Extraction and plant availability of cadmium and zinc in a Willamette sandy loam soil Public Deposited

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  • A Willamette sandy shot loam was treated with ZnC1₂, CdC1₂, or sewage sludge to determine the chemical fate of added Cd and Zn in the soil and their influence on the growth of corn and wheat seedlings. Cadmium was added as CdC1₂ at rates of 0, 5, 20, or 80 μg Cd/g of soil and Zn was added as ZnC1₂ at rates of 0, 20, 80, and 320 μg Zn/g of soil. Sewage sludge was added to other samples at rates of 0, 7.81, 31.3 and 125 kg/ha which was equivalent to the addition of 0, 20, 80, and 320 μg Zn/g of soil and 0, 0.39, 1.55, and 6.22 μg Cd/g of soil. The treated soils were incubated for 24 weeks in a growth chamber and plants were grown on sub-samples of each treated soil after 0, 3, 6, 12, and 24 weeks of incubation. Ammonium acetate at pH 7.0, DTPA, and sodiun dithionite- citrate extracts of the soil were analyzed for Cd and Zn. Soil was also sequentially extracted with ammonium acetate at pH 7.0, ethanol-benzene, 0.05 N H₂SO₄, 0.1 N NaOH, dilute HF-HC1, 0.1 N NaOH, and NaOCI and analyzed for Cd and Zn. The yield of wheat and corn decreased with added CdC1₂. The yield of corn was unchanged by addition of ZnC1₂, but the yield of wheat increased with ZnC1₂ up to 80 μg Zn/g of soil. Sewage sludge increased the yield of wheat and corn at all application rates, but no increases in yield were noticed above the lowest sewage sludge application rate. The concentration of Cd and Zn in wheat and corn tissue increased with added CdC1₂, ZnC1₂ or sewage sludge. The concentration of sodium dithionite-citrate extractable Cd and Zn did not correlate with the iron oxide content of the soil, but was probably a measure of the citrate extractable Cd and Zn. The addition of sewage sludge to the soil initially raised the pH from 5.3 to 6.7, but it then decreased to 4.6 during the incubation period. In the sequential fractionation procedure, almost all the added Cd and Zn were extracted in the ammonium acetate fraction or in the 0.05 N H₂SO₄ fraction. Ammonium acetate exchangeable Cd or Zn increased during the incubation period in soil treated with sewage sludge. This increase resulted from the decomposition of the added organic material with the subsequent release of Cd and Zn to the soil solution, and the decrease in pH of the sewage sludge treated soil during the incubation period. The concentration of ethanol-benzene extractable Cd and Zn increased in the middle of the incubation period and then again decreased. Even though the Zn and Cd concentrations during the incubation period changed they were always a small percentage of the total added. Much of the remaining Cd and Zn was extracted by the 0.05 N H₂SO₄ treatment. The extractable Zn and Cd concentration of the 0.1 N NaOH fraction was very low when the extraction followed the acid treatment. The Cd and Zn complexed by the organic matter was replaced by hydrogen ions in the acid solution, leaving little to be extracted by 0.1 N NaOH. When 0.1 N NaOH was followed by 0.05 N H₂SO₄, a considerable portion of the added Cd and Zn was extracted in the 0.1 N NaOH fraction. The concentration of Cd and Zn in this 0.1 N NaOH fraction decreased during the incubation period. The concentration of exchangeable Cd and Zn increased during the incubation period, but a corresponding increase in plant uptake was not observed. A considerable portion of the Cd and Zn are complexed by the organic matter of the soil. An acid extractant releases organically complexed Cd and Zn into solution.
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