A group of substances that are natural products of earth surface environments. Probably the most widely distributed organic carbon–containing materials in terrestrial and aquatic environments, they are dark-colored, predominantly aromatic, acidic, hydrophilic, molecularly flexible polyelectrolytes. Humic substances constitute 70–80% of the organic matter in inorganic soils and are formed from the chemical and biological degradation of plant and animal residues and from synthetic activities of microorganisms.