Pesticide Levels and Fluctuation in Soil Bacteria in an Irrigated Field and a Pond of Southwest Texas

Abstract

The Presidio area in Texas is now being studied in detail for the effects of pesticides2 upon the ecosystem. A typical irrigated field was studied for the effects of pesticides on bacterial populations from June through August 1966. Bacterial population numbers were found to fluctuate directly with the influences of irrigation. Soil that was not irrigated but had adequate moisture for bacterial growth yielded an inverse relation between pesticide levels and bacterial numbers. A connection between bacterial populations and pesticide breakdown was suggested. Microbial numbers and pesticide levels were examined in a stagnate body of water located adjacent to cultivated fields. Pesticide levels and bacterial counts were found to vary directly with silt washed into the pond from adjacent areas following substantial rainfalls. Pesticide persistence in soil has been a topic of study for several years. DDT and other chlorinated hydrocarbons have been reported to remain in soil for up to ten years by Ginsburg and Reed (1954), Lichtenstein and Schultz (1959), Lichtenstein and Polivka (1959), MacPhee et al., (1960) and Woodwell and Martin (1964). Little breakdown of DDT in soil during the first year after application was shown by Jones (1952). However, he presented data showing that breakdown after the first year occurred in soils with high organic matter and bacterial counts. Bollen et al., (1954) and Martin et al., (1959) presented data which showed that the total number of bacteria in soil had not been affected even though pesticide residues at phytotoxic levels for higher plants had been reached. Ahmed et al., (1958), 1 Technical Article No. 7228, Texas Agricultural Experiment Station, College Station, Texas. This research was supported in part by Grant AP 28-02 from the Division of Air Pollution, Bureau of State Services, Public Health Service. 2 Abbreviations used in this paper are: DDT = 2,2-bis(p-chlorophenyl)-1,1,1trichloroethane; DDD = 1,1-Dichloro-2,2-bis(p-chlorophenyl) -ethane; DDE = 1,1dichloro-2,2-bis(p-chlorophenyl) ethylene; methyl parathion =0,0-dimethyl O-pnitrophenyl phosphorothioate; ethyl parathion 0,0-diethyl 0-p-nitrophenyl phosphorothioate.