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Determination of Silica in Citrate-Bicarbonate-Dithionite Extracts of Soils¹

ABSTRACT

A procedure was developed for the determination of Si directly in citrate-dithionite extracts of soils. The procedure, which provides for noninterference by citrate, dithionite, Fe, and P was developed for measurement of the absorbance of the reduced ß-molybdosilicic acid in the working range of 5 to 25 µg of Si in 50 ml. Interference by unoxidized dithionite was eliminated by bubbling air through the extract. The interference by citrate was prevented by an excess of molybdate; that by Fe and P by tartaric acid addition.

The citrate-dithionite treatment released, per gram of soil, 3 to 174 mg of Fe₂O₃, 0.8 to 42 mg of Al₂O₃, and 0.1 to 3.8 mg Si(OH)₄. The water-soluble silica ranged from 0 to 26 µg of Si(OH)₄ per ml of solution before removal of reductantsoluble Fe₂O₃ and Al₂O₃; it increased into the range of 5 to 58 after removal. At any given pH of Henry soil (in the pH range of 4 to 7), the water-soluble silica increased markedly as a result of previous citrate-bicarbonate-dithionite extraction. The release of Fe, Al, and Si that occurred with the various soils on reduction indicates a possible mechanism by which these constituents may increase sufficiently to support the well-known formation of montmorillonite from soluble and alluviated materials moved into positions of slow drainage. These constituents in solution would also favor preservation of any montmorillonite present in soil parent materials.

¹ Contribution from the Department of Soil Science, University of Wisconsin, Madison. Supported in part by National Science Foundation Grant GP4144 and GA1108-Jackson. Presented before Div. S-2, Soil Science Society of America, Washington, D.C., Nov. 9, 1967.

² Graduate Research Assistant, Project Associate, and Professor of Soil and Water Sciences, respectively.

Received for publication December 29, 1967. Accepted for publication April 15, 1968.