Effect of reducing conditions on P sorption of soils

The phosphate sorption (P_sor) capacity of soils increased when the soils were reduced (Willet and Higgins, Aust. J. Soil Res., 16, 319–326, 1978). The present study aimed at the elucidation of this mechanism using Na₂S₂O₄ and 5 different soils. The P_sor of the 5 soils increased with the addition of a small amount of Na₂S₂O₄. Fe(II) was released from the soils with the addition of the same small amount of Na₂S₂O₄. Furthermore, when the amount of FeCl₂ corresponding to the amount of Fe(II) released along with the small amount of Na₂S₂O₄ was added, the P_sor of the soil increased. However, the P_sor of the lowland soils, of which the hydrous Fe oxide content was lower than the others, decreased when the amount of Na₂S₂O₄ addition was increased up to 150–200 g kg⁻¹. Based on these results, the following process is inferred for the increase in the P_sor of the soils when they are reduced. Hydrous Fe oxide in soil takes the form of very fine, high-density particles and reacts with P mainly on their surface. When a small amount of Na₂S₂O₄ is added, the hydrous Fe oxide is partially reduced, dissolved and finally re-precipitates with P by oxidation with O₂ from the air during the experiment.