Friday, 14 July 2006

Properties of Red Oxisols on Basalt in Thailand.

Saowanuch Tawornpruek1, Irb Kheoruenromne1, Anchalee Suddhiprakarn2, and Robert J. Gilkes3. (1) Department of Soil Science, Faculty of Agriculture, Kasetsart University, Chatuchak, Bangkok 10900, Thailand, (2) Agriculture Faculty, Soil Science Department, Kasetsart University, Chatuchak, Bangkok, 10900, Thailand, (3) School of Earth and Geographical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, 6009, Australia

A pedogenic analysis of properties of red Oxisols on basalt under different climatic regimes in Thailand was carried out to ascertain their similarities and differences. These soils are Kandiustox in Northeast Plateau and Kandiudox in Southeast Coast Thailand having characteristics typical of highly weathered soils. They are very deep (> 2 m depth), generally acidic (pH 4.3-5.5), clayey, having low cation exchange capacity and negative ΔpH values. Their profile development is rather similar being Ap-Bto-Bo for both Kandiustox and Kandiudox. These soils generally have low bulk densities (0.79-1.25 Mg m-3) and low available water for plants (3.0-12.7 weight%). The fabric of the soils indicates that there are more well-rounded micropeds in Kandiustox than in Kandiudox and the sizes of microaggregates in these soils are somewhat different ranging from 20-1200 μm in Kandiustox and 40-2000 μm in Kandiudox. The iron oxide concentration in microaggregates is more variable in the Kandiudox possibly indicating a more complex pedogenesis in these soils as compared to that in the Kandiustox. Kaolin, quartz and hematite are the dominant minerals in all of these red Oxisols. Goethite is the minor mineral found only in the Kandiudox. Differences in mineralogy also indicate the influence of different climatic conditions. Soil properties that relate to soil fertility status particularly organic matter content (1.9-45.2 g kg-1), available phosphorus (0.5-98.6 mg kg-1) and available potassium (3-49 mg kg-1) are moderately low to high in the topsoil and low to very low in the sub-horizons caused by land use managements and those are similar for both the Kandiustox and Kandiudox profiles. Both groups of Oxisols have low values of cation exchange capacity which is inconsistent with the dominance of low activity clays (kaolin, sesquioxides) and the Kandiustox profiles have lower mean CEC (5.8-9.7 cmol kg-1) than the Kandiudox (6.4-18.1 cmol kg-1) profiles. However, it is clear that the exchange properties of these soils largely depend on their variable charges. This is indicated by ΔpH of which the Kandiustox having greater negative charges than those of the Kandiudox, reflecting more intensive weathering of the latter soils.

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