Effect of Clay, Soil Organic Matter, and Soil pH on Initial and Residual Weed Control with Flumioxazin

Greenhouse studies were conducted to evaluate the effects of soil organic matter content and soil pH on initial and residual weed control with flumioxazin by planting selected weed species in various lab-made and field soils. Initial control was determined by planting weed seeds into various lab-made and field soils treated with flumioxazin (71 g ha⁻¹). Seeds of <i>Echinochloa crus-galli</i> (barnyard grass), <i>Setaria faberi</i> (giant foxtail), <i>Amaranthus retroflexus</i> (redroot pigweed), and <i>Abutilon theophrasti</i> (velvetleaf) were incorporated into the top 1.3 cm of each soil at a density of 100 seeds per pot, respectively. Emerged plants were counted and removed in both treated and non-treated pots two weeks after planting and each following week for six weeks. Flumioxazin control was evaluated by calculating percent emergence of weeds in treated soils compared to the emergence of weeds in non-treated soils. Clay content was not found to affect initial flumioxazin control of any tested weed species. Control of <i>A. theophrasti, E. crus-galli</i>, and <i>S. faberi</i> was reduced as soil organic matter content increased. The control of <i>A. retroflexus</i> was not affected by organic matter. Soil pH below 6 reduced flumioxazin control of <i>A. theophrasti,</i> and <i>S. faberi</i> but did not affect the control of <i>A. retroflexus</i> and <i>E. crus-galli</i>. Flumioxazin residual control was determined by planting selected weed species in various lab-made and field soils 0, 2, 4, 6, and 8 weeks after treatment. Eight weeks after treatment, flumioxazin gave 0% control of <i>A. theophrasti</i> and <i>S. faberi</i> in all soils tested. Control of <i>A. retroflexus</i> and <i>Chenopodium album</i> (common lambsquarters) was 100% for the duration of the experiment, except when soil organic matter content was greater than 3% or the soil pH 7. Eight weeks after treatment, 0% control was only observed for common <i>A. retroflexus</i> and <i>C. album</i> in organic soil (soil organic matter > 80%) or when soil pH was above 7. Control of <i>A. theophrasti</i> and <i>S. faberi</i> decreased as soil organic matter content and soil pH increased. Similar results were observed when comparing lab-made soils to field soils; however, differences in control were observed between lab-made organic matter soils and field organic matter soils. Results indicate that flumioxazin can provide control ranging from 75–100% for two to six weeks on common weed species.