Relating visual evaluation of soil structure to other physical properties in soils of contrasting texture and management

The sustainability of agricultural systems depends on the evaluation and monitoring of soil use and tillage in order to mitigate soil degradation. The visual evaluation of soil structure (VESS) was developed to provide a quick, simple and easily understood test to enable researchers, farmers and consultants to score soil quality. In this paper we test the hypothesis that soil structural quality, as specified by VESS (Sq), is sensitive enough to identify differences in structure, resulting from soil management, in and between layers of topsoil. The Sq score ranges from 1 (good) to 5 (poor soil structure). Improvements have already been made to this method, but we wished to test the validity of Sq results compared with other indicators of soil physical quality. Our aims were (1) to evaluate the usefulness of VESS to compare layering of topsoil structure under different soil management and (2) to identify which soil physical properties Sq most closely relates to. We chose to work on soils of contrasting texture in response to criticism that the test works well only on medium-textured soils. In our first experiment, we assessed Scottish soil from native forest that had never been cropped and from arable soils just after harvest so where there was a visible difference between soil tracked or not tracked during harvesting operations. Soil qualities measured were soil resistance to penetration (SR), bulk density (Bd) and air permeability (Ka). In our second experiment we compared the least limiting water range (LLWR) with VESS in a Brazilian Oxisol under no-tillage. VESS showed the differences between the treatments and layers of topsoil. Sq increased with SR and Bd but decreased with air permeability. Results for LLWR showed that for Sq ≥ 3.5, the LLWR was zero, indicating soil physical condition highly restrictive to plants. Harvest is a time of significant soil compaction and the VESS test detected compaction even where it was not visible at the surface and as such may prove useful in diagnosing and remediating compaction and assessing suitability for minimum tillage.