熱帯の退化した黒色の土壌について : カンボディア大湖東北地域に分布する

抄録

Dark colored soil on basalt plateau in Kg. Thom, Cambodia has some resemblance with the tropical black and dark colored soils. On the plateau this soil occur at the middle and lower part, surrounding rhodic Latosol on the top of the same plateau (100 m). The soil used in this study is one developed at the depression on the plateau of about 40 m above sea level. The climate of this region is of tropical one with well defined rainy and dry seasons, and the precipitation is 1400 mm to 1500 mm. The soil has the following horizon sequence; Apg-A_<l2>g-BCg-C_1g-C_2g. Abundant pisolites (buck-shot concretions) are found characteristically in this soil. The Apg has a deep dark color, but the A_<l2>g slightly faded, and the BCg is stained by the abundant rusty mottles, while the C_1g and C_2g are gray. It is presumed that iron has translocated from Apg to BCg horizon under natural hydromorphism or rice cultivation. Due to severe weathering the contents of clay-size particles are generally high throughout horizons, being especially high in bottom horizons, and the contents of silt and fine sand are higher in upper horizons than in lower horizons. So-called "slickenside" was not observed in this soil probably because of the survey in rainy season. Pisolites abundant in this soil was identified to be well-crystallized Goethite. High crystallinity may be due to long time aging under the influence of the periodical appearence of surface and stagnant water (hydromorphic laterization). As compared with typical tropical black soil, this soil has relatively low CEC and base saturation and small amount of exchangeable Ca and Mg. They, however, increase gradually with depth, which suggests some degree of leaching of bases from upper to lower horizons. Clay mineral of this soil is mainly metahalloysite with minor amount of halloysite except for Apg horizon. In this soil was not detected montmorillonite which occur usually as dominant clay mineral species in tropical black and dark colored soils. Although this soil has nearly the same morphological feature as tropical black soil (Grumusol, Regur and Tirs etc.), it has some important deviations, as stated above, in base-status and in clay mineral composition from the typical ones. This would suggest the soil is at advanced stage of the degradation process. From the above results, this dark colored soil with pisolite is interpreted to be a degraded tropical black soil which is superimposed by characteristics caused by the present hydromorphism or paddy soil formation.