Use of Dilute Oxalate to Recover Exchangeable Aluminum Immobilized by Sulfate Salt Addition to Rhodic Kandiudult Subsoil

Abstract

While SO₄ salts have been shown to be effective in reducing exchangeable aluminum (Al) and solution Al activity in acidic subsoils, few attempts have been made in the laboratory to recover exchangeable Al immobilized by the suggested mechanisms of Al–OH–SO₄ mineral precipitation or co-sorption on sesquioxide surfaces with SO₄ and cations added in the salt (salt sorption). Some low-molecular-weight aliphatic acids such as oxalate are well-known for their ability to increase the solubility of Al in soils and oxalate has often been identified as the dominant or one of the dominant organic anions in subsoils. Oxalate is also well-known for its ability to form soluble complexes and a precipitate with calcium (Ca). Therefore, the objective of this study was to investigate the effects of dilute oxalate on the apparent recovery of exchangeable Al immobilized in response to the addition of gypsum (CaSO₄·2H₂O) and langbeinite (K₂SO₄·2MgSO₄). Following a 6-month equilibration in the laboratory, gypsum and langbeinite added at 15 mmol SO₄ kg⁻¹ were equally effective in decreasing exchangeable Al in soil collected from the Bt1 horizon of a Paaloa silty clay (clayey, oxidic, isothermic Rhodic Kandiudult). However, an oxalate loading rate of 16 mmol kg⁻¹ appeared to recover 100% of the exchangeable Al immobilized by langbeinite but only 38% of that immobilized by gypsum. This result was likely due to the formation of an oxalate precipitate or complexes with Ca in the gypsum treatment. It is suggested that the longevity of gypsum effects on reduced exchangeable Al under field conditions may be due in part to the role of Ca in oxalate complexation.

Acknowledgments

Thanks are extended to the undergraduate students in the soil fertility course at the University of Hawaii at Hilo who assisted in all phases of this study.