| Summary: | 博士 === 國立臺灣大學 === 農業化學研究所 === 93 === Ten podzolic soil pedons were selected to study the soil morphology, characteristics, material composition, pedogenesis, elements distribution, and classification in Tai-Ping mountain region of Taiwan. The podzolic soils in the study area can be divided into three classes including Inceptisols, soils with placic horizon, and Ultisols. The selected pedons had an elevation ranging from 1900 to 2100 m and were derived from slate. The vegetation types are dominated by red cypress (Chamaecyparis formosensis Matsum) and Taiwan Chinese fir (Taiwania cryptomerioides Hay). The annual rainfall is about 3000 mm and mostly falls from April to October. The soil moisture regime is udic and the soil temperature regime is mesic. The soil physical properties showed that the illuviation process of clay particle was the dominant pedogenic process which occurred in the study area before the formation of the placic horizon. The chemical properties showed that the Ultisols appeared to be dominated by the organic-Fe types but the placic horizons appeared to be dominated by free iron oxides. On the other hand, the placic horizons are composed of poor crystalline goethite, lepidocrocite, ferrihydrite, and some hematite. All soil pedons are dominated by vermiculite, interstratified vermiculite-illite minerals, and illite. The weathering sequence of clay minerals in the study area is proposed as: illite and chlorite → vermiculite and interstratified vermiculite-illite minerals → HIV and kaolinite → gibbsite.
Lateral eluviation-illuviation occurred between the soils with placic horizon and the Ultisols. This result showed that the relief is the important controling factor of the genesis variation between soils with placic horizon and Ultisols. The genesis of placic horizon in the study area is proposed to have four stages. First stage, the perched water was formed on the surface after heavy rainfall owing to the abrupt textureal change due to strong illuviation. Second stage, the lateral flow which was induced from the slightly sloping area caused the perch water table to fluctuate rapidly thereby ehancing the fast re-oxidation alteration environment. Third stage, the soluble iron was re-oxidized, precipitated, and coated on coarse particles to form the placic horizon at the boundary between the eluvial and illuvial horizons after the perched water was drained. Fourth stage, the organic matter was adsorbed by iron oxide as shown in the characteristic morphology of the upper part of the placic horizon which is black and red in its lower part. The high pH value (>5) of the study area may be one important factor that favored the placic horizon genesis but retarded podzolization. The soil sequence in the study area is as follows: Entisols→Inceptisols→Ultisols or Ultisols with placic horizons.
The results of soil classification can not show the properties of placic horizons in the study area. I propose that a Placudepts should be included in the Great Group of Udepts. Moreover, Spodic Hapludults and Placoaquic Hapludults should be included in the subgroup of Hapludults in Soil Taxonomy. On the other hand, Placic Lixisols and Placic Cambisols should also be included in the subdivisions of Lixisols and Cambisols in the World reference base for soil resources system (WRB system).
Based on the distribution of elements in the soil profiles, the elements can be divided into three classes, such as (1)elements concentrated in the topsoil: sodium, .potassium, magnesium, and manganese; (2)elements concentrated in the B horizon: Gallium and Chromium; and (3)element concentrated in relation to the placic horizon genesis: Indium. Because Indium (In) can be strongly adsorbed in the placic horizon during the formation process, it would be a good indicator for placic horizon genesis.
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