than 2000 S-bearing minerals with S contents ranging from 7 to 53% make S the 13th most abundant element in the earth's crust. Weathering during soil formation occurs very slowly, and primary mineral S is seldom a significant source of plant-available S. Inorganic S exists primarily as SO4~ in calcareous soils of semiarid and arid regions, in volcanic ash, and in recently reclaimed tidal marsh areas. It may be present as gypsum (CaSO4), as a basic aluminum sulfate, or as a contaminant of CaCO3 in concentrations reaching as high as 3000 ^g g_1.
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