フィジー国,ビチレブ島の熱帯多雨林下の土壌

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The Fiji Group consists of over five hundred islands scattered in the South-west Pacific ocean. The Viti Levu island is the main among them, and the area is about 10,400 square kilometers, correspond to 57 percent of the total land area. In the center of Viti Levu island, backbone range of the 1,000 meters class in altitude arranges in the direction from south to north. And, tropical easterly wind (trade wind) dominates in whole year in Fiji. Therefore, the wet zone has appeared in the windward coast (east side), and the dry zone has appeared in the leeward coast (west side) of this island (Fig. 1). The tropical rainforest has usually distributed in the wet zone. The survey was carried on the Nukurua area which is situated in the south-eastern part of Viti Levu island. The area is formed of hilly and rolling lands with 100 to 150 meters above sea-level, consisting of the sediments in the Pliocene to Pleistocene. And, the humic latosols derived from basic sediments have widely distributed in this area. In order to establish the criteria for selecting the proper sites with the proper species originating the soil survey, the humic latosols were subclassified into four soil types, i. e. A-, B-, C- and D-types (Table 1, Fig. 2). The standard for the subclassification was put on the red horizon of over 2.SYR4/8 or 10R4/8, which is regarded as a diagnostic horizon. That is, the following aspects were considered ; whether this horizon exists, and its depth if it exists, the extent of fading of color, the influence of oxidation-reduction, etc. Judging from the experimental results, the soil acidities of surface horizons have genrally weakened with the moving from A-type to D-type. So soil fertility will be expected in order of A<B<C<D (Fig. 4, Table 2). The tendency like this has been also proved by the forest productivity survey. A typical stratigraphic sequence of the humic latosol in the Nukurua area is shown as in Fig. 5. From this survey, the humic latosol is derived from the weathered deposits which are accumulated unconformably on the mudstone of the Tertiary, and that is formed of three fundamental layers, i. e. the degraded layer due to forest humus, red latosolized layer and mottling layer caused by oxidation-reduction process. And, considering from the soil survey, the writer presumed that the oxidation-reduction layer sometimes has changed into the gley layer in depressions, and sometimes has developed into the lateritic soft plinthite layer in ridge of montane land. Since the humic latosols in Fiji are generally heavy clayey and compact, and high in credibility, the soil erosion and the landslide in forest lands will be easily accelerated for the rainy season. So, in Fiji, it is very important to maintain the soil fertility, and to conserve the forest lands from the soil erosion.
1982-06-30

著者

山谷孝一
日本林業技術協会

フィジー国,ビチレブ島の熱帯多雨林下の土壌

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