Behavior of metribuzin and ethylmetribuzin in soil

Graduation date: 1989The behavior of metribuzin [4-amino-6-(1,1-\ud dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] and\ud ethyl-metribuzin (4-amino-6-(1,1-dimethylethyl)-3-\ud (ethylthio)-1,2,4-triazin-5(4H)-one] in soil was studied to\ud determine if differences in herbicide behavior in soil could\ud explain (a) the inconsistent herbicidal activity of these\ud chemicals and (b) the greater activity of metribuzin, as\ud compared to ethyl-metribuzin.\ud Metribuzin had higher activity than ethyl-metribuzin in\ud all soils and in quartz sand. Metribuzin was adsorbed less\ud and moved more than ethyl-metribuzin in all soils. Activity\ud of both herbicides decreased as sand content increased, and\ud activity in quartz sand was lower than in soil when drainage\ud could occur. Herbicide movement was greatest and adsorption\ud was lowest for both herbicides in coarse textured soils.\ud Herbicidal activity increased in a bioassay in which\ud leaching was prevented, suggesting that leaching may be\ud important in the loss of activity of metribuzin and ethylmetribuzin.\ud Phytotoxicity of both herbicides against oats decreased\ud as soil pH decreased from 8.4 to 4.2 in a sandy loam.\ud Herbicide adsorption increased as soil pH decreased. In\ud nutrient solution, pH did not change the phytotoxicity of\ud either herbicide, indicating that pH primarily influences\ud herbicide availability in soil and not plant uptake.\ud In growth chamber experiments, phytotoxicity of ethylmetribuzin\ud decreased as the time between surface application\ud and initial watering increased. Evaporation of metribuzin\ud and ethyl-metribuzin from soil surfaces in field studies was\ud approximately 50% within 24 h, with little additional loss\ud occurring within 12 days. Evaporation of either herbicide\ud from glass was more rapid than from soil, with 75 to 90%\ud loss in 24 h, and up to 99% loss in 12 days. Loss by\ud photodecomposition from soil or glass surfaces was minimal