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26 MAYLAND ET AL. <br />The Se in forages and cereal grains may not differ much among species <br />in areas where available Se levels in the soil are low. Selenium concentra- <br />tions in legumes and grasses vary when these plants are grown on soils where <br />bioavailability of Se to forages is adequate for animal requirements. However, <br />alfalfa usually contains the highest level, which may be several times as much <br />as is in the grass forages (Massey & Martin, 1975). In these same areas, Se <br />levels in wheat (Triticum aestivum L.) grain have been shown to be higher <br />than in oat (Avena sativa L.) or barley. <br />Westermann and Robbins (1974) noted that, in many areas where S is <br />deficient for plant growth, Se may also be inadequate for animal require- <br />ments. Application of fertilizer S to increase forage yields may further lower <br />forage Se concentrations either by direct competition between SO4- and <br />Se04- at the soil root interface, by stimulation of growth and dilution of <br />Se, or both. <br />Williams and Thornton (1972) evaluated, in the glass house, the effect <br />of several soil additives on the plant uptake of Se from soils containing <4 <br />to 230 mg Se/kg. Ammonium sulphate and elemental S markedly decreased <br />the Se uptake by perennial ryegrass and red clover, with the ammonium salt <br />giving the most persistent effect. Both the ammonium and the sulphate ions <br />seem to be involved. Calcium orthophosphate and potassium sulphate some- <br />times increased Se uptake, so use of these fertilizers on Se-toxic soils should <br />be considered with caution. <br />Plant Response to High Bioavailable Selenium <br />Selenium is known to be required for animal health (0.05-0.1 mg Se/kg <br />diet) but is not yet considered as an essential element for plant growth. Several <br />earlier studies strongly suggested that Se was required at least for the ac- <br />cumulator plant species. Trelease and Trelease (1938, 1939) reported a <br />pronounced stimulating effect in the growth of Astragalus racemosus (Pursh) <br />and A. pattersonii (Gray) ex. Brand, grown in nutrient cultures containing <br />up to 9 mg Se/L as selenite. Both species are identified as Se accumulator <br />plants. Astragalus crassicarpus Nutt. (also named A. succulentus by Rosen- <br />feld & Beath, 1964), which is a nonaccumulator, was poisoned by Se at rates <br />as low as 0.3 mg/L. These findings led the Treleases' and others (Rosenfeld <br />& Beath, 1964; Shrift, 1969) to suggest that Se may be an essential element <br />for the Se indicator plants. This apparent Se requirement by the accumula- <br />tor species may be confounded by a Se x P interation, whereby the increased <br />growth in the presence of Se could have been due to a depression of P toxici- <br />ty by selenite (Broyer et al., 1972a). However, evidence for the P x Se inter- <br />action was obtained using solution culture, and proof would be difficult to <br />establish using soil cultures. <br />In an in-depth, solution-culture study of ionic sorption interactions in <br />Astragalus sp., Broyer et al. (1972b) reported the following observations: <br />...selenite tended to depress the sorption of phosphate and the micro- <br />nutrient metals (Cu, Fe, Mn, Zn); phosphate tended to restrict selenite <br />sorption and phosphate tended to enhance the sorption of the macro- <br />