Interactions between agricultural chemicals and their effects on pesticide persistence in soil

The effects of nitrogen fertilizers and selected pesticides on the persistence of soil-applied herbicides alachlor, butylate, and atrazine, and the insecticide carbofuran were evaluated in laboratory and field experiments. Nitrogen fertilizer had a greater effect on pesticide persistence in a Sharpsburg silty clay loam (Typic Argiudoll) at pH 5.8 than at pH 6.7. Persistence of alachlor and carbofuran increased with ammonium nitrate, ammonium chloride and sodium nitrate concentrations in soil. Urea had less effect on alachlor and butylate persistence than nitrogen salts at pH 6.7. At pH 5.8 carbofuran degraded more rapidly in urea amended soil than in untreated soil, while urea increased carbofuran persistence at pH 6.7. Sodium chloride applied at the same rate as nitrogen increased pesticide persistence, but the effect was less than that of the nitrogen salts. A strong inverse correlation existed between pesticide persistence and soil pH changes due to addition of ammonium nitrate, ammonium chloride and urea. Changes in soil pH alone did not account for all observed differences in pesticide degradation. Sodium chloride and sodium nitrate treatments lowered soil pH slightly yet differed in effects on pesticide degradation. Atrazine persistence was not affected by nitrogen fertilizers in the Sharpsburg silty clay loam at pH 5.8 or 6.8. Butylate persistence was not affected by simultaneous nitrogen application, but persistence increased when nitrogen was applied several days before the herbicide. Carbofuran persistence was increased by the presence of 2,4-D in laboratory incubation studies. Terbufos and glyphosate decreased alachlor persistence and butylate increased atrazine persistence in the laboratory incubation studies. These interactions were not confirmed in field studies conducted at two locations with differing soil properties. Pesticide-pesticide and pesticide-fertilizer interactions appeared to be dependent on initial soil pH and form and rate of nitrogen fertilizers. Other variables may include microbial populations, osmotic potential, and organic amendments. Predictions of pesticide-fertilizer and pesticide-pesticide interactions may be difficult without site-specific measurements