Minami-daito island, which is situated in a sub-tropical area, contains red-yellow soil. Recently, grayish white-colored soil was discovered around dead roots in the subsoil on the island. In general, grayish white-colored soil is formed via iron dissolution by a low molecular-weight aliphatic carboxylic acid solution (LACA) in a cold setting; however, there were few studies regarding the formation of white-colored soil in sub-tropical areas. The objective of this study is to elucidate the mechanism of grayish white-colored soil formation in the sub-tropical setting of Minami-daito island. Soil samples (horizons Ap1, Ap2, B1, and B2) were obtained from a pumpkin field on Minami-daito island. Grayish white-colored soil was discovered around dead roots in the B2 horizon, which was also collected. From X-ray analysis, the clay minerals were kaolin minerals, mica, vermiculite, vermiculite–chlorite integrate, and goethite in the A and B horizons; however, goethite was not present in the white-colored soil. This result indicates that the grayish white-colored soil was formed by the dissolution of goethite around dead roots. It is likely that LACA is necessary for the dissolution of goethite in soil. To prove this hypothesis, a leaching test was conducted. Oxalic acid, formic acid, and acetic acid in LACA and samples of the B2 soil horizon were used for the leaching test. Compared with H2O treatment, Al and Fe were dissolved using LACA treatment and oxalic acid solution treatment. From X-ray analysis, it was observed that the gibbsite and goethite peaks obtained using LACA treatment were smaller than that using H2O treatment. In particular, the goethite peak disappeared after oxalic acid treatment. Previous studies have demonstrated that oxalic acid has a high ability to dissolve goethite. It is concluded that the grayish white-colored soil on Minami-daito island was formed via the dissolution of goethite by oxalic acid from plant dead roots.
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