Mineralogical controls on soil health indicators: insights from Scottish national soil survey data

Soil health indicators such as bulk density, water retention, soil carbon stability, cation exchange capacity (CEC) etc. are widely used to assess soil functioning, fertility, and resilience, yet they are often interpreted using empirical proxies such as particle-size clay content. However, many of these indicators are fundamentally controlled by soil mineralogy, as different clay minerals and associated oxides exhibit contrasting surface charge properties, reactivity, and sorption behaviour. Incorporating mineralogical information, therefore, has the potential to improve both the predictive power and interpretability of soil health indicators.

In this study, we use data from the National Soil Inventory of Scotland (NSIS2) to investigate how soil mineralogy influences two key functional indicators: soil carbon stability and CEC. Soil carbon degradation is assessed using the depth-related enrichment factor (ε), derived from δ¹³C profiles and commonly interpreted as a proxy for decomposition intensity. Statistical analyses (Pearson correlation and principal component analysis) were applied to mineralogical data from arable and grassland soils, alongside land use and pH. Our results show that soils enriched in more reactive phyllosilicates (including smectite, illite, and mixed-layer illite/smectite) are associated with lower ε values, indicating reduced apparent carbon degradation and enhanced stabilisation. In contrast, soils dominated by more crystalline, less reactive mineral assemblages exhibit higher ε values. We also demonstrate that predictive models of CEC are significantly improved when mineralogical information is included, compared with models based on particle-size clay and carbon content alone. This highlights the limitations of relying solely on texture-based proxies and underscores the mechanistic role of mineralogy in governing soil properties.

These findings demonstrate that soil mineralogy provides a unifying framework for interpreting multiple soil health indicators (e.g., carbon stability and CEC). Explicitly incorporating mineralogical information into soil monitoring and modelling frameworks can strengthen soil health assessments by moving from descriptive indicators toward mechanistically informed metrics. Further work is needed to refine mineralogical characterisation in organic-rich soils and to support the routine integration of mineralogy into large-scale soil monitoring programmes.