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The fertility of soils with respect to K+ and NH4+ is often difficult to measure, but is essential for achieving effective agronomic practices. This problem is especially important in the Vertisols of Thailand. The purpose of the study reported here was to characterize the composition and layer-charge behavior of Thai Vertisols in order to understand better their K+ and NH4+ fertility. Topsoil and subsoil clay of 12 representative highly smectitic Vertisols from Thailand were studied. Interlayer swelling of smectite with alkylammonium cations, cation exchange capacity (CEC), and chemical composition were determined. These data and the re-expansion on glycerol treatment of Li-saturated, heated smectite demonstrated that high-charge, Fe-rich beidellite is the major clay mineral in these soils. The beidellite has a heterogeneous layer-charge distribution as measured by the alkylammonium method, with mean values ranging from 0.43 to 0.62 charges per half unit cell. The layer charge determined by CEC and structural formula methods for some almost pure smectites is smaller (0.32–0.54) and greater (0.46–0.69), respectively, than determined by the alkylammonium method. The structural formulae of beidellite confirm that the layer charge originates mostly in the tetrahedral sheet.

Effective soil erosion prediction models and proper conservation practices are important tools to mitigate soil erosion in hillside agricultural areas. The Water Nutrient and Light Capture in Agroforestry Systems (WaNuLCAS) and Water Erosion Prediction Project (WEPP) models are capable tools in soil erosion simulation in the conventional and conservation cropping systems in hillslopes. We calibrated both the models in maize monocropping and simultaneously validated them in maize-chili intercropping with Leucaena hedgerow for nine rainfall events in 2010, with the aim to evaluate their performances in runoff and sediment prediction on a skeleton soil in a hillslope, Western Thailand. The results showed that the calibrated WaNuLCAS model poorly predicts runoff prediction in the validation. In contrast, the calibrated WEPP model had a better performance in runoff prediction in the validation. For sediment prediction, the calibrated WaNuLCAS model predicted sediment yield better than the calibrated WEPP model in the validation because the WEPP model shows more variability of the sediment yield in the calibration (5.84 kg m –2 ) than the WaNuLCAS (5.18 kg m –2 ). Thus, the WEPP model was more suitable for runoff prediction than sediment prediction in the monocropping system, whereas the WaNuLCAS model was better suited for sediment yield prediction than runoff prediction, especially in complex intercropping systems.