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Phosphate Fixation by Montmorillonite¹

ABSTRACT

The factors involved in fixation of phosphorus, added as Ca(H₂PO₄)₂ · H₂O, by montmorillonite clay were investigated. The influence of the free iron and aluminum oxides, which accompany this clay, on the fixation was determined by measuring fixation before and after removal of these oxides. It was found that the free iron and aluminum oxides accounted for most of the fixation against weak acid extraction. No appreciable fixation against water extraction occurred when the free iron and aluminum oxides were removed and the clay was H-saturated.

The influence of saturation of the clay with the exchangeable cations usually found in soils on phosphorus fixation was investigated. Ca-saturation clay, treated to remove free iron and aluminum oxides, fixed large amounts of phosphorus against water extraction. However, this phosphorus was recovered by weak acid extraction. The evidence obtained in this investigation indicates that the phosphorus fixed by Ca-saturated clay is fixed as calcium phosphate complexes, and not by a H₂PO₄-Ca-clay bonding as has been proposed. Clays which were saturated with Na, K or Mg, fixed only small amounts of phosphorus against water extraction.

¹ Joint contribution. Contribution 510, Department of Agronomy, Kansas Agr. Exp. Sta., Manhattan, and Wisconsin Agr. Exp. Sta., Madison. Part of a thesis submitted to the faculty of the University of Wisconsin by the senior author in partial fulfillment for the Ph.D. degree. Presented before Div. II, Soil Science Society of America, St. Paul, Minn., Nov. 9, 1954.

² Formerly Graduate Fellow, University of Wisconsin, now Assistant Professor of Agronomy, Kansas State College; and Professor Emeritus of Soils, University of Wisconsin, respectively.

Received for publication December 6, 1954.