Evaluation of soil physical quality index S for some tropical and temperate medium-textured soils

The soil physical quality (SPQ) index S can provide inconsistent designations of SPQ and has a lack of consistency with other physical indicators for some soils. The aim of this study was to compare the suitability of S in identifying SPQ against 12 SPQ indicators, including water-release-related indicators, physical properties, and visual examinations. This study was conducted on medium-textured soil samples taken from tropical and temperate soils. Comparisons of SPQ class and relationships between indicators were used to judge the S SPQ designation. For the studied soils, S classified SPQ in the same way as other indicators when the condition of the soil was optimal or degraded but not when it was intermediate. This demonstrates that the proposed critical limits for S are not generally valid and do not apply for all soil conditions. Porosity parameters from the water release curve were more consistent indicators of SPQ than S. Our work also demonstrates that scores from visual examinations have at least similar resolution (P > 0.05) to the other indicators of SPQ evaluated. The use of S as an indicator to be considered as part of a minimum data set of indicators of SPQ assessment is less viable when other indicators such as bulk density, porosity, and visual examination are much more easily determined and more consistent than S. Therefore, it is too ambitious to consider that a unique indicator such as the S index could be used to evaluate SPQ as such.