In 1998 it was revealed that the application of chlorate was able to induce flowering in longan trees. Since then potassium and sodium chlorates have been used in most longan plantations all over Thailand and some neighboring countries. The maximum rate in the early years of application was approximately 500 g for a medium size tree. An environmental impact study on the effects of chlorate application was carried out in the year 2000. It was found that if farmers apply once a year chlorate residue is unlikely to accumulate over the years and the long-term effect on soil environment is nil. However, due to reasons being investigated, the application rate of the substance has gradually been increased. In 2003 the maximum application for a medium size tree was approximately 2,000 g which is beyond the scope of the above mentioned study. Consequently, there has been concern on the accumulation and leaching of residues through soil profiles and the risk of ground water contamination in the long run. Thus, a re-study on the environmental impact has been done. Movement and degradation of the heavily applied chlorate in four control plots on four representative soils were monitored over a period of two years. Samples of these four profiles taken at various depths, ranging from 0 to 200 cm, have been incubated with chlorate and chlorate degradation has been monitored under laboratory conditions. Additionally, residual chlorate on 42 commercial longan plantations was monitored. The results suggest that organic matter content is the most important factor influencing chlorate degradation. The degradation was observed to be very slow in soils with less than 0.2% of organic matter, regardless of their textures. It was also found that chlorate leached relatively fast in well drained sandy soils. Therefore, two years after application significant concentrations of the residues were found at a depth of 150 - 230 cm in sandy soils of the control plots. Equal concentrations were found at the same depth under the tree canopies in the commercial orchards with these kinds of soil. In loamy and clayey soils significant chlorate residues were found at a depth of less than 100 cm. Consequently, methods to control the application rate have to be investigated. Furthermore, measures to eliminate chlorate residues after flowering of longan trees should be imposed in order to eliminate long term impacts and leaching of residues into the ground water in the long run. Application of any form of sugars was proven to be able to enhance the degradation of chlorate residues. Details on the correlation between chlorate degradation and various soil properties, the movement of residual chlorate within different soil types in both the control plots and farmers' plantations were presented. Keywords: chlorate, leaching , sub-soils, longan.