Herbicide and metabolite movement in different soils as studied by computer assisted microlysimeters

Leaching behaviour of the herbicides metamitron, metolachlor, and terbuthylazine and some of their main metabolites has been studied in different soils. A computerised microlysimeter apparatus was designed for observing the behaviour of those substances in soil cores under automated irrigation and controlled water regime. Undisturbed soil columns from long-term no-tillage and plough tillage field plots were used, and varying irrigation intensities from 4 up to 10 mm/day were applied. Results suggest that an appropriate application of the herbicides according to the label will not result in their release via the leaching path, regardless of the tillage system which can determine physical, chemical, and biological conditions of the soil. However, higher application rates or spills cause excessive leaching losses which can be explained by non-linearity of adsorption processes and non-equilibrated conditions. Furthermore, the irrigation regime (rainfall intensity and timing in relation to the application date) is decisive for the movement of herbicides in soil. Metabolites like desethyl-terbuthylazine and desamino-metamitron, which are more mobile that their parent compounds, can be leached very easily by the percolating water.