Abstract

Five Dutch soils (a medium loess, a slightly acidic sandy soil, a slightly alkaline sandy soil, a slightly acidic loamy peat and a slightly acidic clay peat) were selected according to their expected vulnerability to phytotoxicity after repeated paraquat application. Biological availability of paraquat was determined by bioassays with lettuce. A reduction of 25% in fresh weight of the sprouts was regarded as phytotoxic. In three out of five soils phytotoxicity was observed at paraquat concentrations of less than 100 mg.kg-1. No relation between phytotoxicity and the cation exchange capacity of the soils was found although in literature cation exchange is mentioned as the main sorption mechanism. Clay content appeared more important for sorption than the organic matter content of the soils. Phytotoxicity was found to be relatively low in soils having a high CaCO3 content. This phenomenon, however, can not be explained yet.