Oxidation, or reddening of the soil, derives from the very slow oxidation of iron-bearing minerals in rock particles, and usually results in a thin coating of iron oxides on mineral grains. The youngest soils have colours resembling those of rock, but as soil age increases the intensity of oxidation and reddening and the depth of oxidation both increase, with alteration extending to beyond 1 m in depth. Salinization, or the accumulation of salts, is widespread, and is a consequence of high evaporation rates, which typically exceed precipitation. The salts in the soils may form distinct horizons, and are predominantly derived from atmospheric transport. Clear geographic and climate-related differences in soil salt content, as well as age-related differences in salt abundance, are found (Claridge and Campbell 1977; Campbell and Claridge 1987). Salt accumulation is essentially linear with time (Bockheim 1979), and chemical weathering is insignificant by comparison.
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