Soil clay–humus complexes. I. Alkali dissolution, TEM, and XRD studies

Abstract

Clay-humus complexes from 5 different soils (Entisol, Alfisol, Vertisol, and 2 Mollisols) were isolated by aqueous dispersion. Humic acid (HA) extractable from the complexes by alkali solutions ranging from pH 8 to 14, before and after acid pretreatment, were studied. Extraction by alkali solutions alone was not very effective except with the Alfisol sample. However, pretreatment by acid greatly increased extractability in all samples except the Alfisol. Maximum extraction of HA occurred at pH 12. In the Alfisol sample, clay-HA linkage is probably through hydrogen or other monovalent cation bridges which can be broken by alkali treatment. In Entisol and Mollisol II samples, cation bridging by higher valent, acid-extractable cations is dominant; both these modes are evidenced in the Mollisol I sample. The Vertisol sample has the strongest clay-HA links. TEM studies revealed a reduction in size of the montmorillonite particles in the Vertisol clay-humus complex (compared with humus-free clay); destacking and dispersion of montmorillonite occurred on HA complexation as observed previously for in vitro systems. Relative intensity of the 15Å XRD line was much weaker in the Vertisol clay-humus complex than in the clay, lending further support to the fact that that c-stacking is disrupted during complexation of smectite with humus.