Two soil toposequences in Northeast Thailand under tropical savanna climate were studied with emphasis on the spatial variation of soil development on landscape as affected by pedogenesis processes. Soil profiles were sampled at six positions; summit, shoulder, upper midslope, lower midslope, footslope and toeslope for the Nam Phong toposequence developed on sandstone in the upper part and shale in the lower part. Four profiles developed on residuum and colluvium derived from basalt on crest, backslope, footslope and valley floor were sampled on the Khon Buri toposequence. Field morphological, physical, chemical and mineralogical analyses according to standard method were employed. The result of the study revealed that there is an Ultisol-Alfisol-Entisol and an Oxisol-Ultisol-Vertisol sequence for Nam Phong and Khon Buri toposequence, respectively. All soils are deep but the C-horizon appears in deep horizons of the soils on midslopes of both landscapes which is probably due to specific erosion at each position. Their colors range from reddish on the uplands to grayish with some redoximorphic features at the lowest position of the landscape, which indicates differences in drainage condition. The spatial distributions of clay minerals are systematic as kaolin is the major clay mineral of the upslope soils while 2:1 clay minerals dominate clay fraction of the soils on the lowest position. A presence of hematite and goethite in the upslope soils reflects intense weathering and their occurrence is controlled mainly by redox conditions and pedoclimate. Only little amount of goethite is present in soils at low position where poorly drainage prevail, reflecting a seasonally reduced and iron oxide-poor environment. Soils on the Nam Phong toposequence have texture ranging from loamy sand to silty clay whereas soils on the Khon Buri toposequence are clayey as influenced by their parent materials. However, there is an increase of fine materials downslope. Except for their texture, the physico-chemical properties of upslope soils within each toposequence are not distinctly different. Dominant characteristics of upslope soils are acidic condition with low exchangeable bases corresponding to low cation exchange capacity. However, these upslope soils and those on lowest positions of toposequences are significantly different for most of their properties. Soils on the lowest positions are alkaline, having high exchangeable bases and high cation exchange capacity. Leaching and eluviation-illuviation are considered the main pedogenic processes in the soils on the higher positions whereas cumulization is the major pedogenic process in the lowest position of the catenary sequence. This study clearly shows the effects of relief on soil development in both of soil toposequences. The relief provides variation of environmental conditions occurring on landscape which related to soil properties change. The soils on higher positions with well-drainage show highest development as controlled mainly by leaching and eluviation-illuviation processes. The soils on the lower positions where poorly drainage prevails, are less developed as influenced by water activity. The soils on the midslope of the landscape with steeper slopes have intermediate characteristics as controlled mainly by erosion. In addition, there is an oxidation-reduction environment related to soils drainage condition influencing type of iron oxide mineral and soil color. However, considering the soils on Nam Phong toposequence which derived from different parent materials, it is clear that the parent material has more effect on soil development and their properties particularly on the type and amount of their clay content.
Keywords: topography, pedogenesis, tropical soils