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Reactions Between a Podzol Fulvic Acid and Na-montmorillonite¹

ABSTRACT

The interlamellar adsorption of fulvic acid by Na-montmorillonite was pH-dependent and decreased with increase in pH. At pH 2.5, 40 mg of clay adsorbed 33.2 mg of fulvic acid. Interlamellar adsorption was rapid and increased with increasing fulvic acid concentration until approximately 100 mg of the latter had been added to 40 mg of clay. While distilled water desorbed only 24% of the fulvic acid in the organic matter-clay complex, O.1N NaOH solution desorbed 93% of the adsorbed fulvic acid. From the experimental data it appeared that fulvic acid was adsorbed in and on the clay mainly in the undissociated or slightly dissociated form and, to a minor extent, as Na-salt. Desorption experiments showed that a decrease in interlamellar spacing of 1A was associated with the expulsion of approximately 3.0 mg of fulvic acid. It was estimated that slightly more than one-half of the fulvic acid was adsorbed in the interlamellar spaces of the clay.

The controversy in the literature as to whether or not soil organic matter is adsorbed in the interlamellar spaces of 2:1 type expanding silicate minerals appears to a considerable extent to be related to disregard by many investigators of the effect of pH.

Key Words: soil organic matter • interlamellar adsorption on Na-montmorillonite • pH • X-ray diffraction

¹ Contribution no. 227, Soil Research Institute, Canada Department of Agriculture, Ottawa, Ontario.

² Senior Research Scientist and Research Scientist, respectively. The authors thank N. M. Miles for technical assistance with X-ray diffraction analysis, G. F. Morris for carbon analyses and I. Hoffman for running the samples on the thermobalacne.

Received for publication March 13, 1967. Accepted for publication May 31, 1967.