Graduate Thesis Or Dissertation
 

Influence of soil moisture on movement of terbacil in soil

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  • Previous field research with terbacil (3-tert-butyl-5-chloro- 6-methyluracil) has shown that early fall applications to dry soil performed better than did late fall or early winter applications to wet soil. Field and greenhouse studies were conducted to determine the effect of soil moisture content at time of application and the effect of moisture applied after herbicide application on the leaching of terbacil. A field trial was initiated during the summer in which terbacil was applied to saturated and dry soil and allowed to dry one week before sprinkler irrigation. Terbacil was also applied to saturated and dry soil which was irrigated immediately. Three-and-one-half inches of irrigation water were applied. Of the three test species, rape (Brassica napus L.) was very sensitive, oats (Avena sativa L.) was moderately sensitive, and corn (Zea mays L.) was relatively tolerant to terbacil. The field bioassay showed that terbacil was the least effective when applied to dry soil with a one-week interim before irrigation. Herbicidal activity was noticeably better when terbacil was applied to wet soil or to dry soil if irrigated immediately. Bioassays of core samples taken from the plots indicated that terbacil sprayed on dry soil one week before irrigation was the least subject to leaching. Terbacil applied to dry soil and irrigated immediately appeared to leach the greatest distance. Leaching studies in the greenhouse were conducted with soil columns consisting of two layers of soil. The bottom nine-inch layer was saturated and the top two-inch layer, to which terbacil was applied, was either saturated or air dry. Ten inches of water were added immediately, three days, and seven days after application of the herbicide. Bioassay tests showed that terbacil leached the greatest distance when water was applied immediately. When water was applied seven days after terbacil applications, slightly greater leaching of terbacil occurred when applied to wet soil than to dry soil. Analysis of the leachates by gas-liquid chromatography showed that less terbacil was leached through columns in which water was applied three days after application compared to those receiving water immediately after application. However, the speed with which water moved through the columns was quite variable. Analysis of leachates from soil columns in which the entire soil mass was at one moisture level, saturated or air dry, showed that more terbacil leached through the wet soil than the dry soil when ten inches of water was applied. Again, less terbacil was found in the leachates from columns leached three days after terbacil application compared to those leached immediately. Core samples were taken in April from a field trial in which terbacil had been applied in October and December. The October treatments had given excellent control of winter-germinating weeds while the December treatments failed to give satisfactory control at any rate. Bioassays of the core samples showed no marked differences in depth of leaching of the terbacil from the two dates of application. These results would indicate that the large differences in weed control that have consistently been observed between early fall and winter applications of terbacil are not due to differences in depth of leaching.
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