日本の赤色土壤の生成時期・生成環境にかんする二, 三の考察

抄録

Both stratigraphical and pedological exmaminations reveal all of surveyed reddish soils found in Northeast Japan being Relict Red Soils formed during past warmer periods, probably, interglacial stages or substages, in the Pleistocene. Reddish soils in West Japan, which have hitherto been regarded as “zonal soils” by several pedologists, also have been made under the similar condition to them.
Though the precise datings of their formations are not fully established, at least three warmer periods may be assigned to their datings, that is, pre-Shimosueyoshi, Shimosueyoshi and post-Shimosueyoshi stages, among which more cases examined indicate their datings to the Shimosueyoshi stage, possibly Riss/Würm interglacial. The upper parts of the Relict Red Soil profiles are subjected to modification toward other genetic soil types to be developed under recent bio-climatic environments.
Soil geographical extension of recent zonal soil patterns in the world, especially of those in the coastal region of the Chinese continent (compiled by Ma and Ven, 1958) toward the circumference of Japan suggests that typical zonal Red Soils find their home distribution in the Amami Islands, the southern end of Japanese territory, and the southern regions. Most parts of West Japan except along the Pacific coast seem to be characterized by zonal soils akin to the Yellow Brown Forest Soils in China, and East Japan, by ones similar to the Brown Forest Soils, unsaturated subtype proposed by Fridland (1953).
The genetical environments of the Relict Red Soils in Japan are probably not so far from those suggested above, that is, being represented by middle part of the subtropical humid climatic zone with mean annual temperature of 20°C or more, annual precipitation of 1, 500-3, 000mm., and annual cumulative temperature of more than 5, 000°C, on the one hand, and by evergreen broadleaved forests of Machilus Thunbergii, Distylium racemosum, Shiia Sieboldii, Cyclobalanopsis stenophylla etc., on the other hand.
Paleontological evidences so far obtained seem to offer no positive proof to this assumed environments of the Relict Red Soils, being too warmer than shown by the formers. It is reserved for further studies whether the apparent discrepancy is caused by ill preservation of appropriate fossil indicators or by over-evaluation of the mean annual temperature effect as a soil forming factor. The cumulative temperature and precipitation factors might be worthy of appreciation in the latter possibility, if at all.