RESIDUAL ACTIVITY OF PARAQUAT IN SOILS II. ADSORPTION AND DESORPTION

Glasshouse studies showed that low doses of paraquat inhibited the germination of Lolium perenne L. broadcast directly onto the paraquat-sprayed surfaces of a sphagnum and a peat soil, but that higher doses were necessary to produce phytotoxic symptoms on mineral soils, a compost and a loam. On all soils residual activity increased rapidly with increasing dose once the minimum phytotoxic dose was reached. On a sandy soil, residual activity increased almost linearly from the lowest to the highest dose applied. At 9·0, 4·5 and 2·24 kg/ha phytotoxicity on a compost was not affected by changes in the volume of application, but at 1·68 kg/ha and lower, reducing the volume from 562 1/ha to 281 and 112 1/ha resulted in increased phytotoxicity. Phytotoxic residues were eluted from paraquat-treated compost surfaces by percolating de-ionized water up soil columns but residual activity was not removed from the eluted surfaces. Surface irrigation of paraquat-treated surfaces with water previously percolated through columns of untreated soil reduced residual activity by 45%.