Characteristics and classification of Kuroboku soils distributed in the northern rim of the Aso caldera, southern Japan

Soils with non-allophanic Kuroboku characteristics have been found by previous surface (0-15 cm depth) soil surveys to be widely distributed on the Kuju Plateau and on the northern rim of the Aso caldera on the island of Kyushu, southern Japan. The soils from the Kuju Plateau have been analyzed and confirmed to be non-allophanic or allophanic of Kuroboku soils according to the Unified Soil Classification System of Japan (the Second Approximation; USCS2), but those from the northern rim of the Aso caldera had not yet been fully characterized. We therefore conducted full profile descriptions (from 0 to - 100 cm) of the soil at two sites in the northern rim of the Aso caldera, one in natural grassland and the other in pasture, and then determined the physical and chemical properties of all horizons for classification of these soils under the USCS2. In USCS2, a diagnostic feature of a non-allophanic Kuroboku soil is the contiguous thickness of soil in which the Alsub(p)/Alsub(o) ratio is >=0.5 (Alsub(p): sodium pyrophosphate-extractable aluminium [Al]; Alsub(o): acid ammonium oxalate-extractable Al). If such soil layer is thicker than 25 cm, the soil is classified as non-allophanic Kuroboku soil. At the natural grassland site, the Alsub(p)/Alsub(o) ratio of the A horizon was 0.55, but the thickness of the horizon was only 24 cm; therefore, this Kuroboku soil was classified as allophanic. At the pasture site, the Alsub(p)/Alsub(o) ratios of the Ap (0-15 cm depth) and A (15-27 cm depth) horizons were 0.48 and 0.41, respectively, i.e., < 0.5. This Kuroboku soil was also classified as allophanic. Although the characteristics of the Kuroboku soils from the two sites in the northern rim of the Aso caldera were close to non-allophanic, both soils were classified as allophanic Kuroboku soil according to the prevailing taxonomy.