Sorption-desorption and column leaching of strychnine with soil

Sorption-desorption of rodenticide strychnine by soil and its leaching through soil columns were studied on 4 typical soils of south-east Queensland. All 4 soils showed a high tendency to sorb strychnine, with the sorption rate higher for clay soils. The sorption capacities of the 4 soils are in the order Kingsthorpe > Warra > Oakey > Roma, which is also the order of decline in their clay contents. The desorption process also closely followed the clay content of the 4 soils. The 2 clay soils of Kingsthorpe and Warra not only sorbed a higher proportion of the applied strychnine at any application rate, they also showed a greater resistance to releasing their sorbed strychnine compared with the 2 silty clay loam soils. The effects of pH and organic matter content on the sorption-desorption of strychnine were inconclusive due to the dominant influence of clay content and the narrow range of these characteristics provided by the soils under investigation. The 2 clay soils of Kingsthorpe and Warra required a significantly higher number of pore volumes of leaching solution to pass through their respective columns for the concentration of strychnine in the effluent to approach that of the leaching solution, compared with the 2 silty clay soils of Oakey and Roma. The pore volumes of the leaching solution necessary for this point to be reached were not in the order of their clay contents, but when the cumulative volume of solution was used instead of the pore volume, the trend followed the clay content of the soils closely. Pore volume may not be an appropriate characteristic for assessing the leachability of strychnine through soil columns when the soil's clay fraction is of an expanding type, as it is the case for Kingsthorpe soil. No desorption or leaching of strychnine took place in any of the 4 soils aged with a range of strychnine concentrations.