Six volcanogenus dark red soils were examined and compared with a paleoclimatical red soil and a burnt soil (reddened under a forest fire). Volcanogenus dark red soils contained iron oxides both inside and on the surface of their mineral particles, whereas in red and burnt soils, the mineral particles had iron oxides only on their surface as weathering products. Volcanogenus dark red soils contained better-crystalline hematite than those of red and burnt soils, and their XRD peaks were so sharp that they could be recognized without any concentrating treatment. Grinding test showed that the grain size of hematite was a possible factor in the darkness of red color. The observation of parent materials in thin sections showed that the dark-reddening process includes deuteric and thermal alterations. The features of volcanogenus dark red soils mentioned above should be helpful for classifying red- and dark red colored soils.
Sclerotium grain is the resting body of ectomycorrhizal fungi found in forest soils. A melanic-spherical shape in approximately 1-2mm diameter characterizes the external feature of the grain, and a hollow structure with honeycomb transverse wall appears inside the grain. In our previous studies, we reported a high aluminum concentration inside sclerotium grains and suggested the close relationship between the status of active aluminum and the distribution of these grains in Andosols (Watanabe et al., 2001; 2002). Here we examined the chemical properties focused on active aluminum and carbon in several nonallophanic Andosols under forest vegetation for further discussions on the regulating factor of the distribution of the sclerotium grains. In each studied soil profile, the mean weight of sclerotium grain (mg grain^<-1>) had a tendency to increase with the content of exchangeable aluminum, content of total organic carbon and nitrogen, carbon content of humic acid extracted from soils. The ratio of Alp to total organic C(Al_p/T-C) showed a strong negative correlation between the mean weight sclerotium grains regardless of profiles. The bonding ratio of carbon and aluminum in soils was assumed to be one of the factor influencing the development of sclerotium grains.