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32 MAYLAND ET AL. SELENIUM AS A FERTILIZER Selenium fertilization is briefly discussed to provide information to in- terested agronomists and animal nutritionists. The clay fraction and organic matter content in soil play a major role in the bioavailablilty of Se, especially selenite, which is readily sorbed to these soil constituents. Selenate is less readily sorbed in finer-textured soils or soils with higher organic matter concentrations. Consequently, Selenate is usual- ly absorbed from soils by plants more readily than is selenite (Carter et al., 1969). Greater absorption of Selenate may also result from differences in root permeability or that selenite is reduced to elemental Se on the root surface. Although sulfate-S exerts little effect on selenite uptake, it competes with the absorption of selenate (Gissel-Nielsen, 1973; Westermann & Robbins, 1974). The development of a safe and effective Se fertilizer must consider the chemical form, its solubility under the range in soil pH and redox poten- tials, and the rate and amount of Se uptake by the plant. The application of lime or companion fertilizer containing phosphate, sulfate, or N can also affect Se concentrations in the herbage (Gissel-Nielsen, 1973; Milchunas et al., 1983). These materials may contain small amounts of Se, or they may stimulate root growth and subsequent uptake of soil Se (Carter et al., 1972; Westermann & Robbins, 1974). Application of these nutrients often stimu- late forage yields, thereby diluting the Se concentration in the forage (H.F. Mayland, 1975 unpublished data). Fertilizers made from rock phosphates may contain as much as 200 mg Se/kg (Rader & Hill, 1935; Robbins & Carter, 1970; Senesi et al., 1979). These concentrations are generally much higher than those measured in fertilizers derived from rock carbonates, synthetic N fertilizers, and even potassium sulfate (Senesi et al., 1979). Thus, some fertilizer materials may contribute Se for use by higher plants. Selenium-deficient soils are often S-deficient, especially for the produc- tion of legumes. The application of sulfate fertilizers can simultaneously stimulate plant growth and compete with soil selenate for absorption sites on the root and transport mechanisms in the plant. Increasing the concen- trations of sulfate-S in the forage also may have detrimental effects on Se availability to the animal (Hintz & Hogue, 1964; Pope et al., 1979). There is some risk associated with the application of selenates to acid and neutral soil because of their high bioavailability (Reuter, 1975). Ex- perience has shown that the use of selenate fertilizer results in much higher levels (sometimes toxic to animals) of Se in the first cut of forages following fertilization (Tables 2-3 and 2-4). Selenium concentrations in the forage, fertilized with a selenate source, usually tend to decrease sharply with each subsequent harvest (Carter et al., 1969; Giessel-Nielsen & Bisbjerg, 1970; Ylaranta, 1983).