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Zinc Sorption, Desorption, and Fractions in Three Autoclaved Soils Treated with Pyrophosphate

Dep. of Renewable Resources, Macdonald College of McGill Univ., Ste. Anne de Bellevue, Quebec, Canada, H9X 1C0

* Corresponding author.

ABSTRACT

Pyrophosphate (PP) is used as a P fertilizer. Unlike orthophosphate, PP can sequester soil Zn. In addition, PP sorption may affect Zn sorption and have implications for fertilizer practice. Pyrophosphate effects on Zn solubility were examined with autoclaved top- and subsoil samples from three Quebec soils. Soils were equilibrated sequentially with PP solutions, then with Zn solutions, and finally with solutions containing neither P nor Zn. The effects of PP sorption on soil cation exchange capacity (CEC), PP desorption, and Zn sorption and desorption were determined. Zinc from treated soils was extracted sequentially with KNO₃ (Zn_KNO3), NaOH (Zn_NaOH) and concentrated HNO₃ + H₂O₂ (Zn_HNO3) solutions. One mmol sorbed P kg^–1 as PP resulted in CEC increases of 0.52 to 0.89 mmol_c kg^–1 soil. Increased Zn sorption was equivalent to 7 to 12% of the increase in CEC induced by sorbed PP in the Uplands (Typic Haplorthod), and 0.3 to 2.8% in the St. Bernard (Typic Eutrochrept) and the Dalhousie (Typic Humaquept) soil samples. Sorbed PP reduced Zn desorption in the Uplands and St. Bernard soils but not in the Dalhousie soils. Positive correlations between sorbed PP and extractable Fe or Al materials, clay, and organic C suggested that PP sorption was related to surfaces of Fe materials (especially crystalline Fe), or clay-organic complexes. Zinc addition improved correlations between desorbed PP and soil parameters. Added PP at 90 mmol P kg^–1 soil reduced Zn_KNO3, by about 50% (except with the Uplands subsoil), while added PP increased Zn_NaOH and Zn_HNO3 in all soils, indicating that Zn sorption and PP-Zn interactions were mainly related to Fe or Al oxides, and increased Zn sorption with PP was related to increased specific sorption sites.

Received for publication October 17, 1988.