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Soil Science Society of America Journal Abstract - DIVISION S-2-SOIL CHEMISTRY

Magnesium- and Silicon-Induced Phosphate Desorption in Smectite–, Palygorskite–, and Sepiolite–Calcite Systems


This article in SSSAJ

  1. Vol. 63 No. 5, p. 1167-1173
    Received: May 4, 1998

    * Corresponding author(s): mermut@sask.usask.ca
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  1. H. Shariatmadaria and
  2. A. R. Mermut *a
  1.  aSaskatchewan Centre for Soil Research, Dep. of Soil Science, Univ. of Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N 5A8, Canada


Carbonates are one of the most important group of minerals that fix phosphates in soils. Magnesium and silicon are known to reduce the P fixation of calcite. The effect of Mg and Si addition on P sorption–desorption of a mixture of 90% clay and 10% calcite was studied. The mixtures were montmorillonite–calcite (Mont–CaCO3), palygorskite–calcite (Pal–CaCO3), and sepiolite–calcite (Sep–CaCO3). Concentrations of 0.0, 5.0, and 10.0 mg L−1 of dissolved Mg or Si were introduced into P sorbing systems. The chemical speciation of P sorption solutions was predicted using the SOILCHEM computer program. Desorption of sorbed P on different samples was studied in successive extractions with 0.01 M KCl. The P sorption of Mont–CaCO3, Pal–CaCO3, and CaCO3 were similar (≈10 cmol kg−1). Addition of 10.0 mg L−1 Mg or Si sharply reduced the P sorption of calcite to <1 cmol kg−1, while it induced only a 20% reduction in P sorption of the mixtures. Magnesium and silicon treatments increased the desorption of sorbed P. The Sep–CaCO3 mixture showed a P sorption 60% lower than that of pure CaCO3 The desorption of sorbed P by Sep–CaCO3 was far above that of other clay–CaCO3 combinations. Addition of Mg and Si inhibited the formation of Ca–phosphates, which would otherwise make major contributions to P retention in all samples used in this study. Phosphate sorbed by silicate clays was the second major fraction of retained P, limiting the effects of Mg and Si treatments. As a slow release source of Mg and Si, sepiolite increased the desorption of P sorbed by CaCO3 This has an important implication for P availability in arid soils containing sepiolite. This mineral may be mixed thoroughly with commercial P fertilizers before P application into soil.

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