Interactions of heptachlor (a chlorinated hydrocarbon insecticide) and soil microflora

The effect of heptachlor on soil microorganisms was studied under field, greenhouse, and laboratory conditions. Growth of many soil microorganisms was inhibited on culture media containing heptachlor. At a concentration of 25 ppm, heptachlor was bactericidal to 63% of the bacteria tested. Heptachlor, at 100 ppm in agar media used for isolating microorganisms from soil, prevented the development of 89% of the bacteria, 81% of the actinomycetes, and 50% of the fungi that appeared on isolation plates without heptachlor. After heptachlor was added to soil, fungal populations declined and bacterial populations increased. Numbers of bacteria were related to amount of heptachlor added; higher concentrations in soil resulted in larger populations. A selective increase in numbers of fungi which would grow on media containing heptachlor at 100 ppm occurred in soils amended with heptachlor in amounts ordinarily used in field practices (1 Ib./A.), but a similar increase of heptachlor-resistant bacteria occurred only in soils amended with much higher amounts of heptachlor. One bacterium. Bacillus cereus, isolated from soil was found to degrade heptachlor to its parent compound, chlordene.