Clay-metal-humus complexes

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. 

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