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This article in SSSAJ

  1. Vol. 52 No. 6, p. 1585-1589
    Received: June 2, 1987

    * Corresponding author(s):
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Phosphorus Adsorption by Kaolinite and Montmorillonite: II. Organic Anion Competition

  1. U. Kafkafi,
  2. B. Bar-Yosef ,
  3. Rivka Rosenberg and
  4. G. Sposito
  1. Dep. of Field Crops, Faculty of Agriculture, P.O. Box 12, Rehovot, Israel
    Inst. of Soils and Water, Agricultural Res. Organization, The Volcani Ctr., Bet Dagan 50250, Israel
    Dep. of Soils and Environmental Sciences, Univ. of California, Riverside, CA 92521



The mechanisms by which roots affect the release of phosphate from soil surfaces into the solution is not clear. This study was undertaken to investigate the competition of several agents, reported as root exudates, with P on adsorption sites on kaolinite and montmorillonite. The agents studied were acetate, bicarbonate, citrate and oxalate, and the amino acids phenylalanine and alpha-amino isobutyric acid. The adsorption or desorption of P on the clays in the presence of the noted agents at various fixed pHs and known ionic strength, and the effect of slurry density on P adsorption, were determined. Citrate, bicarbonate and oxalate decreased, and acetate and the amino acids increased P adsorption to the clays. A decrease in the solution-to-clay ratio from 100 to 10 L/kg resulted in enhanced P adsorption. The unifying principle that explains these findings is that P adsorption on a clay depends on the extent of the depression of the negative potential of the clay platelets. Bicarbonate and citrate anions at pH > 8 compete with P for active Al sites on the clay. The increased P adsorption in the presence of amino acids is suggested to stem from the fact that H-bonding of the amino groups causes a decrease in interlamellar spacing, and produces quasicrystals. The quasicrystals reduce the spillover of negative charge from the surface to the edges and allow closer approach of the phosphate ion to the Al site.

Contribution from the Agricultural Res. Organization, no. 1969-E 1987 Series.

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