Extended terbutryn persistence and herbicidal activity effected by superphosphate fertilizer

Microorganisms play a major role in the degradation and detoxification of most soilapplied pesticides. Any interference with soil microbial activity may affect the persistence as well as the effectiveness of the pesticide. The objective of this study was to investigate the effect of granulated superphosphate fertilizers on terbutryn (2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-l,3,5-triazine) degradation and on its herbicidal activity. Concentrations exceeding 1% of superphosphate in the soil slowed down terbutryn degradation; a concentration of 3% completely inhibited terbutryn degradation for nearly 60 days. When terbutryn was impregnated on the surface of superphosphate granules, the concentration of the fertilizer that inhibited terbutryn degradation was reduced from 3% to 0.1%. Bioassays with mustard seedlings confirmed the results obtained by chemical analysis of terbutryn. The decrease in the rate of terbutryn degradation was not specific to superphosphate. The inhibition of degradation could be attributed mainly to the concentration of the salt in the soil solution, expressed as electrical conductivity values, and not to the pH of the soil or to the type of the salt. Terbutryn was found to be quite stable on the surface of the superphosphate granule. At the concentrations tested, superphosphate did not affect root growth. The optimal weight of the impregnated granule for extending terbutryn activity was 125 mg, containing 0.2% terbutryn. A greenhouse experiment confirmed the findings that fertilizers could serve as potential inhibitors of terbutryn degradation.