Integrating soil structure with organic matter and clay content - a new framework for rating soil resilience

Soil organic matter (SOM) level is a key factor determining soil resilience and condition; however, the optimal level of SOM in soils increases with clay content. Previous work has promoted the concept of Clay to Soil Organic Carbon (SOC) ratios to derive critical thresholds of soil degradation. Here, we analyse a dataset comprising 485 soil samples across England & Wales, where Visual Estimation of Soil Structure (VESS) scores was used alongside measurements of Clay and SOC contents (0–25 cm depth) to integrate soil structure into a new data‐driven Rating framework of soil condition and resilience. SOM (calculated as SOC/58%) increased with clay content. Both SOM and VESS scores were significantly better under grassland. For similar soil structure scores, SOM and clay content best fitted to diminishing power relationships (curves). Based on VESS scores, empirical bands are derived to rate the SOM as Maximum under grassland, Very Good, Good, Moderate, Deficient or Very Deficient at any given clay content. Most grassland sites were classified in the top three categories, whereas arable land spanned all six. On arable land, the existing Clay:SOC ratio framework classed 86% of heavy‐textured soils as ‘Degraded’ whereas the new Rating system categorises only 36% as Deficient. Conversely, on lighter land the Rating bands identified more Deficient soils than the Clay:SOC relationship. Across all soil types, Clay:SOC ratio classed 46% as Degraded whereas the Rating bands judged 11% as Deficient plus 12% as Very Deficient. 34% were rated Good or Very Good. VESS score was Good or Very Good in 37% of cases, while 26% were inadequate (VESS > 3.0), of which 14% were 4.0 or worse. Under grass leys, Clay:SOC ratio designated 12% as Degraded whereas the Rating only found 7% as Deficient; 3% had VESS 4 or 5. 59% of leys and 73% of permanent grassland had VESS 1 or 2 and only 2% had VESS 4 or 5. A substantial part of the variation in VESS scores is not captured by the Clay‐v‐SOM relationship which we attribute to management (cultivation and crop rotation). Structure was generally worse on sites with a history of minimal/shallow tillage. Conversely, cultivated soils could have good structure despite Moderate Rating. The new Rating categories are proposed as a robust and practical framework to describe soil structural condition and resilience for farmers and advisers to inform management guidance, including whether continuous reduced cultivation is appropriate or not, and where SOC should be raised, in contrast to soils already high in SOC that are unlikely to benefit from (nor retain) increased Carbon inputs. We estimate that an increase of 13 t/ha C in the topsoil is sufficient to restore a Very Deficient soil to Moderate Resilience. The data and new framework proposed in this study indicate that soil structure assessment needs to be integrated with SOC and clay for a more relevant metric of soil degradation and resilience. The methodology was developed on soils in England and Wales but has the potential to be used on soils worldwide.