Magnesium- and silicon-induced phosphate desorption in smectite-, palygorskite-, and sepiolite-calcite systems

Carbonates are one of the most important group of minerals that fix phospha tes 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 mix ture of 90% clay and 10% calcite was studied. The mixtures mere montmorillo nite-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 dissolv ed 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 extr actions with 0.01 M KCl. The P sorption of Mont-CaCO3, Pal-CaCO3, and CaCO3 were similar (approximate to 10 cmol kg(-1)). Addition of 10.0 mg L-1 Mg o r 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 mixt ure showed a P sorption 60% lower than that of pure CaCO3. The desorption o f 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 wou ld otherwise make major contributions to P retention in all samples used in this study. Phosphate sorbed by silicate clays was the second major fracti on of retained P, limiting the effects of Mg and Si treatments. As a slow r elease source of Mg and Si, sepiolite increased the desorption of P sorbed by CaCO3. This has an important implication for P availability in arid soil s containing sepiolite, This mineral maybe mixed thoroughly with commercial P fertilizers before P application into soil.