An appreciation of the formation and properties of humic substances is needed to understand their interactions with cations, and the influence of those interactions within the natural environment. This chapter presents a summary of current knowledge, emphasising those aspects of most relevance to cation binding. More detailed, wider-ranging, information can be found in the books of Aiken et al. (1985a), Hayes et al. (1989), Averett et al. (1989), Beck et al. (1993), Stevenson (1994), Piccolo (1995) and Gaffney et al. (1996). Jones & Bryan (1998) reviewed the colloidal properties of humic substances. Thurman (1985) is an essential text on the organic matter of natural waters, and includes much information on humic substances.

Natural organic matter and humic substances

Most (99.95%) of the Earth's carbon is held in sedimentary rocks, to and from which it cycles on geological time scales. The remaining carbon (about 4 × 1019g) is in the biosphere or shallow subsurface, 90% of it in the form of carbonate dissolved in seawater, and about 9% in organic forms (Table 2.1). In soils, natural waters and their sediments, the majority of the organic carbon is present in humic matter.

Stevenson (1994) considers natural organic matter in soils to consist principally of litter (macroorganic material lying on the soil surface), the light fraction (plant residues within the soil proper), soil biomass (predominantly microorganisms living in the soil), and stable humus. Of these pools, the last predominates in most agricultural, forest and moorland soils.