A simplified representation of the chemical nature and reactions of soil humus

An understanding of the chemistry and reactions of humic acids and related substances is crucial to an understanding of the nature and properties of soils. A comprehensible structural representation of humic substances is presented and the simplified model is used to illustrate: (i) the way in which stable humus is formed from lignin; (ii) forms of organic N in soils; (iii) the interaction of humus with clay minerals; and (iv) the important roles played by soil organic matter, including contribution to the cation-exchange capacity, binding of pesticides, and formation of complexes with micronutrient cations. The approach is of value for teaching soil science at the undergraduate level. T BULK of the organic matter in most soils consists of a series of brownto black-colored substances referred to by such names as humic acids, fulvic acids, and humins. In classical terminology, humic acid is the material extracted from soil by alkaline solutions and which precipitates upon acidification; fulvic acid is the extracted material that remains in solution upon acidification. A portion of the soil organic matter cannot be F.J. Stevenson, Dep. of Agronomy, 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801; R.A. Olsen, Plant and Soil Science Dep., Montana State Univ., Bozeman, MT 59797. Received 22 Aug. 1988. "Corresponding author. Published in J. Agron. Educ. 18:84-88 (1989). solubilized by base extraction and is called humin. The various fractions are not specific compounds for which discrete structural formulas can be given. Rather, they represent an extremely heterogeneous mixture of substances that range in molecular weight from as low as several hundred for fulvic acids to over 300 000 for humic acids and humin (Stevenson, 1982). The inconvenience and awkwardness of writing structural formulas and reactions for high-molecular-weight organic molecules is readily apparent. In some textbooks on soil chemistry, one or more "type" structures are given for humic and/or fulvic acids; in other cases, empirical formulas are provided (e.g., C2.2H2.2O). More frequently, humic subtances are merely described as being exceedingly complex and of unknown composition. With respect to both teaching and learning, this state of affairs is unfortunate. In order to explain the nature and properties of humic substances to students, it would be advantageous to express their basic chemical structure in terms of a simple structural unit or building block, and to use this structural unit to demonstrate the important chemical reactions that humus undergoes in soil. In this paper, a simple dimeric unit is proposed as a structural representation of humic substances for instructional purposes. The simplified model is then used to illustrate the more important aspects of humus chemistry and reactions. Using this model, some precision is sacrificed for clarity, simplicity, and usefulness. The treatment is oriented more towards the undergraduate student rather than to graduate students or researchers. 84 J. Agron. Educ., Vol. 18, no. 2, 1989 BASIC CONCEPTS OF STRUCTURAL UNITS IN HUMIC SUBSTANCES Basically, humic substances in soil are high-molecularweight compounds (macromolecules) that have been chemically structured through the linking together of individual building blocks or monomers (i.e., quinones and phenolic acids). To some extent, chemical reactions of the macromolecules are similar to those of the monomers from which they were formed, primarily because they both contain similar types of reactive functional groups (i.e., COOH and phenolic OH). The functional groups are attached to a framework or matrix that can be visualized as being comparable to a six-sided chain-like fence, except for being linked together in a more haphazard way. Also, the humus matrix contains imperfections and broken links. In humus, the six-sided basic unit is the benzene or aromatic ring to which functional groups and various aliphatic substances are attached.