Soil clayhumus complexes. II. Bridging cations and DTA studies
Nayan Ahmed,
Chandrika Varadachari and Kunal Ghosh
Australian Journal of Soil Research
40(4) 705 - 713
Published: 14 June 2002
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+, Fe3+/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 Fe3+/2+ and Al3+, which are well correlated to HA extracted. The extract from Vertisol sample contains little Fe3+/2+ or Al3+ and major bonding is through Ca2+. In Mollisol I and II, Ca2+, Mg2+, Fe3+/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.
Keywords: Alfisol, Vertisol, Entisol, Mollisol, ligands, correlation.
https://doi.org/10.1071/SR01046
© CSIRO 2002