Alfalfa growth and absorption of cadmium and zinc from soils amended with sewage sludge

Abstract

Safe application of sewage sludge to soil used for food chain crops requires careful management of the heavy metals that pose a potential health hazard. Soils in pots were amended with various chemicals and sewage sludge. Alfalfa ( Medicago sativa L.) was grown in the amended soils to measure yield and shoot concentrations of Cd and Zn. The four soils were a strongly calcareous Millville silt loam (Typic Haploxeroll), a slightly calcareous Kidman loam (Typic Haplustoll), a slightly acidic Sinks silt loam (Cumulic Cryoboroll), and an acidic Franklin Basin silt loam (Pachic Paleboroll). Soils were amended with 20 Mg ha −1 dry weight of sewage sludge; some soil samples also received additions of CaCl 2, CaO, CaCO 3, or HCl. Pots were set on benches outside after alfalfa germinated. Adding 20 Mg ha −1 sludge provided 4.6 kg ha −1 Cd, which exceeds the proposed US EPA criteria maximum of 0.5 kg Cd ha −1 per year. Alfalfa was grown on all soils and was harvested three times. The Cd concentrations in dry alfalfa tops ranged from 0.21 mg kg −1 for controls to 0.64 mg kg −1 for sludge-amended soils. Neither Cd nor Zn reached phytotoxic concentrations in the alfalfa. In most treatments Cd levels in alfalfa equalled or exceeded the 0.5 Cd kg −1 that is permissible in animal feed. During the rapid growing period between the first and second cuttings, Cd and Zn concentrations in alfalfa were as much as 50% lower than concentrations during periods of slower growth. The total Cd and Zn recovered in alfalfa grown on the two slightly acidic soils was about 35–50% higher than from the non-acid soils. Adding lime to the acid soils significantly reduced Cd and Zn concentrations in tissues.