the surface layer of the Paleudalf under no tillage, were greater for saturation degrees up to 30%, compared to the other saturation degrees.For the Haplortox, for the bulk density range from 1.30 to 1.45Mgm-3, the increase in the degree of saturation from 46-60% to 61-75% represented a decrease in σ'p values of 1.6 times for the no tillage, at the two depths, and of 2.4 times for the surface layer under conventional tillage.The values of σ'p decreased in a logarithmic way, with the increase of the saturation degree.
soil compaction; no tillage; compression index; preconsolidation pressure