Broadscale on-farm sampling suggests extensive soil physical degradation in Scotland

Soil structural degradation has been found to be widespread across different countries, including from a 2015/2016 winter survey across multiple catchments in Scotland. Winter is when soil structure can be most degraded, and this work found an extreme storm event exacerbated soil structure degradation over time. The previous research was limited to a simple Visual Evaluation of Soil Structure (VESS), so a repeat study in 2022/2023 at the same locations tested the hypothesis that soil structure degradation would recover over time, and that VESS would relate to quantitative soil physical data of penetration resistance, and bulk density, hydraulic conductivity and water retention characteristics from intact soil cores (2-7 cm depth). This research therefore aimed to explore the extent of soil structure degradation and its resilience over time, comparing visual and quantitative methods. Across 42 separate fields, three replicate samples were taken from in-field locations, three from degraded regions with visible damage to the soil surface from heavy traffic, and three from less disturbed field margins. From VESS scores, 59.5% of in-field and 78.5% of margin locations had a good soil structure, compared to 11.1% for degraded soils. This pattern continued for the quantitative core data. For bulk density, in-field soils were 8.3% denser, and degraded soils were 12.7% denser than margins, which was also reflected in porosity. Furthermore, organic carbon content was 10.6% less for in-field and 11.3% less for degraded compared with field margins. Much of the in-field soils had no degradation from VESS scores, but indicators of erosion and structural damage from quantitative soil core data were found. Land use also significantly impacted soil structure, with saturated hydraulic conductivity being highest for in-field soils, which is likely due to tillage practices. Grasslands presented the least degraded physical structure, with significantly greater porosity compared to stubble, ploughed soils and winter cereal cropland. Although we found VESS to be a valuable and rapid tool, data from quantitative measurements found more structural degradation, demonstrating that VESS scoring alone may not provide a holistic assessment of soil structural degradation. These results emphasise the need for improved land management practices in Scotland to maintain good soil structure and retain land productivity.