Uptake of phorate by lettuce

AbstractFollowing experimental and commercial applications to soil of a granular formulalation of phorate (O,O‐diethyl S‐ethylthiomethyl phosphorodithioate), residues in the soil and in lettuce were determined by gas‐liquid chromatography. When applied by the bow‐wave method as a continuous logarithmically‐changing dose ranging from approximately 0.9 to 16.0 kg a.i. ha−1, the proportional rate of oxidation in soil of phorate sulphoxide to phorate sulphone was inversely related to dose. Ten weeks after application, total phorate residues in the soil had declined by about 35% at all dose levels. Residues in mature lettuce, from the 1‐5 kg ha−1 dose‐range, comprised the parent and oxygen analogue sulphoxides and sulphones; the relative proportions of the individual metabolites were independent of dose. Over this dose‐range, total residue concentrations in the crop became proportionally slightly greater with increasing dose. When single doses of 1.1, 2.0 or 2.2 kg a.i. ha−1 were applied at drilling, the total residue concentrations in the lettuce declined from 5 mg kg−1 in seedlings from some treatments to <0.05 mg kg−1 at harvest. In plants raised in peat blocks containing 10 or 20 mg a.i. per block, however, residues in seedlings totalled 45‐47 mg kg−1 and declined to only 0.7 mg kg−1 at harvest. It was concluded that bowwave applications of phorate when field‐sowing lettuce were unlikely to lead to unacceptable residues in the harvested crop, but that residues in lettuce raised in phorate‐treated peat blocks may be unacceptably high.