Sorption of Pyrazosulfuron-Ethyl by Soils of Different Agro-Climatic Zones of Southern Karnataka

Sorption studies were conducted with pyrazosulfuron-ethyl using batch equilibration technique and HPLC analysis. Four different paddy growing soils of Kathalagere, Mandya, Mudigere and Mangalore soil of Southern transitional zone, Southern dry zone, Hilly zone and Coastal zones of Southern Karnataka were used in the study with a pH of 7.3, 6.9, 6.1 and 5.8 and organic matter (g kg^-1) of 12.9, 10.9, 6.4 and 10.4, respectively. The sorption isotherm (n) values were 0.104, 0.101, 0.085 and 0.165 for Kathalagere, Mandya, Mudigere and Mangalore soils, respectively. The sorption isotherm for pyrazosulfuron-ethyl was non-linear in all soils, as explained by the Freundlich equation (n < 1.0), indicating differential distribution of site energies for sorption. In general adsorption isotherms fitted well with Freundlich equation (r^-2 > 0.96), and Freundlich constants 'K', 'Koc' and 'K_c' values were in the range from 5.45 to 9.52, 1099.46 to 1436.12 and 32.24 to 49.07 in different soils and described in the order of Kathalagere > Mandya > Mudigere > Mangalore, which is also in the order of decreasing organic matter content in the soils. The coefficient of determination showed an improvement up to 96 per cent using quadratic functions, suggesting a better goodness of fit in preceding pyrazosulfuron-ethyl sorption compared to the linearised form of the Freundlich equation. The cumulative desorption of five days of pyrazosulfuron-ethyl in these soils were determined and it ranged from 22.3 to 41.1 mg kg^-1 in Kathalagere soil, 25.6 to 49.4 mg kg^-1 in Mandya soil, 37.1 to 51.9 mg kg^-1 in Mudigere soil and 39.9 to 55.7 mg kg^-1 in Mangalore soil. The desorption was also in decreasing order of Mangalore > Mudigere > Mandya > Kathalagere soils. The sorption of pyrazosulfuron-ethyl increased with increasing organic matter content and was significantly correlated with organic matter content of the soils where as clay content did not greatly affect the sorption as indicated by low regression coefficient.