Microstructure and water retention of Oxisols in Thailand

The moisture characteristics and related pore size distributions of Thai Oxisols were determined for 11 representative pedons. These soils are Kandiustox and Kandiudox located in the North-east Plateau, South-east Coast, and Peninsular Thailand. They are generally acidic (pH 5-6), clayey, have low cation exchange capacity (5-15 cmol(c)/kg) and have low bulk densities (0.77-1.36 Mg/m(3)). The microstructures of these soils are mostly granular with compound packing 10-1500 mu m voids between aggregates. Water retention curves for all soils are similar with amounts of water retained at 33 kPa (field capacity) ranging from 22 to 43 (% weight) and 1500 kPa (permanent wilting point) ranging from 17 to 34%. Water available to plants is 3-12%. There is a strongly bimodal pore size distribution with the modal macropore and micropore diameters calculated from the soil moisture retention characteristic curves being about 100 mu m and 0.02 mu m. The pore size distribution determined by N-2-BET method indicates a modal micropore of about 0.01 mu m. The total volume of pores >5 mu m estimated by image analysis of scanning electron micrographs ranges from 20 to 50% with most of the variation in porosity being due to large differences in macropore (>75 mu m) volume.