Zinc, copper, and nickel availabilities as determined by soil solution and DTPA extraction of a sludge‐amended soil

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Abstract
Extracting sludge‐amended soil with DTPA does not always give a reliable measure of plant‐available heavy metals. The major purpose of this greenhouse pot study was to help explain why. Two anaerobically digested sludges from sewages treated with either Ca(OH)2or FeCl3were applied to 3‐kg samples of a Mollic Albaqualf previously limed with Ca(OH)2rates of 0, 2.5, and 10g/pot that resulted in pHs in the check pots of 5.4, 6.2, or 7.7 after the first harvest. Sludge rates provided 0, 200, 40, 800, and 1600 mg Zn kg‐1of soil. Two consecutive crops of soybeans (Glycine MaxL.) were grown for 42 d each in the greenhouse. DTPA‐extractable, soil‐solution, and plant concentrations of Cu2+, Ni2+, and Zn2+were measured. Dry matter yields were depressed due to salt toxicity, while DTPA‐extracted Cu2+correlated with plant uptake of Cu2+for both sludges. DTPA‐extracted Ni2+also correlated with plant Ni2+from the Ca(OH)2‐sludge‐amended soil, although DTPA‐extracted Ni2+did not correlate with plant uptake of Ni2+from the FeCl3‐sludge‐amended soil, DTPA‐extracted Zn did not correlate with plant uptake of Zn2+from any sludge‐amended soil. Soil‐solution composition correlated with plant uptake of Cu2+and Ni2+in both sludges; it also correlated with plant uptake of Zn2+from FeCl3‐sludge‐amended soil but not from Ca(OH)2‐sludge‐amended soil. DTPA extraction probably failed with Ni2+and Zn2+because of (i) its ineffectiveness at low pH, (ii) the inability of DTPA to buffer each soil extract near pH 7.3, and (iii) increased amounts of soluble chelated micronutrients at higher sludge rates and higher soil pHs. Soil‐solution composition seemed to fail only where micronutrient cations in solution probably were present largely as organic chelates