Genesis and classification of Red soils derived from marl in Miyako Island were studied based on their physico-chemical and mineralogical properties. The results are as follows: 1) In Miyako Island, the Red soil derived from marl had A-Bt1-Bt2-Bt3-C1g-C2 horizon sequence. The argillic horizon (Bt1-Bt2-Bt3) is blight brown to red brown (7.5YR5/6 or 5YR4/8) in color, and has strongly subangular blocky structure. Soil reaction of Bt horizons is strongly acid (pH4.7-4.8) and the degree of base saturation is lower than 35%. From these characteristics, this red soil is classified as Haplic Acrisol (WRB) or Typic Kanhapludult (Soil Taxnomy). 2) The activity ratio of free iron oxides(Fe_o/Fe_d) ranges from 0.03 to 0.16, and the crystallinity ratio of free iron oxides[(Fe_dFe_o)/Fe_t] ranges from 0.28 to 0.70. The values of (Fe_d-Fe_o)/Fe_t of A and Bt horizon are higher than 0.5, therefore this soil is considered to belong to Red-Yellow soils. 3) Clay mineral composition of A and Bt horizon mainly consists of hydroxyl interlayered vermiculite (HIV), kaolin minerals, goethite and gibbsite, while that of C horizon consists of smectite and mica minerals. The existence of Red-Yellow soils (Haplic Acrisols) derived from marl in Miyako Island may lead to the following conclusion: the soil evolution on marl in Ryukyu Islands proceeds from immature soils (Calcaric Regosols) through Brown Forest soils (Calcaric and Orthieutric Cambisols) to Red-Yellow soils (Haplic Acrisols), and suggesting a part of Miyako Island had already emerged from under sea level before the last interglacial period.
