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DIVISION S-2-SOIL CHEMISTRY

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

^a Saskatchewan Centre for Soil Research, Dep. of Soil Science, Univ. of Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N 5A8, Canada

mermut{at}sask.usask.ca

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.

Abbreviations: BET, Brunauer-Emmett-Teller • EC, electrical conductivity • Mont, montmorillonite • Pal, palygorskite • Sep, sepiolite

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