Laboratory studies utilising radio-isotopic techniques were conducted to determine the adsorption, desorption and mobility of endosulfan and methamidophos in the sandy-loams of the Cameron Highlands and the clays of the Muda rice-growing areas. High Freundlich adsorption distribution coefficients [K ads(f)] for endosulfan (6.74 and 18.73) and low values for methamidophos (0.40 and 0.98) were obtained in the sandy-loams and clays respectively. The observed K_(oc) values for endosulfan were 351 (sandy-loam) and 1142 (clay), while K_(oc) values of 20.8 (sandy-loam) and 59.7 (clay) were obtained for methamidophos. Log K_(ow) of 0.40 and 1.25 were calculated for endosulfan and -1.96 and -1.21 for methamidophos in the sandy-loam and clay, respectively. Desorption was common to both pesticides, but the desorption capacity of methamidophos from each soil-type far exceeded that of endosulfan. Soil TLC and column studies showed that in both soils methamidophos was very mobile, while endosulfan had negligible mobility. The adsorption of both pesticides was not influenced by pesticide mixtures, but it was affected by temperature, pH and fertiliser (NPK).
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