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 or 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 Na-saturated 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.