|Welker, William - FORMER USDA EMPLOYEE|
Submitted to: Weed Technology Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 29, 1999
Publication Date: N/A
Interpretive Summary: This experiment provided a view of the persistence of soil residues of three herbicides that have been commonly used in fruit culture for more than 30 years. Diuron did not move more than 40 cm into soil, and it did not accumulate. Terbacil was more persistent than diuron, but, like diuron, significant residues were not found below 60 cm. In contrast, simazine or simazine degradation products were found in the deepest soils that were sampled, 100 cm. Long-term use of diuron and terbacil was safe, but simazine may be problematic. After fifteen annual applications, herbicide residues were present but were not accumulating.
Technical Abstract: Diuron, simazine, and terbacil were applied in the field each year from 1981 to 1995. Soil was sampled at selected times after herbicide application in 1993, 1994, and 1995 to determine herbicide residue changes with time and soil depth. Diuron was found mainly in the upper 20 cm of soil and diuron residue concentration decreased expoentially with time. A small residue, less than 1% the concentration after application in summer, was present in the following spring. Terbacil was found in soil below the upper 20 cm. However, terbacil concentration decreased markedly immediately below 20 cm. Terbacil degraded more slowly than diuron and residues in spring were below 30% the level of the previous summer. Simazine plus hydroxysimazine soil residue concentrations were present in all depths to 100 cm and were higher at these depths than diuron or terbacil. Simazine plus hydroxysimazine residues in spring were nearly 40% %the level after application the previous year. With all three herbicides, soil residues were greatest in the upper 20 cm of soil during the 2 to 3 weeks following application. The data confirmed that diuron did not leach whereas simazine can migrate through the soil. Terbacil migrated intermediately relative to diuron and simazine. After fifteen annual applications, herbicide residues were present but were not accumulating. Nomenclature: Diuron, N'-(3,4-dichlorphenyl)-N,N-dimethylurea; simazine, 6-chloro-N,N'-diethyl-1,3,5-triazine-2,4-diamine; desethylsimazine, 2-chloro-4- (ethylamino)-6-amino-s-triazine; di-desethylsimazine, 2-chloro- 4,6-diamino-s-triazine; hydroxysimazine, 2-hydroxy-4,6-bis(ethyl-amino)-s- triazine; terbacil, 5-chloro-3-(1,1-dimethylethyl)-6-methyl-2,4(1H,3H)- pyrimidinedione.