Calcium silicate slag is a by‐product of the electric furnace reduction process for producing phosphoric acid from phosphate rock (PR). Since slag contains trace amounts of radium (Ra), and uranium (U) as contaminants from the PR, a greenhouse pot experiment was conducted to determine if plants absorb Ra from slag applied to soil. Slag at rates equivalent to 0 and 22 mt/ha was mixed with Mountview silt loam (Typic Paleudults) limed to pH 5.8 and 7.2. Three clippings each of fescue (Festuca arundinaceaSchreb.), and Swiss chard (Beta vulgarisL.), and one harvest of wheat (Triticum aestivumL.) for grain and straw were grown on separate series of treated soil, and plant samples were analyzed for radioactivity due to Ra uptake. Samples of sugarcane (Saccharum officinarumL.) forage and extracted juice from field experiments in Florida testing this slag as a Si source also were analyzed for radioactivity.