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24 MAYLAND ET AL. Some plants growing on seleniferous soils accumulate surprisingly low levels of Se. White clover (Trifolium repens L.), buffalograss (Buchloe dac- tyloides [Nutt.] Engelm.), and grama (Bouteloua sp.) are poor accumula- tors of Se. On the other hand, high sulfur (S)-containing plants like the Brassica sp. (mustard, cabbage, broccoli, and cauliflower) and other Cruci- ferae are relatively good concentrators of Se (NAS-NRC, 1983). Absorption of Se and S by plants may be correlated (Shrift, 1973). Plant Response to Selenium Species Gissel-Nielsen and Bisbjerg (1970) evaluated Se uptake by agricultural plants from various Se compounds (Table 2-3). They found in greenhouse and field studies that over a 2-yr period, red clover (Trifolium pratense L.), barley (Hordeum vulgare L.), and white mustard (Sinapis alba L.) absorbed <0.1°l0 of the applied elemental Se, about 1.0% of the SeOz and Selenite forms, and 12 to 60% of the Se added as selenate (Table 2-3). In another study, Se uptake by alfalfa, barley, and sugar beet (Beta vulgaris L.) was one-third or less of that taken up by the Cruciferous mustard. Uptake by red clover decreased by a factor of six over the eight successive harvests regardless of the Se source. Selenium concentrations in clover grown on the selenate sources were 25 to 50 times that obtained from Selenite, and the decrease in selenate uptake with time was about four times greater. In another study, Gissel-Nielsen (1973) again reported that more Se was taken up from selenate than Selenite by barley and red clover. With subsequent cropping, there was a greater decline in uptake from the 'SSe labeled sodi- um selenate source than from the labeled sodium Selenite source. Adding sulfate (SO4-), at rates of 0 to 375 mg S/kg soil, greatly decreased the up- take of Se from the selenate source but had a lesser effect on Se uptake from the Selenite source. The ratio of Se in roots to Se in tops shows that Se is more readily translocated from the roots when taken up from added selenate than from added Selenite. Table 2-3. Selenium solubility in water and relative uptake of Se by plants from differ- ent sources labeled with 75Se in pot experiments using a loamysand having 2.8% or- ganic matter, 5.7 pH, and 0.12 mg Selkg (adapted from Gissel-Nielsen &Bisbjerg,1970-. Se source Solubility of Se in cold watert Se added to soil Uptake relative to added Se Clover Barley Mustard g/L mg/kg % Se i 2.5 0.005 0.02 0.07 Se02 i 0.5 1.0 0.9 1.2 KZSe03 22.4 0.5 1.0 1.1 1.3 Na2Se03 s 0.5 1.0 1.0 1.1 BaSe03 0.05 0.37 0.9 0.9 0.9 FeSe03 i 0.35 1.1 1.0 1.1 CuSe03 i 0.30 0.8 0.8 0.7 KZSe04 390 0.50 24. 12. 24. BaSe04 0.03 0.10 63. 27. 61. CuSe04 68 0.13 53. 28. 48. t i = insoluble, s =slightly soluble.