土壌およびAl-スメクタイトの交換性Alとリン酸イオンの反応

抄録

There are known contradictory reports with regard to the reactivity of phosphate with exchangeable Al. When 10 mgP/l of KH₂PO₄ was added to soils, no significant correlation was obtained between the exchangeable Al contents and the decrease in phosphate sorption due to removal of exchangeable Al. On the other hand, almost one to one relationship was obtained between exchangeable Al contents and the decrease in phosphate sorption due to removal of exchangeable Al when 0.189 M of ammonium phosphate of pH7.0 was used. The objective of this study is therefore to elucidate the reactivity of Na₂HPO₄ and NaH₂PO₄ with exchangeable Al in soils.
Using 2 soils with different clay mineral composition and an Al³⁺-saturated smectite, the following conclusions were obtained.(1) Na₂HPO₄ selectively reacted with the exchangeable Al under the coexistence of active Al and Fe such as Al-humus and hydrous Fe oxides.(2) Low concentration of NaH₂PO₄ (i.e. 5mM or less) hardly reacted with exchangeable Al and mostly sorbed by active Al and Fe.(3) A decrease in the exchangeable Al of Al³⁺-saturated smectite was much smaller than those for 2 soils, appearing that the exchangeable Al scarcely reacted with NaH₂PO₄. On the other hand, the decrease in H₂PO₄-sorption after removal of Al³⁺ indicated that some of the exchangeable Al reacted with NaH₂PO₄. These results were possibly due to the formation of ion pair like Al H₂PO₄²⁺in the interlayer of smectite.
Thus, the above-mentioned contradictory reports can be explained by the fact that the reactivity of exchangeable Al with phosphate is strongly dependent on ionic species of phosphate.