Dissipation of some organochlorine insecticides in cropped and uncropped soil

Dissipation of four organochlorine insecticides, viz. aldrin, HCH, chlordane and heptachlor was studied in a sandy loam soil with and without crops during a period of 10 cropping seasons. Dissipation of all chemicals followed first-order kinetics ( 2 = 0·537-0·976) with almost similar persistence in cropped and uncropped soils for all the insecticides. The average half-lives, ( t 1 2 values) for total residues of aldrin, HCH, chlordane, and heptachlor in cropped treatments were 80·7, 58·8, 93·2, and 110 days. Their respective values in fallow plots were 78·4, 83·8, 154, and 116 days. None of the parent compounds or their isomers could be detected below the 20 cm depth at the termination of the experiment. Highest residue concentrations were observed in the surface 10 cm layer in fallow plots, but in the deeper (10–20 cm) layer in cropped plots. Analysis of plants and grains showed significant residues of all the chemicals. Degradation of these compounds in cropped and uncropped plots is discussed with regard to their volatilization, microbial degradation, leaching, and plant uptake.