Worker environment research. IV. The effect of dust derived from several soil types on the dissipation of parathion and paraoxon dislodgable residues on citrus foliage.

Parathion sorbed to dust can persist as dislodgable residues on citrus leaves. Semi-logarithmic plots of parathion dislodgable residue data were initially linear for the 6 soils studied and then a distinct change to lower rates occurred with 5 of the soils. As initial rates and the residue level at the rate change are dependent on the soil, foliar dust is implicated as a causative factor in the non-uniformity of residue dissipation rates. Soils can influence the conversion of parathion to paraoxon. Paraoxon levels differed greatly with the type of soil and were highest with Pike's Peak clay. Thus, its use in a parathion formulation could produce relatively high levels of paraoxon dislodgable residues. It is significant that, second to Pike's Peak clay, the highest total dislodgable residue level was found with Visalia silt loam which was collected from a grove where a worker poisoning episode had occurred in 1974.