Comparison of extractants for estimating heavy metal availability in mid-Atlantic coastal plain soils
The effectiveness of four soil extractants - diethylene triamine pentacetic acid (DTPA) at pH 7.3 and 5.3, Mehlich 1 (0.05 N HCl + 0.025 NH2SO4), and Mehlich 3 - for estimating labile Zn, Cu, Mn, Pb, Ni, and Cd in Coastal Plain tobacco soils in Maryland was evaluated. Stepwise multiple regression procedures were used to relate extractable metals and soil characteristics to metal concentrations in tobacco leaves. The four extractants were significantly different with respect to the quantities of metals extracted and, except for Cu, the magnitude of differences in amounts of metal extracted typically varied by 100% among the four extractants. The relative ranking among the four extractants varied among the six metals with no single extractant appearing consistently high or low. Mehlich 1 extracted larger quantities of Zn, Cu, Mn, Ni, and Cd and smaller amounts of Pb than Mehlich 3. Also, Mehlich 1 extracted larger amounts of Zn, Mn, Ni, and Cd and smaller amounts of P b than DTPA pH 7.3. DTPA pH 7.3 extracted larger amounts of Cu and smaller amounts of Zn than DTPA pH 5.3. Regression analyses relating soil metals, pH, C.E.C., and clay content to plant metal concentrations showed the four procedures to be highly significant (P<=0.01) for Zn, Mn, Ni, and Cd; however, DTPA pH 5.3, Mehlich 1 and 3 exhibitedSquare R for Cu which equalled only 0.14, 0.17, and 0.10, respectively. None of the four procedures were significant for Pb.