RESIDUAL ACTIVITY OF PARAQUAT IN SOILS I. FACTORS AFFECTING PERSISTENCE

Summary. Phytotoxic residues were recorded on soil surfaces sprayed with paraquat (1,1'-dimethyl-4,4'-bipyridylium-2 A ) at 1 and 2 Ib/ac a.i. Phytotoxicity was greater on organic than on mineral soils though on both the residues persisted unchanged for at least 14 days and were greater with the higher dose.
Germination of Brassica napus L. was only slightly affected and most seedlings did not show phytotoxic symptoms until after cotyledon expansion. The cotyledons of Trifolium repens L. showed symptoms on emergence from the testa. By contrast, grains of Lolium perenne L. either failed to germinate on paraquat-treated soils or showed reduced germination.
Seedlings of T. repens were susceptible; both root and shoot systems were affected as they grew through layers of soil previously treated with paraquat. Ground limestone appeared to reduce the phytotoxicity of paraquat under some circumstances. Phytotoxic residues were not reduced by surface irrigation but disappeared if the soil surface was disturbed mechanically.