Humic Fractions and the Nature of Organic Materials in Intimate Association with Soil Clays
Michael H.B. Hayes, Guixue Song, Andre J. Simpson, 2009. "Humic Fractions and the Nature of Organic Materials in Intimate Association with Soil Clays", Carbon Stabilization by Clays in the Environment: Process and Characterization Methods, David A. Laird, Javiera Cervini Silva, Yona Chen, Claire Chenu, Françoise Elsass, Javier M. Gonzalez, Michael H.B. Hayes, David A. Laird, Alain Plante, Andre J. Simpson, Guixue Song, Jorge Tarcjotzly, Michael L. Thompson, I. Virto, Robert L. Wershaw
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For any consideration of the organic matter in intimate association with the surface of soil clays it is appropriate to consider first the totality of the organic matter in the soil. The term “soil organic matter” (SOM), according to Stevenson (1994), refers to “the whole of the organic matter in soils, including the litter, the light fraction, the microbial biomass, the water-soluble organics, and the stabilized organic matter (humus)”. Nowadays, the term natural organic matter (NOM) is widely used for the natural organic components in soils, sediments, and waters.
On the basis of earlier definitions by Kononova (1966), Hayes and Swift (1978) considered the complete soil organic fraction to be made up of live organisms and plants within the soil, and their undecomposed, partly decomposed, and completely transformed remains. However, they regarded SOM to be a more specific term for the non-living components that may be described as “a heterogeneous mixture composed largely of products resulting from microbial and chemical transformation of organic debris.” The transformation, or the humification process, gives rise to the final product, humus, a mixture of substances with some resistance to further degradation. This definition is similar to that of Stevenson who considered humus to be the total of organic compounds in soil exclusive of undecayed plant and animal tissues, their partial ‘decomposition’ products, and the soil biomass. Stevenson partitioned SOM into the “active” (or labile) and the “stable” pools. The “active fraction” contains the macro-organic (or particulate) matter and the