Soil dissipation research

The exponential decay equation [1] effectively describes halosulfuron dissipation (Figure 3). First-order dissipation rate constants (k) for halosulfuron were less (i.e. slower dissipation) for soil under low density polyethylene mulch (0.07) than for bare soil (0.10). Halosulfuron dissipation for bare soil dropped to undetectable levels by 27 and 28 days after treatment in two studies, respectively. This trend was similar for soil under low density polyethylene mulch. From equation [2], the DT50 for bare-soil was 6 to 7 daysverses soil under low density polyethylene mulch which was 10 days. Although the first-order rate constants were not significantly different between bare soil and soil under LDPE mulch, the DT50 was 3 to 4 days longer for soil under low density polyethylene mulch. Dermiyati and Yamamoto (1997b) reported halosulfuron half-lives of 7 to 98 days depending on soil moisture and temperature regimes.

S-metolachlor dissipation was well described by the exponential decay equation [1] and for bare soil and soil under low density polyethylene mulch (Figure 3). First-order dissipation rate constants for S-metolachlor were less for soil under low density polyethylene mulch (0.2) than for bare soil (0.4). S-metolachlor dissipation was rapid for bare soil and soil under low density polyethylene mulch dropping to undetectable levels by 44 days after treatment. Rapid dissipation has been previously noted for metolachlor with sandy soil under moist soil conditions (Weber et al., 2003). In one experiment, S-metolachlor dissipation was biphasic, dropping to less than 400 ug/kg of soil at 7 days after treatment, yet was detectable at 44 days after treatment for both soil scenarios. While the DT50was 2 and 5 days for bare soil and soil under low density polyethylene mulch, respectively, dissipation was slower in one year as compared to another. This could be attributed to an equilibrium that was reached with S-metolachlor where soil adsorption had occurred, and then desorption of the parent was observed over time (Patakioutas & Albanis 2002). Data indicated that low density polyethylene mulch decreased the rate of dissipation of S-metolachlor versus bare soil which could extend its herbicidal activity.

Sulfentrazone dissipation varied but was slower than halosulfuron and S-metolachlor (Figure3) and had the longer DT50. Overall, the exponential decay equation [1] adequately described the sulfentrazone dissipation. Sulfentrazone dissipation first order rate constants were equal on average, with 0.055 for soil under low density polyethylene mulch and 0.050 for bare-soil. Half-lives were 16 days for bare-soil and 13 days for soil under low density polyethylene mulch. While counter intuitive, this could be due to increased temperature regimes that have been noted under polyethylene mulch (Peachey et al., 2001), that could have accelerated dissipation. Ohmes et al. (2000) previously noted that sulfentrazone dissipation followed first-order kinetics in Tennessee soils. They reported varying dissipation with DT50 ranging from 24 to 118 days. Variation in sulfentrazone DT50 has been noted from 2 (Collins et al., 1999) to 302 days (Vencill, 2002 b).

These studies indicate that halosulfuron-methyl and S-metolachlor dissipation was more rapid for bare-soil than soil under low density polyethylene mulch. However, sulfentrazone dissipation was variable. For bare-soil and soil covered with low density polyethylene mulch, dissipation of halosulfuron and S-metolachlor were biphasic (Figure 3).

Time after application (d) Time after application (d)

Time after application (d)

Fig. 3. Halosulfuron, s-metolachlor, and sulfentrazone dissipation for bare soil and soil covered with low density polyethylene mulch.

Time after application (d)

Fig. 3. Halosulfuron, s-metolachlor, and sulfentrazone dissipation for bare soil and soil covered with low density polyethylene mulch.

Sulfentrazone dissipation was slower (Figure 3) than halosulfuron or metolachlor. This indicates that sulfentrazone could provide residual Cypress species control when preemergence applied to vegetables but could also result in carryover problems to subsequent plantings.

Time after application (hours)

Time after application (hours)

Fig. 4. Glyphosate and paraquat dissipation from low density polyethylene mulch for dry and wash off conditions over time.

Time after application (hours)

Fig. 4. Glyphosate and paraquat dissipation from low density polyethylene mulch for dry and wash off conditions over time.

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