Organochlorine Insecticide Residues in Soybean Plant Tops: Root vs. Vapor Sorption1

Soybeans were grown in specially constructed pots to determine the amount of residue contamination in plant tops from 20 ppm surface or subsurface soil‐applied 14C‐labeled p,p'‐DDT, dieldrin, endrin, or heptachlor. Regardless of the insecticide treatment, plant tops were contaminated by both root sorption and foliar sorption of vapors emanating from the soil. Root sorption was the major source of contamination by dieldrin, endrin, and heptachlor, though vapor sorption of dieldrin was nearly as great. Foliar contamination by vapor sorption of DDT residues was nearly seven times greater than for root sorption. Foliar contamination from vapor sorption of residues from all four insecticides was near the same order of magnitude, about 6.5 ppm plant dry weight, whereas contamination from root sorption from endrin was 38, heptachlor 21, dieldrin 11, and DDT 1 ppm. Plant parts were assayed by several methods. Although residue concentrations were different for each insecticide, both the apparent plant degradation and mobility of the insecticide, or degradation products, were: DDT ≥ heptachlor > endrin > dieldrin where root sorption was the source of residue. One heptachlor degradation product, in addition to heptachlor epoxide, two endrin degradation products, and p,p'‐DDE and p,p'‐DDD were found.