The Fate of Phorate in Soils1

Abstract

Studies were conducted to determine the hydrolysis, oxidation, insecticidal efficiency, persistence, and the degree of binding of phorate, formerly designated as Thimet® ( O,O -diethyl S-(ethylthio) methyl phosphorodithioate) in three soils and quartz sand. Phorate applied as a soil treatment in the field was more available to cabbage and potatoes grown in a sandy soil than in a clay-loam soil as indicated by insect-control data and anticholinesterase assay. A measurement of p32-labeled phorate uptake by peas from three soils and quartz sand showed that the largest amounts of toxicant were taken from quartz sand and followed by lesser amounts from a sandy soil, day loam, and muck, in that order. Extraction, column chromatographic and partitioning techniques showed that soil applications of phorate are partially oxidized, hydrolyzed, and bound to the soil. Chloroform extractions of radioactivity from the soils at 7, 14, and 28 days after treatment were identified as phorate plus a mixture of the phosphorodithioate sulfoxide and the phosphorodithioate sulfone of phorate. Only a very small amount of the radioactivity extracted from the soils could be identified as hydrolysis products. A large portion of the radioactivity remained bound to the soil and could not be identified. Radioactive phorate added to quartz sand was rapidly hydrolyzed, but no oxidation products were detected. A study of phorate volatilization from the soils showed that within an hour after treatment the sandy soil, silt loam, and muck had lost respectively, 25%, 20%, and 18% of the radioactivity applied. However, after this initial loss, very little or no volatilization occurred. Phorate was lost rapidly from quartz sand and a steel surface with less than 10% of the radioactivity remaining 24 hours after treatment.