Refining the Walkley–Black Correction Factor for Soil Organic Carbon Estimation in Highly Weathered Tropical Soils of Western Ghats

Abstract

Background and aim

Accurate estimation of soil organic carbon (SOC) is essential for assessing soil health, guiding land management, and making informed decisions on climate change mitigation strategies. Most studies on soil carbon rely on the Walkley–Black (WB) method due to its simplicity, but it underestimates SOC, especially in highly weathered tropical soils.

Methods

This study evaluated the WB correction factor (WBCF) by comparing SOC estimated from WB and the carbon-nitrogen (CN) analyzer method for 563 samples collected across the Western Ghats region of India.

Results

The results showed that WBCF increased with depth from 1.27 at 0–15 cm to 2.84 at 100–150 cm, indicating that the oxidation efficiency of WB method decreased with depth due to the accumulation of recalcitrant and mineral-bound organic matter in sub-soils. Correlation and segmented regression analysis identified a critical breakpoint at CEC/clay ratio of 0.21, below which soils exhibited high correction factors (WBCF) and low SOC recovery. Similarly, a threshold at 50% clay content marked a shift in recovery behavior. The correction factor calculated as the inverse of the regression slope was 1.62 for soils with low CEC/clay ratio (<0.21) and 1.29 for those with ≥0.21, indicating a strong influence of clay reactivity on SOC recovery. Similarly, soils with high clay content (≥50%) had a higher correction factor (1.54) compared to soils with <50% clay (1.34), indicating the underestimation of SOC by the WB method in clay-rich soils due to greater organic matter stabilization.

Conclusion

These findings underscore the need for depth and mineralogy-specific correction factors, as the conventional WB factor substantially underestimates SOC in highly weathered, oxide-rich, low-active tropical soils.