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52 ELRASHIDI ET AL. Selenium is one of the most widely distributed elements of the earth's crust. Most of Se (average is 0.09 mg/kg) occurs associated with sulfide minerals or as metal selenide (NAS-NRC, 1983). Sedimentary rocks are considered the major parent material in most agricultural soils having averages ranging from 0.08 to 1.0 mg Se/kg (Rosenfeld & Beath, 1964). Swaine (1955) reported that the Se content of most soils lies between 0.1 and 2 mg/kg. Soils developed from cretaceous shale (i.e., areas in the western USA) tend to be high in Se. Trelease (1945) collected 500 soil sam- ples from these areas. He found that the average Se content was 4.5 mg/kg with a maximum of 80 mg/kg. Only a small fraction of total Se in soils is dissolved in solution. Adri- ano (1986) reported that water-soluble Se varied between 0.3 and 7%a of the total Se in soils. Byers et al. (1938) measured water-soluble Se in more than 100 soil samples; water-soluble Se was <0.1 mg Se/kg for most of the sam- ples. Workman and Soltanpour (1980) stated that many cultivated soils con- tain <0.05 mg Se/kg. Soils that supply sufficient Se to produce toxic plants are commonly referred to as toxic seleniferous soils (NAS-NRC, 1983). Toxic seleniferous soils are usually alkaline in reaction and contain free calcium carbonate (Lakin, 1961). They occur in regions of low rainfall. However, several seleniferous areas were found in humid regions of Colombia and Ireland (Rosenfeld & Beath, 1964). Identification of the chemical forms of Se in soils is difficult because of the small amounts of the element present and the complex composition of soils. The forms of Se considered to be present in soils are selenates, selenites, elemental Se, selenides, and organic Se compounds (Lakin, 1961). The chemical forms and the solubility in soils depend mainly upon the oxidation-reduction potential (Eh) and the pH (Cary et al., 1967; Geering et ai., 1968). The low solubility of metal selenides favor their formation and persis- tence in agricultural soils. Although redox potentials indicate that selenide would be oxidized to selenite in most soils, the rate of oxidation is probably sufficiently slow to stabilize this form of Se under some soil conditions (Alla- way et al., 1967). Elemental Se is present in small amounts in some soils (Byers et al., 1938; Trelease & Beath, 1949; Fleming, 1980), and it may be an intermediate product in the oxidation of selenides or the reduction of selenites. Allaway et al. (1967) reported that elemental Se is a transitory constituent of neutral and acid soils during the reduction of selenites in acidic environments. Mea- surable amounts of elemental Se may be formed in soils due to bacteria and fungi. These microorganisms can reduce selenates and selenites under suita- ble conditions to elemental Se (Rosenfeld & Beath, 1964). Soluble selenates are the major form of Se in well-aerated alkaline soils (Lakin, 1961). selenates are not present in appreciable quantities in acid or neutral soils. The large fraction of Se in acid soils may occur in selenite forms of low solubility. Geering et al. (1968) indicated that solid-phase Se present in acid and neutral soils may be a ferric hydroxide-ferric selenite adsorption complex.