Synthesis and Degradation of Natural and Synthetic Humic Material in Soils

Soil humus is composed predominantly of two types of substances, humic substances and polysaccharides. Humic acids are the major extractable component of soil humic substances; fulvic acids are usually a small component of soil; and soil humin is a major nonextractable component. Humic acids are complex macromolecules consisting of an array of aromatic and aliphatic structures. Soil polysaccharides, nonhumic components, are discussed in this chapter because of their important role in soil structure and aggregation. They consist of a variety of sugar units of both plant and microbial origin. Decomposition of fresh organic residues is discussed in terms of the fate of the constituent carbon. Before residues can be considered a part of the true soil humus, they undergo profound transformation and no longer resemble the original material. An overview of some current hypotheses on the formation of humic acids and soil polysaccharides is presented along with possible mechanisms for their increased resistance to biodegradation. Use of model humic acid-type molecules have aided in elucidating some of the possible ways by which humic acids are synthesized and protected in the soil environment. Information on the characterization of humic acids and soil polysaccharides that has accumulated through the use of various degradative and nondegradative procedures is examined.