Dieldrin and Heptachlor Residues in Soybeans 1

Abstract Where soybeans [ Glycine max (L.) Merr.] are grown on land previously planted to maize ( Zea mays L,.), the soybean crop may be contaminated with residues of organochlorine insecticides applied to the soil for control of soil‐borne predators attacking maize. Experimental findings on the decrease of contamination with time are meagre. In 1970, soybeans were grown on experimental fields that had received 5 kg/h of dieldrin, mixed to the 7.5‐cm depth in 1970, 69, 68, 67, and 66. Residues in the soybeans declined rapidly over the first 2 years after application, but showed little further change in the 3rd through the 5th year. At harvest, beans from the lower half of the plant contained 0.31 ppm from the 1970, 0.132 ppm from the 1969, and 0.038 ppm from the 1968 applications. Beans from the upper half contained ½ to ¼ of these amounts. Heptachlor residues, principally heptachlor epoxide, from a parallel treatment in 1969 were less than the dieldrin. The data confirm that soybeans should not be planted for 2 years after soil has been treated with dieldrin.