ABSTRACT: Agricultural areas in the region of Turuçu, on the Southeast Hillside in the state of Rio Grande do Sul (RS), Brazil present many examples of soil physical degradation. Accordingly, strategies aimed at evaluating and ameliorating the structural quality of soils should be developed to ensure the sustainable use of these areas. This study aimed to evaluate the least limiting water range (LLWR), S-index, compressibility parameters, organic carbon and the organic matter granulometric fractions of a Udalf under different agricultural uses. Soil samples with disturbed and non-disturbed structure from the 0.00 to 0.10 m layer were collected on farms in the Southeast Hillside, Turuçu region, RS, for analyzing soil under: i) conventional system of corn after tobacco crop, (CT); ii) conventional system of corn after native field (CNF), iii) native field (NF) and iv) native area (NA). Both the CT and the CNF systems had a negative influence on the values of bulk density, soil porosity, least limiting water range, compressibility parameters and total organic carbon, coarse fraction carbon and carbon associated with minerals.

Management systems may influence the structural quality of soils. Tensile strength (TS) and friability (F) are indicators of soil structural quality. The aim of this study was to evaluate the TS and F of an Alfisol under different management systems. The treatments were as follows: (i) soil under conventional system with growing maize after tobacco cultivation, (ii) soil under conventional system with growing maize after use as pasture, (iii) soil under natural pasture, and (iv) a natural area with predominance of spontaneous vegetation. TS and F were evaluated at depths of 0.00-0.05 and 0.05-0.10 m. The water content of soil aggregates, soil particle-size distribution, total organic carbon, carbon in the coarse fraction and carbon associated with minerals were also determined. The increase in clay content and soil organic carbon influences the values of TS. The lowest TS was for the soil under maize cultivation after tobacco in the conventional system. Soil under natural area in the 0.05-0.10 m layer was classified as slightly friable, while other systems were classified as friable. Evaluations of the structural quality of soils under management systems can be performed using TS. However, F was not efficient in detecting changes between the different management systems.