Adsorption-desorption, persistence, and leaching behavior of dithiopyr in an alluvial soil of India

Investigations were undertaken to determine the adsorption-desorption, persistence and leaching of dithiopyr (S,S'-dimethyl 2-difluoromethyl-4-isobutyl-6-trifluoromethyl pyridine-3,5-dicarbothioate) in an alluvial soil under laboratory condition. The adsorption-desorption studies were carried out using batch equilibration technique. The mass balance studies showed that 83-97% of the pesticide was recovered during adsorption-desorption studies. The results revealed strong adsorption of dithiopyr in alluvial soil with K-d values ranging from 3.97-5.78 and Freundlich capacity factor (K-F) value of 2.41. The strong adsorption was evident from the:hysteresis effect observed during desorption. The hysteresis coefficients ranged from 0.17-0.40. The persistence studies were carried out at two concentrations (1.0 and 10.0 mug g(-1) level) under field capacity moisture and submerged condition by incubating the treated soil at 25 +/-1degreesC. In general, dithiopyr persisted beyond 90 days with half-life varying from 11.5-12.9 days under different conditions. The rate of application and, moisture regimes had no overall effect on the persistence. The leaching studies carried out in packed column under saturated flow condition revealed that dithiopyr was highly immobile in alluvial soil. Only small amounts (0.02-0.04%) were recovered from leachate whereas major portion (99.9%) remained in top layer of the soil column. The data suggest that strong adsorption of dithiopyr will cause a greater persistence problem in the soil. However, the chances of its movement to ground water will be negligible due to its immobility.