Reactions of Selenite‐Selenium Added to Soils That Produce Low‐Selenium Forages

Alfalfa ( Medicago sativa L.) absorbed Se in concentrations potentially toxic to livestock from four of seven soils treated with radioactively labeled Se at a 2.5‐ppm rate as a selenized concentrated superphosphate. When the Se rate was 0.5 ppm, only the first few crops from one soil contained toxic amounts. Subsequent alfalfa crops from all seven soils receiving 0.5 ppm and six of seven soils receiving 2.5 ppm contained nearly constant, nontoxic levels of Se that would protect animals from Seresponsive diseases. After 1 year of cropping soils in pots in a greenhouse, up to 40% of the residual Se could be removed by isotopic exchange with selenite. Further fractionation indicated that organic Se compounds and elemental Se or selenides are also probably present in the residual Se.