Red soils and Yellow soils in Okinawa Island have the problem of severe hardening in dry conditions. It is important for the improvement and management of these soils to make the factors and mechanisms that affect the soil hardening clear. For this purpose, the properties and the degree of hardening of 43 topsoil samples in upland fields in the central area of Okinawa Island were determined. The results showed that the clay content and pH values were positively related to the degree of hardening. It is well known that clayey soils become very hard by air-drying because of the remarkable shrinkage, however, the pH-dependency of soil hardening has not been reported. Some investigations on the cause of the pH-dependency suggested that some physico-chemical conditions such as charge on the surface of soil particles in high-pH conditions, and the remarkable shrinkage of alkaline soils due to a large amount of calcium ions, enhance the hardening of these soils. The pH-dependency of hardening was also observed in the Red soils and Yellow soils in Kyushu Island, and this phenomenon probably occurs in soils with properties similar to the Red soils and Yellow soils in Okinawa, such as some Ultisols, Alfisols or Inceptisols in tropical, subtropical or temperate regions in the world.