奄美諸島徳之島の表層灰白化黄色土

抄録

The soil studied occurs in Tokunoshima Island of humid subtropical Amami-Islands in Japan. It is a residual soil derived from granite rock (biotite), which occupies frequently nothern mountain foot in Tokunoshima Island. The soil is characterized by its bleached surface horizon and yellowish orange colored lower horizon. The soil profile feature is similar to that of the bleached Red yello soil (" Feichisha") or the Surface gleyed red and yellow soils in the Okinawa Island. This soil, however, does not have rusty mottlings in lower illuvial horizons. 1) Morphology The soil profile is described as follows ; L : 0+2 cm, A_1/A_2 : 0-6 cm, CoSL, a little humus, grayish brown (7.5 YR 6/2), granular structure (as a clod of white grayish sand particles). A_3 ; 6-21 cm, SCL, bright yellowish brown (IO YR 7/6), subangular blocky and granular structure, field hardness (readings of cone-penetrometer) 24. B_1t : 21-53 cm, bright brown (7. 5 YR 5/6), field hardness 28. B_2t/C : 53-130 cm, SL, mosaic of light gray (7.5 YR 8/2) and light yellow orange (7.5 YR 8/6), crack-like structure, field hardness 29. C : 130-150 cm, lower boundary is indistinct or diffuse, LCoS, the same color and structure as above, biff use boundary. 2) Physico-chemical properties pH (H_2O) value of the soil is just a little higher than other related yellow or yellowish brown colored soils. The values of exchangeable base and CEC are low throughout the profile, among which, however, the A_1/A_2 horizon has more or less higher exchangeable bases compared with the undering illuvial horizons, probably because of biological cycles. The content of free iron oxides of higher activity (Tamm's Feo) is rather high in the A_1/A_2 horizon compared with those in the lower ones, and the content of free iron oxides of lower activity (of more stability, Fed-Feo) is highest in the B_1t horizon. The NaOH-titration curve of the A_1/A_2 horizon shows nearly vertical curve indicating a weak buffer action, but that of the Bit horizon shows a lower angle curve indicating a strong buffer action (Fig. 4.). Clay content in the A_1/A_2 and C horizons is very low, and the Bxt horizon has the highest content. Fine clay content also shows the same vertical distribution pattern as total clay. In the distribution ration (volume) of three physical phases (solid, liquid and gas) the solid phase ratio is not very different in all horizons, though a little higher in the C horizon. In the A_3 horizon the liquid phase ratio is very small, but the gas phase ratio is large. On the contrary, but the gas phase ratio is large. On the contrary, the liquid phase in B_1t horizon is highest in all horizons, and gas phase ratio is small. According to thin-section observation, the B_1t horizon has a best oriented fabric (ommisepic ?) and B_2t/C horizon is also abundant in oriented clay. On the basis of the micromorphological observation and particle-size distribution against depth, it can be said that this soil has developed through illimerization (lessivage) processes. This soil similar to Yellow brown forest soil of warm temperate zone in that it has large amount of Al-vermiculites, which is absent in Red and Yellow soil of subtropical Ishigakishima Island. On the other hand, this soil resemble the subtropical Red or Yellow soils in that it lacks 14Å/10Å interstratified minerals, which is common in soils of warm temperate rigions. In conclusion, this soil has clay mineral compositon transional between yellow brown forest soils in warm temperate climate and Yellow or Red soils in humid subtropical climate. The surface bleached Yellow soils can be considered to belong to Yellow soil which occur widely under humid subtropical climate environment.