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.