Clay humus complexes

Ahmed, N., Varadachari, C., and Ghosh, K. (2002). Soil clay-humus complexes. II. Bridging cations and DTA studies. Aust. J. Soil Res. 40, 705-713. 

Varadachari, C., Mondal, A. H., and Ghosh, K. (1991). Some aspects of clay-humus complex-ation Effect of exchangeable cations and lattice charge. Soil Sci. 151,220-227. 

Jung (1943) was one of the earlier workers on clay-humus complex formation. His results indicated that sesquioxides of Fe and A1 play an important part in binding the humus to clays. Khan (1951) also emphasized the importance of Fe and A1 that exist in the crystal lattice of clays and help tq bind the humus very tightly to montmorillonite-type minerals. In a later publication Khan (1961) considered the composition of humic substances in soils and the manner in which they are bonded to the soil minerals. 

Stevenson, F. J. (1994). Clay-organic complexes and formation of stable aggregates. In Humus Chemistry Genesis, Composition Reactions. 2nd Ed. John Wiley Sons, New York, pp. 429 152. 

Information concerning clay-metal-humus complexes has come from studies using sonic vibration to examine soil aggregates. The concepts derived from these studies have been summarized by Bremner (1968) as follows ... 

Mechanism (1) is particularly important in peat and other organic-rich sediments, where clay and metal complexes are present in very low amounts in relation to the humus component. A typical example of humic substances bound by polyvalent complexes (item 2) is the Spodosol. These soils have developed under climatic and biologic conditions that have resulted in the mobilization and transport of considerable amounts of iron, aluminum, and organic matter into the B horizon. This illuvial horizon is a rich source of fulvic acids, which are readily separated from the sesquioxides by mild extractants. 

Al, and Mn oxyhydroxides, and organic matter as humus) are deposited. Deposition of Fe, Al, and Mn oxyhydroxides is favored by the breakdown of metal complexing organic acids produced in the O and A horizons. Horizon is enriched in clay minerals, may also include precipitated calcite and gypsum, and contains highly weathered, altered materials relative to underlying bedrock. B horizon may be a few cm to 2 m thick. 

The fluoride ion chemisorbs on clays and oxides by ligand exchange of surface OH", a reaction favored at low pH and on oxide and silicate minerals of low crystallinity. Fluoride, a hard base, has a particular affinity for a hard acid. Soluble AP -fluoride cationic and anionic complexes are quite stable, and can dominate the speciation of dissolved aluminum in low-humus soils. The mobility of A1 can be increased by the presence of F soluble complex formation with A1 may explain the rather high solubility and mobility of F in acid soils. 

Aeration, water-holding capacity, and permeability are all improved by humus. The frequent addition of easily decomposable organic residues leads to the synthesis of complex organics (e.g., polysaccharides) that bind soil particles into aggregates. The intimate association of clay-sized particles (layer silicates) with humus via cation (e.g., calcium, magnesium, aluminum, iron) bridges also promotes aggie-... 

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