Molybdenum, sulfur, and other trace elements in farm soils and forages after sewage sludge application

Farm application of sewage sludge products (biosolids) can potentially increase soil concentrations of macronutrients [especially phosphorus (P)], micronutrients [including sulfur (S), copper (Cu), zinc (Zn), molybdenum (Mo)] and heavy metals. In the present study, sampling and elemental analysis by ICP of soils and forages from 13 farms in Upstate New York that had applied sludge showed some elements to be significantly elevated (relative to controls) in the soils and crops of several farms. Specifically, total soil cadmium (Cd), chromium (Cr), Cu, vanadium (V), nickel (Ni), P, S, and Zn on several farms were significantly higher than the average for farms of this region, although the estimated soil loadings of regulated heavy metals from sludge application were no more than a few percent of the EPA 503 permitted loadings. For most elements, the higher soil concentrations did not result in significantly higher average forage concentrations. Nevertheless, the average forage S concentration was significantly increased on every farm that had applied sludge, most farms had significantly elevated average forage Zn, and three farms had significantly elevated average forage Mo contents. The highest forage Mo was found on several farms that had made a single application of an alkaline-stabilized sludge product. Forage Mo content was significantly correlated to two soil properties: pH and CaCl2-extractable Mo. Contrary to expectation, averaged over all farms, Mo contents of grass forages (2.4 +/- 1.9 mg/kg) were not generally lower than Mo contents of the forage legumes, alfalfa (2.0 +/- 2.2 mg/kg) and red clover (2.6 +/- 2.4 mg/kg). More intensive sampling of a single alkaline sludge-amended field revealed high variability in Mo content of alfalfa (0.67-7.4 mg/kg), soil pH, and CaCl2-extractable Mo in the soils. The alfalfa Cu/Mo ratio was strongly correlated to both soil pH and CaCl2-extractable Mo at the site of sampling. A laboratory experiment demonstrated that liming acidic (pH 5-6) sludge-amended soils to about pH 7 increased Mo solubility by about 10-fold, while having little effect on soluble S. It is concluded that, where sewage sludges are used as farm fertilizers, the greatest short-term risk to livestock is hypocuprosis induced by higher forage content of Mo and S.