ENVIRONMENTALLY STRATIFIED SOC BENCHMARK RANGES FOR SOUTH AFRICAN SOILS

Soil organic carbon (SOC) is widely used as an indicator of soil condition, but interpretation of SOC values requires consideration of climate and soil texture. This study developed environmentally stratified SOC benchmarks for South African soils using the aridity index (AI) and soil texture (fine fraction: clay + silt). The framework was developed using 8,753 georeferenced mineral topsoil profiles from the national Land Type Profile Database. SOC distributions were analysed within climate-texture strata using the 10th percentile (SOC₁₀), mean SOC (SOCmean), and 90th percentile (SOC₉₀) to derive empirical benchmark ranges for non-cultivated soils. SOC₉₀ increased from 0.83% in arid, coarse-textured soils to 4.80% in humid, fine-textured soils, indicating substantial differences in upper benchmark SOC concentrations across climate-texture environments. Cultivated soils generally had lower mean SOC concentrations than the non-cultivated benchmark soils across the climate-texture classes, with mean SOC reductions ranging from 0.06 to 0.46 percentage points. Degraded soils showed variable SOC responses across climate-texture classes, including substantial SOC reductions in some environments and considerable overlap with benchmark ranges in others. The proposed framework provides an empirical basis for evaluating SOC across the heterogeneous landscapes of South Africa and may support soil monitoring and carbon sequestration initiatives.