Significance of hydrous iron oxides in enhancing P sorption of soils with the addition of sodium hydrosulfite (Na₂S₂O₄)

Phosphorus sorption (P_sor) of soils is affected by redox conditions. It has been shown that P_sor of lowland soils at a pH value of about 4.3 increases when a small amount of sodium hydrosulfite (Na₂S₂O₄) is added and decreases when an excess amount of (Na₂S₂O₄) is added to the mixture of a soil and P solution. Hydrous Fe-Al oxides, manganese dioxide (MnO₂) exchangeable Ca, models of reactive components with P in soils, were examined to identify the factors responsible for the increase of P_sor in lowland soils when a small amount of Na₂S₂O₄ was added. For clarifying the contribution of the hydrous Fe-Al oxides, goethite and 7 hydrous Fe-Al oxides (Fe/Al atomic ratio: 1/0, 5/1, 2/1, 1/1, 1/2, 1/5, and 0/1) were used. The P_sor of all these materials increased when they were treated with a small amount of Na₂S₂O₄ although the increase was the smallest for the hydrous Al oxide among the 7 oxides. Thus, the hydrous Fe oxides, and Al oxide to a smaller extent, play an important role in the increase in P_sor of the lowland soils treated with a small amount of Na₂S₂O₄ The P_sor of the hydrous Fe oxides was not appreciably affected by the addition of MnO₂. The increase in P_sor of the Ca-saturated Hachirogata soil was almost the same as that of the Nasaturated Hachirogata soil, indicating that exchangeable Ca did not affect appreciably the increase of P_sor in reduced soils at a pH value of about 4.3.