COPPER FERTILIZATION TO PREVENT MOLYBDENOSIS ON RETORTED OIL-SHALE DISPOSAL PILES

Plants growing on retorted oil shale disposal piles often have Mo concentrations high enough (> 8 mg kg-1) and Cu/Mo ratios low enough (< 2.0) to cause molybdenosis in grazing ruminants. High Mo concentrations are present in some plant species in spite of 90 cm of topsoil cover. Copper sulfate was broadcast at rates of 0, 8, 16, and 32 kg Cu ha-1 on plots with 30 and 90 cm of topsoil over Paraho retorted oil shale to determine if Cu fertilization can increase Cu/Mo ratios in plants so that molybdenosis is no longer a hazard. Of the plant species studied winterfat [Eurotia lanata (Pursh) Moquin], northern sweetvetch (Hedysarum boreale Nutt.), alfalfa (Medicago sativa L.), native grasses [Agropyron smithii Rydb., A. inerme (Scribn. and Smith) Rydb., A. dasystachium, (Hook.) Scribn. and Poa ampla Merr.], and introduced grasses [A. desertorum (Fisch.) Schult., A. intermedium (Host Beauv.), and Bromus biebersteinii Roem. and Schult.], all except alfalfa showed increased tissue Cu concentrations during the first growing season. Because of high Mo concentrations, however, only winterfat under high fertilization rates developed Cu/Mo ratios sufficient to prevent molybdenosis (i.e., > 2.0). By the end of the second growing season, increases in Cu concentrations due to fertilization had declined approximately 30%. These results indicate that although Cu fertilization can be used for short-term improvements in Cu/Mo ratios, it should not be relied upon as the sole technique for preventing molybdenosis on retorted shale disposal piles. Instead, maximum feasible topsoil thicknesses should be used and plant species known to concentrate Mo should be eliminated from seeding mixtures.