Metal‐humus complexes in A horizons of Thai and Korean red and yellow soils

Carbon, Al and Fe (C_pyr, Al_pyr and Fe_pyr) were extracted with 0.1 m Na₄P₂O₇ from 26 A horizon samples of tropical Thai and temperate Korean soils (Ultisols, Alfisols, Oxisols and Inceptisols). The soils, except for one Thai Inceptisol, had similar total C (0.35–3.29%) and C_pyr/total C ratios (0.20–0.41). There were approximately linear relationships between total C or C_pyr and clay content; two groups of soils gave different linear relationships. A curvilinear relationship between C_pyr and (Al + Fe)_pyr (milli‐atom kg⁻¹) that can be approximated by an equation: C_pyr= 53 (Al_pyr+ Fe_pyr)^1/2– 24 was also found for most Thai and Korean soils. The above relationships indicated that total C and C_pyr would be close to zero at zero clay or zero (Al + Fe)_pyr. It was inferred that clay‐humus interaction has a primary importance in the determination of humus content in red and yellow soils in tropical and temperate regions and that the main role of clay is to supply Al and Fe that complex and stabilize humus against microbial degradation.