Soil clay–humus complexes. II. Bridging cations and DTA studies

Abstract

Clay-humus complexes, isolated from 5 soils (Entisol, Alfisol, Vertisol, two Mollisols), were extracted with 0.1 n citrate, EDTA, and oxalate at pH 7.0-10.0; amounts of Ca²⁺, Mg²⁺, Fe^3+/2+, Al³⁺, as well as humic acid (HA) in the extract were determined. HA extracted increased with pH and varied with nature of ligand; largest amounts were extracted by EDTA at high pH. In the Entisol clay-humus extract, Ca²⁺ is dominant. In Alfisol sample, Ca²⁺ and Mg²⁺ have little role in clay-HA bonding; apart from monovalent cations, bonding is mainly through Fe^3+/2+ and Al³⁺, which are well correlated to HA extracted. The extract from Vertisol sample contains little Fe^3+/2+ or Al³⁺ and major bonding is through Ca²⁺. In Mollisol I and II, Ca²⁺, Mg²⁺, Fe^3+/2+, and Al³⁺ are all involved in bonding and are highly correlated to extracted HA. Difference in mineralogy determines the difference in bonding strength between Alfisol and Vertisol complexes.

DTA indicates dual bonding modes. A major fraction of HA (in clay-humus complexes) shows thermal destabilisation due to multiple attachments on the clay surface; a small fraction is also thermally stabilised by ionic bonding with Ca²⁺/Mg²⁺ and absence of ring strain in the complex. Only the Alfisol HA does not show thermal stabilisation in the complex.