SOC: Clay Ratio: A Mechanistically-Sound, Universal Soil Health Indicator Across Ecological Zones and Land Use Categories?

28 Pages Posted: 13 Feb 2024

See all articles by Walter Wenzel

Walter Wenzel

affiliation not provided to SSRN

Alireza Golestanifard

affiliation not provided to SSRN

Olivier Duboc

affiliation not provided to SSRN

Abstract

The European Commission has recently launched a proposal for a soil monitoring and resilience directive (“soil monitoring law”, SML), defining the SOC: clay ratio as descriptor of the soil organic carbon (SOC) status, with a ratio of 1/13 separating “healthy” from “unhealthy” soils. Using data of the Lower Austrian soil database, this article explores the mechanistic foundation and applicability of the SOC: clay ratio in the study region. We observe considerable, clay-biased variation of the SOC: clay ratio among agroecological regions because clay content and SOC are driven by different ecological variables, with clay content related to the texture of parent materials, and SOC primarily controlled by amorphous oxyhydroxides of aluminium (Alo) and carbonate equivalent.  Apart from their high structure quality scores, a large share of Lower Austrian topsoils (0 - 20 cm) falls below the SOC: clay threshold of 1:13 deemed to indicate degraded structure, and thus “unhealthy” soil conditions according to the SML proposal. This lack of relation is most striking in cultivated soils but even evident in reference systems (grassland, hedgerows) with generally larger SOC concentrations. Whereas the total porosity is insensitive to the SOC: clay ratio even within the land use categories of an ecologically rather uniform region, it is positively related to SOC in cultivated and hedgerow soils, further elucidating the bias introduced by normalization of SOC to the clay content. Moreover, in fine-textured Chernozems and Chernic Phaeozems with deep organic matter accumulation, topsoil SOC: clay ratios are likely to underestimate ecosystem services such as water retention capacity.Overall, application of the SOC: ratio as descriptor of the SOC status for soil health assessments in the study region lacks mechanistic justification, is not linked to soil structure quality, and is particularly misleading if general thresholds (health criteria) are employed across land use categories and regions of varying ecological conditions. We suggest to benchmark the SOC status only within agroecologically rather uniform regions after stratification by land use. This approach could be further refined by calibrating SOC categories against functional soil properties such as soil porosity within each region. Comparison across larger areas (e.g., Europe) could be performed by transformation to relative scores.

Keywords: soil health, Soil organic carbon (SOC), SOC: clay ratio, Soil structure, Soil monitoring law (SML)

Suggested Citation

Wenzel, Walter and Golestanifard, Alireza and Duboc, Olivier, SOC: Clay Ratio: A Mechanistically-Sound, Universal Soil Health Indicator Across Ecological Zones and Land Use Categories?. Available at SSRN: https://ssrn.com/abstract=4725180 or http://dx.doi.org/10.2139/ssrn.4725180

Walter Wenzel (Contact Author)

affiliation not provided to SSRN ( email )

No Address Available

Alireza Golestanifard

affiliation not provided to SSRN ( email )

No Address Available

Olivier Duboc

affiliation not provided to SSRN ( email )

No Address Available

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