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Holmgren, G.G.S.; Meyer, M.W.; Chaney, R.L.; Daniels, R.B. (1993). "Cadmium, Lead, Zinc, Copper, and Nickel in Agricultural Soils of the United States of America". Journal of Environmental Quality. 22 (2): 335–348. Bibcode:1993JEnvQ..22..335H. doi:10.2134/jeq1993.00472425002200020015x.