So-called "Sandy alluvial soils" are widespread in Cambodia, particularly on old alluvial plain (about 50m above sea level) surrounding Tonle Sap (Great Lake). One of these soils was studied. Parent materials are fluvial deposits derived from Mesozoic sandstone. The climate of this region is of tropical one with well defined rainy and dry seasons, and the precipitation of 1292 mm/year. The profile is stained by rusty mottles from the top to the bottom. It also contains pisolites ("buck-shot" concretions) throughout horizons, which are identified to be well crystallized goethite. Most characteristic feature of this soil is the occurence of compact, yellow brown colored "plinthite" at the lowest part of the profile. Textures are extremely sandy, while the clay particles increase regularly with increasing depth. Humus content is very low even in surface soil. Due to the very sandy texture, exchange capacities are extremely low, exchangeable Ca and Mg are extraordinary small in amount, and the reactions are strongly acid. Primary minerals are almost exclusively quartz particles, and the secondary minerals are predominated by well crystallized kaolinite group with some 2:1 type minerals. This soil obviously has the characters of ground water lateritic soils, of which the forming process, however, ceased to proceed after the elevation of land surface. Vertical changes in soil properties suggest that the present formation is inclined toward red-yellow podsolic soils. Therefore this soil is considered to be a polygenetic soil, in which the characters of red-yellow podsolic soils are superimposed on those of the past ground water lateritic soils.
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