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Formation of a Transient Magnesium-Aluminum Double Hydroxide in Soils of Southeastern Ohio

Dep. of Agronomy, Ohio State Univ., Columbus, OH 43210

H. T. Saif

Al-Furat Center for Studies and Designs Irrigation Projects, Baghdad, Iraq

*Corresponding author.

ABSTRACT

To assess the potential for Mg-Al double hydroxide formation in acid, Al-rich soils of southeastern Ohio under native conditions, laboratory studies were conducted to determine if Mg-Al double hydroxide could be produced with Al-saturated clays and soils of mixed mineralogy common to the region. Aluminum-saturated clays suspended in dilute Mg solutions and titrated with NaOH to pH 9.5 yielded pronounced buffer regions between pH 8.3 and 8.5 attributable to the formation of Mg-Al double hydroxide. Equilibration of the clays titrated to pH 9.5 for 55 d showed a gradual decline to pH 8.0 to 8.4. The decline was attributed to the precipitation of additional Mg-Al double hydroxide caused by the slow release of interlayer hydroxy Al, which showed a correlation with the rate of pH decline. X-ray diffractograms of the equilibrated clays yielded peaks at 0.78 and 0.39 nm that were indicative of Mg-Al double hydroxide. Aluminum-saturated soils equilibrated with Ca solutions at pH 8.4 retained only 50 to 70% as much added cation as soils equilibrated with solutions of combined but equal total quantities of Ca and Mg. Furthermore, the soils equilibrated with Ca and Mg retained 2.4 to 4 times more Mg than Ca. The enhanced retention of Mg was attributed to Mg-Al double hydroxide formation. Sequential leaching of the equilibrated soils with NaOAc at pH 8.2, 7.0, and 5.0 and 0.05 M HCl indicated that Mg-Al double hydroxide was not readily solubilized until the pH dropped below neutral. It was concluded that the pedogenic formation of an ephemeral Mg-Al double hydroxide was a probable consequence of calcareous loess deposition on acid, residual soils during the Wisconsinan glaciation.

Research support provided by state and federal funds appropriate to the Ohio Agricultural Research and Development Center, Ohio State Univ. Journal Article 43–93.

Received for publication April 7, 1993.