Soil Organic Matter

Model Formula: C349H401N26O173S

The description of soil organic matter as specific type models is fraught with speculation since, like snowflakes, very likely no two soil organic molecules are alike. Indeed, it has been proposed that the chemical heterogeneity of humic macromolecules is the key to their persistence in the soil environment populated by carbon-starved microorganisms. Nonetheless, for decades structural models have been proposed as intellectual and educational exercises around which discussion can take place. With the advent of chemical modelling software, three-dimensional models are now possible. Shown here is the model of Schulten and Schnitzer (1997) consisting of humic substances, carbohydrate, hexapeptide, and hydrating water molecules.

Highlighting Features

Humic substances comprises the majority of soil organic matter.

It is partially extractable from soil by strong base and cannot be identified as any other biochemical. It contains a number of acid groups, among them weakly acidic groups of the carboxylic type (pK ~4-5), very weakly acidic groups of the phenolic type (pK ~9), and very, very weakly acidic groups of the alcoholic type (pK ~20).

Carboxylic groups (-COOH)
Phenolic groups (ArOH)
Alcoholic groups (ROH)
Quinonic groups
Ketone groups
Methoxyl groups
Saccharides (sugars)

Part of soil organic matter and are here represented by a trisaccharide occluded in the humic acid.

Proteinaceous material

Here represented by a hexapeptide (AspGlyArgGluAlaLys) occluded by humic acid, is also part of soil organic matter but is not considered a humic substance.

Water is drawn as part of the model substance.
Aromatic carbon is drawn as part of the model substance.
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H.-R. Schulten and M. Schnitzer. 1997. Soil Sci.162:115-130.