Visual examinations and soil physical and hydraulic properties for assessing soil structural quality of soils with contrasting textures and land uses

This study evaluates the use and the ability of visual examinations for assessing soil structural quality (SSQ) in soils with contrasting textures and under different land uses. The study searched for similarities in SSQ class between visual examinations and soil physical and hydraulic properties (soil organic carbon (SOC), aggregate stability, bulk density, porosity, plant available water capacity (PAWC) and unsaturated and saturated hydraulic conductivity), as well as the statistical relationships between them. The visual examinations used were the visual evaluation of soil structure (VESS), the visual soil assessment (VSA), the visual assessment of aggregate stability and the visual type of aggregates index. The latter is proposed as a new visual index for assessing SSQ. Samples were taken on a sandy loam and a silt loam soil, both under cereal monoculture (CM) and permanent pasture (PP), with conventional tillage and no tillage, respectively. Visual examination methods indicated significant differences between CM and PP in the silt loam soil (0.01 < 0.05), which were confirmed by significant differences in soil porosity and PAWC values. Wet sieving and the visual type of aggregates index were similar in identifying differences between land uses in both soils. Measurements of the visual type of aggregates index and of the hydraulic conductivity at different pressure heads were similar in indicating the soil structure condition of the soils. In the silt loam soil, the visual examinations were most related to properties such as SOC, PAWC, aggregate stability and porosity, whereas in the sandy loam soil they were most associated with water flow properties. The present study demonstrated that visual examinations are reliable semi-quantitative methods to assess SSQ and could be considered as promising visual predictors of soil physical properties (0.33 < 0.95). Finally, from the dissimilarities in terms of soil quality found with the VSA, VESS and porosity compare to the amount of SOC, SOC should be used cautiously as a sole indicator for soil structural quality as has been proposed in the literature, because SOC per se is not always well related to soil structural quality. (C) 2014 Elsevier B.V. All rights reserved.