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United States Department of Agriculture

Agricultural Research Service

Title: Humic Substances: Considerations of Compositions, Aspects of Structure, and Environmental Influences

Authors
item Hayes, Michael Hb - LIMERICK UNIVERSITY
item Clapp, Charles

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 1, 2001
Publication Date: N/A

Interpretive Summary: Organic materials in soils are made up of a combination of components in close association; the major components of soil organic matter are the humic substances humic acid, fulvic acid, and humin. These are especially important because they are the most active agents for binding soil particles (aggregation) and for transporting chemical pollutants (pesticides). A major controversy has developed among humic scientists as to the molecular associations and interactive forces involved in humic molecular structures. We have prepared an 'umbrella' paper to present the differing points of view of the various scientists. We are convinced that perceiving the sizes and shapes of the humic macromolecules is of prime importance for an understanding of the basic reactions that occur in natural environments. The impact of uncovering these fundamental scientific concepts will undoubtedly be to allow us to find ways of preventing undesirable chemicals from polluting the soil and ground water.

Technical Abstract: Humic substances (HS) are the major components of the mixtures of materials that compose soil organic matter, and these substances, which are by far the most abundant organic materials in the environment, are themselves complicated mixtures of biologically transformed organic debris. Separation and fractionation techniques are improving, and most of the instruments needed to advance awareness of compositions and aspects of structures are now in place. The broad based definition of humin is especially unsatisfactory. This umbrella term covers a mixture of materials that are insoluble in aqueous systems and contain non-humic components such as long chain hydrocarbons, esters, acids, and even relatively polar structures of microbial origins such as polysaccharides and glomalin that can be associated with the non-polar moieties and with soil minerals, as well as plant components that are highly resistant to decomposition. The remarkable developments in nuclear magnetic resonance are allowing better interpretations of compositions and of aspects of structures. There will emerge from these advances a more fundamental understanding of the functions that humic substances have in such important roles as the stabilization of soil aggregates, the binding of anthropogenic organic chemicals and in the sequestration of carbon from atmospheric CO2. The latter function is especially important at this time because we need to know why some soils sequester more C than others, and why the quality (or compositions and structures) of the HS in some soils are different from those in others.

Last Modified: 9/10/2014