A Novel Multi-Factor Interpretation of Visual Evaluation of Soil Structure in Forest Soils

Visual evaluation of soil samples is a powerful and efficient method to assess quality and stability of soil aggregation. For cropland and pasture (open land), it was shown that a good score assigned to the visual evaluation is correlated to low bulk density (no compaction) and high ratio of soil organic carbon (SOC) to clay content (a measure for the stabilization of organic carbon. However, in forest soils, aggregate formation conditions are different due to lower mechanical disturbance, higher biological activity, higher organic input of different quality, enhanced weathering, and the absence of liming. Under these conditions, we expect the link between visual score, bulk density, and SOC-clay ratio to be less consistent, which challenges the applicability of the visual evaluation method for forest soils.

To quantify and interpret the visual evaluation method for forest soils, 27 forest sites in Switzerland were sampled across a wide range of texture, organic carbon, and acidity levels. Various soil structure-related properties were measured directly in the field (score of visual evaluation) and in the lab (macropore geometry and soil hydraulic properties). No general relationship between visual scores and basic soil health parameters (porosity, SOC-clay ratio, saturated hydraulic conductivity) was found, which illustrates the fundamental difference between forest and open land soils. In a next step, the parameter space defined by basic soil properties (texture, SOC, and pH) was subdivided into subregions using cluster analysis. Initial results indicate that VESS and basic soil health parameters correlated but varied across pH- and clay-defined subregions. Interpretation of VESS therefore requires consideration of basic soil properties, yet in combination the method provides a promising field-based assessment of soil structural health.