EXTENDED TERBUTRYN PERSISTENCE AND HERBICIDAL ACTIVITY EFFECTED BY SUPERPHOSPHATE FERTILIZER

Microorganisms play a major role in the degradation and detoxification of most soil-applied pesticides. Any interference with soil microbial activity may affect the persistence as well as the effectiveness of t he pesticide. The objective of this study was to investigate the effec t of granulated superphosphate fertilizers on terbutryn ino)-4-(ethyla mino)-6-(methylthio)-1,3,5-triazine) degradation and on its herbicidal activity. Concentrations exceeding 1% of superphosphate in the soil s lowed down terbutryn degradation; a concentration of 3% completely inh ibited terbutryn degradation for nearly 60 days. When terbutryn was im pregnated on the surface of superphosphate granules, the concentration of the fertilizer that inhibited terbutryn degradation was reduced fr om 3% to 0.1%. Bioassays with mustard seedlings confirmed the results obtained by chemical analysis of terbutryn. The decrease in the rate o f terbutryn degradation was not specific to superphosphate. The inhibi tion of degradation could be attributed mainly to the concentration of the salt in the soil solution, expressed as electrical conductivity v alues, and not to the pH of the soil or to the type of the salt. Terbu tryn 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 extendi ng terbutryn activity was 125 mg, containing 0.2% terbutryn. A greenho use experiment confirmed the findings that fertilizers could serve as potential inhibitors of terbutryn degradation.