Abstract
Some Hachirougata reclaimed soils exhibit high phosphate sorption coefficients comparable to Andisols though the former soils contain much less active Al and Fe than the latter soils. It was revealed that exchangeable Ca^<2+> and Mg^<2+> of these soils play a significant role in the measurement of phosphate sorption coefficients in addition to active Al and Fe. With the addition of ammonium phosphate, exchangeable ions are released and Ca^<2+> and Mg^<2+> react with phosphate to give precipitates of CaHPO_4・2H_2O and MgNH_4PO_4・6H_2O, respectively. These crystalline phosphates were identified with X-ray diffraction analysis of precipitates formed by the addition of ammonium phosphate solution to soil extract with NH_4Cl solution. These phosphates contain P and Ca^<2+> or Mg^<2+> in the molar ratio of 1:1. The 1:1 relationship was found between the amounts of exchangeable Ca^<2+> + Mg^<2+> and the difference in phosphate sorption coefficients between original soils and Na^+-saturated soils. This fact quantitatively confirms the contribution of exchangeable Ca^<2+> + Mg^<2+> to phosphate sorption coefficients of Hachirougata reclaimed soils. Phosphate availability in the Hachirougata reclaimed soil to plants was higher than that in an Andisol.
