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Persistence of Dinitramine and Pendimethalin in the Soil
Thesis Abstract:
The study of different soil types, application rates, and moisture levels on persistence of dinitramine and pendimethalin were evaluated in pot experiments under greenhouse conditions.
Higher residual phytotoxicity from the herbicide was obtained in clay loam soil over clay and fine sandy loam soils at 50-110 days because of its lower soil organic matter content. After three months, herbicide residues were very minimal in all three soils. Dinitramine was lost faster than pendimethalin at 50-80 days after application. However, both herbicides were almost undetectable after three months.
With sorghum bioassay, the general ranking of recovered pendimethalin residues was clay > clay fine > sandy loam. With gas liquid chromatography, the ranking was changed to fine sandy loam > clay load > clay. This was because the bioassay measured plant response to the herbicide, hence, only amounts in the solution were detected whereas with gas liquid chromatography, absorbed herbicide molecules could be extracted and quantitatively measured.
Higher rates of application consequently gave greater and more prolonged residual phytotoxicity and higher residues in all sampling periods than the recommended rate. Applying four times over the normal dose gave reduciotns of 62 percent in sorghum growth and 60 percent in dry matter yield three months after treatment. At twice the recommended rate, it took more than 80 days to bring down phytotoxicity to 50 percent.
Residues were very negligible at the end of three months when the recommended rate was used. Pendimethalin was again more persistent than dinitramine although both were almost undetected at three months at the lowest rate of application.
Loss of dinitramine and pendimethalin under low soil water levels was very slow. Marked residual phytotoxicities were still apparent at 110 days with four-day interval watering (19.56%) and with two-day interval watering (7.98%) in contrast with the daily watered treatment (2.98%). Dinitramine was again more readily lost than pendimethalin, particularly in the frequently wet soil.
Leaching was not a principle means of dinitramine or pendimethalin loss in high soil water levels with most of the residues still present in the upper 5 cm soil layer, even if water was applied daily at 100 percent field capacity.