Manuring effects on visual soil quality indicators and soil organic matter content in different pedoclimatic zones in Europe and China

A study was carried out to assess if the visual soil assessment method (VSA) would allow recognizing differences between soils receiving organic matter (OM) amendments and similar control soils, by the observation of visual soil quality indicators’ score. 36 practices were identified across 8 pedoclimatic zones. These fields/plots were paired with nearby control fields/plots, without OM amendments, sharing similar farming features. A survey, comprising a VSA of the soil structure status, surface ponding, signs of erosion, earthworm counts and soil stability (slake test), complemented by measurements of soil organic matter (SOM) and permanganate oxidizable organic carbon (LOC) content, soil pH, penetration resistance and texture, on soils of both management system groups (OM addition and Control), was performed in 2016. Correlations of the visual soil quality indicators’ score with SOM, LOC, other soil properties and climate variables and indices were calculated within each group; the correlations between soil properties, and between soil properties (SOM and LOC) and climate variables were also calculated. A statistically significantly higher proportion of soils of the OM group had a good score for “soil structure and consistency” and “soil porosity”. These differences are not directly explained by non-inherent soil properties. No statistically significant Spearman’s correlation coefficients were observed between “soil structure and consistency” and either soil properties or climate variables; concerning “soil porosity”, distinct statistically significant correlations were observed between the two groups with different climate variables and indices. Correlations between the scores of the visual soil quality indicators and climate variables were found to follow the same directions of correlations of LOC content with the same climate variables, although the latter correlations were weak. Mean SOM and LOC content, were slightly higher in the OM group, although differences were not statistically significant. A high linear correlation between LOC (mg/g) and SOM (%) (r = 0.65, n = 26) exists within the Control group, but not within the OM group (r = 0.20 and n = 26). When the relationship of SOM and LOC content with visual soil quality indicators’ score was studied, statistically significant correlations were only observed between SOM and “earthworm count” within the Control group (rs = 0.44), and between LOC and “soil colour” within the OM group (rs=-0.52). Both LOC content and LOC status (ranked as a function of LOC content and soil texture), had only negative statistically significant correlations with visual soil quality indicators’ score, questioning their worth as stand-alone soil quality indicators.