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Thai Oxisols in land subjected to various use with two types of soil moisture regimes were analysed for some chemical properties such as pH, cation exchange capacity (CEC), carbon concentration, and content of Fe, Al, and Mn oxides in crystalline, noncrystalline, and organic form. We also determined clay content, bulk density (BD), aggregate size distribution, mean weight diameter (MWD), water stable aggregate, and strength in large (LMA; > 2000 µm) and small macroaggregates (SMA; 250-2000 µm). Type and amount of minerals including kaolin crystal size were also evaluated. Soil samples of surface (0-5 cm) and subsurface (5-20 cm) layers were collected from the perennial crops: durian (Durio zibethinus) and rubber (Hevea brasiliensis) plantation; and annual crops: maize (Zea mays), sugarcane (Saccharum officinarum), and cassava (Manihot esculenta) plantation. Two sites in each land use were chosen in this study. Clay content did not differ significantly for all soils. BD was significantly higher for annual crops. Mean pH ranged from 3.95-5.48 and 3.99-5.49, and mean CEC ranged from 8.1-13.4 cmol/kg and 3.9-7.1 cmol/kg for surface and subsurface, respectively. Carbon concentration was 2.37-4.09 and 1.15-1.36% for surface and subsurface, respectively. The values were lower for annual crops than for perennial crops for both surface and subsurface layers. All forms and contents of Fe, Al, and Mn oxides were significantly higher for perennial crops than for annual crops except for content of crystalline Fe which was not statistically different. These oxide contents correlated positively to MWD, carbon concentration, and LMA strength with r > 0.70. The perennial crop soils had higher average MWD (0.92-1.31 nm), amount of LMA (17.0-41.2%), LMA strength (5.8-9.0 N), and SMA strength (2.2-3.2 N) than those of the annual crop soils. Microaggregates (28.2-39.2%) were more abundant for annual crops. Amount of SMA (41.3-49.1%) did not differ significantly between land uses. The amount of LMA and MWD depended largely on the concentration of carbon with r = 0.96 and 0.94, respectively. Kaolin crystal size was significantly smaller for perennial crops (82.0-114.5 nm) than for annual crops (140.0-163.0 nm). Kaolin crystal size of both types of land use were negatively correlated to aggregation, carbon, and oxides content with r < 0.70.

Effects of land use on some physical, chemical, and mineralogical characteristics of Thai Oxisols