Extractability of Cadmium, Copper, Nickel, and Zinc by Double Acid Versus DTPA and Plant Content at Excessive Soil Levels

Cadmium, copper, nickel, and zinc were applied as sulfate salts to samples of surface horizons of three Maryland soils in the greenhouse at rates of metals equivalent to those in 0 to 896 dry metric tons/ha of Washington, D.C., digested sewage sludge. The sludge was also applied at a rate of 224 dry metric tons/ha. Two pH regimes, approximately 5.5 and 6.5, were maintained. Metals were extracted by the DTPA (diethylenetriaminepentaacetic acid buffered at pH 7.3) double acid (0.05N HCl, 0.025N H₂SO₄) extractants. Correlations were determined between extractable soil metals and metal content of two crops of corn (Zea mays L.) each grown for 30 days, but at times of 1 or 13 mo after making the chemical amendments to the soils. Highly significant associations, r values ranging from 0.658 to 0.998, were obtained between the quantities of Cd, Cu, Ni, and Zn extracted by the two extractants. DTPA tended to extract smaller quantities of all metals and tended to be more indicative of reduced plant Zn and Cu uptake under limed conditions than double acid. Cadmium remained the most extractable after 14 mo by either extractant when expressed as a percent of the applied metal that was extractable. Overall metal extractability on this basis followed the order Cd > Cu > Zn > Ni, independent of soil type or extractant. Sludge-applied metals were approximately half as extractable as sulfate salt-applied metals at comparable rates of application, but followed the same order of relative extractability. Plant content ofCd, Cu, and Zn was significantly correlated, at the 1% level, with extractability of the respective metal for both extractants for metal salt-amended soils. Correlations from the sludged soils were poor and generally not significant. Numerically higher and generally more significant correlations were obtained when the data were separated by lime level into two regression equations compared to a combined regression. The primary cause of this effect was the greater metal content of the plant tissue per unit of extractable metal on the low vs. the high pH soils, except for Cd, producing two distinct regression equations. This relationship was more pronounced with the first crop than with the second.