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United States Department of Agriculture

Agricultural Research Service

Title: Sizes and Shapes of Humic Substances

Authors
item Clapp, Charles
item Hayes, Mhb - UNIVERSITY OF LIMERICK

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 15, 1999
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). This paper focuses on the specific topic of the sizes and shapes of the humic macromolecules. 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. Uncovering and understanding these fundamental scientific concepts will allow us to find ways of preventing undesirable chemicals from polluting the soil and ground water.

Technical Abstract: This series of papers focuses on considerations of the sizes and shapes of humic macromolecules. The initiative arose because of the divergent views held by participants at a Symposium/Workshop held under the auspices of the Soil Science Society of America and the International Humic Substances Society in Anaheim, CA in October, 1997. Three different viewpoints were expressed there, and each satisfied its proponent. One suggested that humi substances (HS) are macromolecular and assume random coil conformations in solution, a second proposed that HS are molecular associations of relatively small molecules held together by weak interaction forces, and a third considered that HS are in solution as micelle or "pseudomicellar" structures. Viewpoints two and three could broadly be considered to be under the same umbrella. We have considered the experimental procedures which have led the proponents to their points of view, and we do not consider it to be our brief to side in favor of one or another of the concepts. There is much work still to be done before a definitive answer is found. We sympathize with the fact that the various concepts arose from studies of different soils using different experimental techniques, and we share the view that the matter/controversy can best be resolved when data are available from applications of the same advanced techniques and procedures to a variety of HS from different sources. We are convinced that awareness of the sizes and shapes of humic molecules is of fundamental importance for an understanding of the many basic interactions which take place in the soil environment. This series of papers should stimulate researchers to pool their views, skills, and instruments to find a solution for this fundamental aspect of humic structures.

Last Modified: 9/29/2014
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