Nature And Forms In Soil

Trace metals in soil originate from (1) parent materials from which the soils are formed, (2) contamination from impurities in soil amendments such as mineral fertilizers or biosolids, and (3) atmospheric deposition of natural (e.g., wind-eroded soil or volcanic materials) or anthropogenic (e.g., fossil fuel combustion products) particulates. The main source of trace metals is the parent material because the amounts added, even in high fertilizer use agroecosystems, are generally too low to have an influence on the total amount present. Some sites that have received high rates of biosolid amendments over a long period of time may have soil concentrations of trace metals many times higher than that of the geochemical background (Table 15.10).

Micronutrients and trace metals in soil can be water-soluble ions or water-soluble complexes with organic or inorganic ligands. They may be present as exchangeable cations on clay mineral surfaces. Some trace elements (e.g., Cu2+) are retained by clay minerals or oxides even in the presence of an excess of Ca2+. Trace elements bound in this manner are called "specifically adsorbed." Organic matter is also a substrate from adsorption and complexation of trace metals. And finally, trace

TABLE 15.10 Mean Concentrations of Some Trace Metals in Igneous and Sedimentary Rocksa

Mean concentration (pg g ')

Igneous rocks Sedimentary rocks

TABLE 15.10 Mean Concentrations of Some Trace Metals in Igneous and Sedimentary Rocksa

Mean concentration (pg g ')

Igneous rocks Sedimentary rocks

Element

Granite

Basalt

Shale

Sandstone

Carbonate

Fe

13,700

77,600

47,000

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