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SELENIUM SOLUBILITY, 5PECIATION, AND TRANSFORMATIONS 53 A number of investigators (e.g., Rajan & Watkinson, 1976; Singh et al., 1981) reported that Se is adsorbed on soil surfaces, particularly on sesqui- oxide minerals. Some scientists, however, suggested that sparingly soluble selenites, i.e., ferric selenite and basic ferric selenite, may be present in acid soils (Lakin, 1961; Rosenfeld & Beath, 1964). It is still not clear whether Se in soil solution is controlled by an adsorption-desorption mechanism or byprecipitation-dissolution reactions. The effect of Eh and pH on the solubility, speciation, and transformation of Se in cultivated soils is important. Recently, Elrashidi et al. (1987) devel- oped solubility relationships for 83 Se minerals and soluble species in soils. The objective of this report is to discuss some of the results reported in the literature regarding the solubility, speciation, and transformations of Se in soils. SELENIUM SPECIATION Solution Species Selenium in soil solution can be present in any of the following species: Selenate(VI) Se04-, HSe04 ,and H2Se0° Selenite(IV) Se03-, HSe03 ,and H2Se0° Selenide(-II) See-, HSe-, and H2Se° In Fig. 3-1, the negative log of electron activity in solution (pe) is plotted as a function of pH for some redox couples that could be important in the chemistry of Se in soils. The stability lines were developed from reactions reported in a previous study (Elrashidi et al., 1987). The redox couples O2(g)/H20(1) and H +/H2(g) give the redox limits of natural aqueous en- vironments. The expression Eh(volt) = 0.0592 pe was used to calculate Eh values in Fig. 3-1 after Lindsay (1979). This figure can be used to predict the Se solution species that might be present in soil depending on pH and pe. In the region between the line of O2(g)/H20 couple and the selenate/selenite couples (i.e., Se04-/HSe03 and Se04-/Se03-), Se in solution would be expected in the selenate form. Between selenate/selenite and selenite/selenide couples, the selenite form would be the major species in solution. Between the lines of selenite/selenide and H2O/H2(g) couples, Se in solution would be mainly in the selenide form. Elemental Selenium and Metal-Selenides At low redox the formation of elemental Se or a metal selenide mineral can be expected in soils. Consequently, species in solution would be in equilibrium with the most stable solid phase. The redox at which elemental Se and metal Selenides are formed in soils depends on the total Se in solution and pH.