INFLUENCE OF AGRICULTURAL PRACTICES ON THE LEVELS OF DDT AND ITS RESIDUES IN SOIL

DDT residues in soils resulting from past applications of DDT to pasture are of concern due to the uptake and accumulation of these residues by stock. In this paper, we report the results of a study of DDT residues in the soils (Lismore stoney silt loam) of long-term irrigation and superphosphate application trials set up approximately 40 years ago. Irrigation at a rate sufficient to maintain mean moisture in the top 100 mm of soil during the summer at 27.3% resulted in a total DDT residue level of around 40% that of unirrigated soil (mean summer moisture = 13.8%). Superphosphate treatment resulted in lower soil levels of p,p'-DDT and total DDT residues compared to the unfertilized control. Possible causes of these effects are discussed. In addition, the profiles of DDT residue decline in the samples taken from the irrigation trial since 1959 are presented. These data suggest that appreciable levels of DDT residues (mainly as p,p'-DDE) will be present in the soils for some time.