Abstract
Soil core samples from upland maize field at Mae Ramah district, Tak province were analysed for thorium isotopes using the modified method. The measures Th-232, Th-230 and Th-228 revealed that the rock parent material is the dominant source at these sites while fertilizer is an important supplement. Migration behaviour of Thorium isotopes varying with seasonal and human activities during cultivation period such as ploughing, harrowing, and applying fertilizer resulted in different distribution of Thorium isotopes in soils. It was observed the descending and ascending of Th-232, Th-230 and Th-228 concentrations in maize soils along the 250 meters line transect sampling. The highest Th-232 found in the upland maize soils was 11.76 Bq/kg or 5344.56 Bq/m2 while the lowest was 1.88 Bq/kg or 289.40 Bq/m2. Erosion and deposition of topsoil in the site should be the most possible reason for the significantly lower thorium isotopes values and vice versa along downhill soils. These results demonstrated that Thorium isotopes could be applied as an ideal substitute of Cs-137 for soil erosion study in the future. Further study is to collect additional line transect sampling including increase with the distance.
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