Critical visual thresholds of soil structure for guidance in the management of subtropical soils

The Visual Evaluation of Soil Structure (VESS) is a practical and participatory methodology for assessing soil quality, but it was originally developed for temperate climate regions. This study reports the relationship between VESS and quantitative indicators for Oxisols in southern Brazil. The soil structure in areas with different uses and management practices (croplands and cultivated and permanent pastures) was evaluated using VESS and quantitative indicators. The visual scores (Sq), which range from 1 to 5, were significantly related to soil bulk density (R² = 0.72), macroporosity (R² = 0.75), saturated soil hydraulic conductivity (R² = 0.42), degree of compactness (R² = 0.72), and least limiting water range (R² = 0.73). The critical limits for macroporosity (0.10 m3 m-3), bulk density (1.4 Mg m-3), least limiting water range (LLWR = 0), and degree of compactness (90%) corresponded to an Sq of 2.6, 3.1, 3.3, and 3.5, respectively. We conclude that VESS correlates well with quantitative indicators, and scores between 3 and 3.5 may serve as a reference for decision-making in the management of Oxisols in Southern Brazil under a wide range of use and management conditions.