What is a good level of soil organic matter? An index based on organic carbon to clay ratio

Summary

This paper validates a simple, practical index for assessing soil organic matter based on the ratio of soil organic carbon to clay content, using data from 3,809 sites across England and Wales sampled between 1978 and 1983. The authors establish threshold values for this SOC/clay ratio that distinguish between degraded, moderate, good and very good soil structural conditions, with land use emerging as the strongest predictor of the ratio. The index offers a method for national and regional soil monitoring programmes using two routinely measured soil properties, though the authors note that land use, soil type, precipitation and pH together explain only 21% of variance in the ratio.

UK applicability

This index was explicitly developed and validated using the National Soil Inventory of England and Wales, making it directly applicable to UK soil assessment, monitoring and management at national, regional and sub-regional scales. The findings provide evidence-based guidance on appropriate SOC/clay thresholds for arable, grassland and woodland soils under UK climate and soil conditions.

Key measures

Soil organic carbon (SOC) concentration, soil clay content, SOC/clay ratio, soil structural condition classification, land use type, soil type, annual precipitation, soil pH

Outcomes reported

The study validated an index based on soil organic carbon (SOC) to clay ratio thresholds for assessing soil structural condition across 3,809 sites in England and Wales under arable, grassland and woodland uses. The index identified specific SOC/clay ratio boundaries (1/8, 1/10, 1/13) that distinguish between 'very good', 'good', 'moderate' and 'degraded' soil structural conditions, with prevalence of degradation varying by land use (38.2% of arable sites, 6.6% of grassland, 5.6% of woodland).

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