The Effect of Cation-Exchange Capacity on the Retention of Diquat2+ and Paraquat2+ by Three-Layer Type Clay Minerals: II. Plant Availability of Paraquat

Abstract

Paraquat (1,1′-dimethyl-4,4′-dipyridinium dichloride) was adsorbed on varying amounts of Ca-montmorillonite and Cavermiculite. The herbicide-treated clays were added to a synthetic soil medium and cucumber seedlings (Cucumis sativus) were planted. Ca-montmorillonite effectively reduced the activity of approximately 90 to 95% of the adsorbed paraquat leaving only 5 to 10% available to the plants. Approximately 10% was also found to be exchangeable using 5 × 10-3N AlCl3 solutions and is believed to be that which is adsorbed on external surfaces. The results are analogous to those found with other montmorillonites and suggest that the major portion of dipyridinium herbicides that are adsorbed on this clay mineral is probably adsorbed in a biologically unavailable form. Ca-vermiculite reduced the pbytotoxicity of paraquat, but did not remove the phytotoxic effects completely. Substantial amounts of paraquat were also released from this clay mineral using salt solutions. The results are analogous to previous findings with kaolinite clay and suggest that the adsorption of this herbicide by soils in which vermiculite and kaolinite are the dominant clay minerals is probably largely in a biologically available form.