火成岩の風化に關する研究

(第17報)鳥取地方の火山性土壌について (1)

抄録

(1) The volcanogenous soils derived from quartz biotite hornbrende andestic ejecta, which accumulated on the pyroxene andesite soils, basalt soils, soils from tertiary formation, and soils from diluvial series, are widely distributed in Tottori-district. (2) The author studied on the volcanogenous soils at Tomari-mura, Tohaku-gun, and at Kando, Tottori-shi, Tottori Prefecture. The soil profiles studied consisted of yellow or brownish-yellow lower horizon (which the author conveniently calls “yellow horizon”) derived from lapilli, brown B horizon derived either from volcanic sand or from volcanic ash, and dark coloured A horizon derived from volcanic ash. Under the yellow horizon there is reddish-brown pyroxene andesite soil. The humus content in the B horizons was about 2%, and that in the A horizons was about 11%. The B horizons are clayey, and the A horizons are clay-loamy. (3) From the amount of silica and alumina which were dissolved by 10% NaC0₃ solution, it was recognized that the free alumina contents in the soils were 2_??_4% and that there was no horizon markedly enriched in free alumina. (4) In extracting silica and alumina from the volcanogenous soil with 5% KOH solution, the ratio SiO/Al₂O₃ becomes constant when the quantity of this solution is larger than 120cc per g of the sample. 5% KOH solution dissolved the free alumina, com-plex A₁ (allophane-like substance) almost completely, and complex A₂ (halloysitic substance) partly. The volcanogenous soil contains a large amount of silica and alumina, which are soluble in 5% KOH solution, and the ratio SiO₂/Al₂O₃ in this case is very low (0.88_??_1.14). This is a marked characteristics of this soil.