Relationship between soil erosion and physical properties of soils at zero-order basins in Okinawa, Japan

Abstract

The erosion of "reddish soil" caused by development projects has damaged the ecosystems of coral reefs around the main island of Okinawa, in southwestern Japan. To clarify the critical condition of soil erosion on hilly lands in Okinawa, we investigated the relationship between microtopographical units of zero-order basins and physical properties of the soils. The microtopographic units were divided into six units consisting four soil groups: 1) crest flats (red soils and surface gleyed red and yellow soils), 2) crest slopes (yellow soils and surface gleyed red and yellow soils) 3) upper sideslopes (yellow soils and surface gleyed red and yellow soils), 4) head hollows (yellow soils), 5) lower sideslopes (immature soils) and, 6) bottomlands (immature soils). From the clay ratio, dispersion ratio, porosity and saturated hydraulic conductivity of the soils, we concluded that the red soils and surface gleyed red and yellow soils in the higher areas have high erodibility, while yellow soils have low erodibility. This conclusion agrees with the observation that the thickness of the A horizon of the former soils decreased at crest slopes and upper side slopes after forest clearing. Therefore, areas of surface gleyed red and yellow soils are especially vulnerable to erosion after forest clearing, but there is little erosion of red soil after forest clearing because it is distributed in mainly flat areas.