Alternative herbicide formulations may have the potential to reduce atrazine leaching. This study was conducted to determine if starch-encapsulation produced using an extrusion process or several acrylic polymer additives reduced atrazine leaching in soil columns packed with Plainfield sand (98% sand and 0.7% organic matter). Three watering regimes were evaluated to determine the effects of water volume and rate of application on atrazine movement When 7.6 cm of water (0.44-pore volumes) was applied over 2 h, polymer treatments reduced atrazine movement from the soil surface by 9 to 21% compared to atrazine without the additives. With increased water volume and time, the effectiveness of several polymer treatments diminished. Acrysol ASE-108 and G110 polymers (mixed with atrazine at a 1:1 ratio) most effectively reduced atrazine leaching over all watering regimes. Starch encapsulation was more effective than any polymer additive in retarding atrazine movement Increasing the water volume from 7.6 to 15.2 cm (0.88-pore volumes) did not increase leaching of starch-encapsulated atrazine. Ninety-nine percent of the starch-encapsulated atrazine was retained in the top 5 cm of the column compared to only 18 and 13% of the dry flowable formulation (DF) when 0.44- and 0.88-pore volumes of water were applied over 2 and 4 h, respectively. When 0.88-pore volumes of water were applied over 12 d, 81% of the starch-encapsulated atrazine was retained in the upper 5 cm of the column compared to only 5% of the DF formulation of atrazine. This study indicates that starch encapsulation reduces atrazine movement to a greater extent than polymer additives and suggests that starch encapsulation may be an effective method of reducing atrazine leaching.

Field and column experiments were conducted to determine the effect of controlled release formulations on weed control and leaching of alachlor and metribuzin on a Plainfield sand. Controlled release formulations including two starch encapsulations of both herbicides and a microencapsulation of alachlor were compared to emulsifiable concentrate and dry flowable formulations of alachlor and metribuzin, respectively. Herbicide movement was measured in laboratory columns and in the field throughout two growing seasons to a soil depth of 91 cm. Soybean injury and weed control were monitored. No significant differences in herbicide movement between starch-encapsulated and emulsifiable concentrate formulations were observed in either field or column experiments. Microencapsulation resulted in the greatest retention of alachlor in the soil surface in field and columns. Compared to the dry flowable formulation, starch encapsulation did not affect metribuzin distribution in the field but reduced leaching in columns. Controlled release formulations did not result in significant differences in weed control and soybean injury compared to the emulsifiable concentrate alachlor and dry flowable metribuzin formulations. Starch encapsulations had a limited effect on alachlor and metribuzin movement. Degradation appeared to be the primary mechanism for herbicide dissipation while leaching losses were minor.

Controlled-release formulations have the potential to reduce atrazine leaching but may reduce herbicide bioavailability and weed control. This research was conducted to determine if starch-encapsulated atrazine produced by the extrusion process could be modified to provide an optimum rate of herbicide release for adequate weed control and reduced leaching potential in sandy soils. The dry flowable (DF) and six starch-encapsulated atrazine formulations of various mesh size (14 to 20 and 20 to 40), clay content (0 and 5%), and extruder processing speed (200 and 400 rpm) were evaluated. All starch encapsulations reduced the amount of atrazine leaching from the surface 5 cm of soil columns by an average of 68% compared with the dry flowable formulation. Compared with the larger starch granules (14 to 20 mesh) 8% more atrazine in the smaller granules leached from the surface 5 cm. Atrazine efficacy was measured using an oat bioassay conducted in the greenhouse. Smaller granules (20 to 40 mesh) provided equivalent efficacy to DF while large granules (14 to 20 mesh) did not. Extruder speed and clay content did not affect efficacy or leaching potential. This study indicates that finer starch granules should result in faster atrazine release and improved weed control while still reducing the leaching potential of atrazine.

A tractor mounted sampler was constructed which allows large soil columns to be obtained rapidly and with minimal physical effort. The machine utilizes the hydraulic system of a tractor to push a steel sampling cylinder containing 30.5-cm id. polyvinyl chloride (PVC) pipe into the soil, after which the hydraulic cylinder is retracted to pull the pipe and enclosed soil from the ground. Columns 68 cm in length were taken with a hydraulic cylinder pressure of approximately 10 MPa. Soil separated evenly at the bottom of the columns, and compaction was measured to be approximately 5%. This is a relatively simple and rapid method for obtaining large soil columns for use in pesticide leaching studies.

Laboratory studies were conducted to evaluate 14C-flumetsulam mobility in two Mississippi soils of varied texture and organic matter content following delays in irrigation. Mobility was evaluated using packed soil columns, 25 cm deep, under unsaturated–saturated flow conditions. Irrigation timings included 0, 3, and 5 d after flumetsulam application. Flumetsulam mobility (defined as the amount collected in leachate) decreased from 45% to no more than 20% of the applied in the Prentiss sandy loam soil when irrigation was delayed 3 or 5 d. With the Okolona soil, flumetsulam recovery in the leachate was 21, 14, and 6%, respectively when irrigation occurred 0, 3, and 5 d after application. Flumetsulam proved to be mobile when irrigation immediately followed application, with 6 to 45% recovered in the leachate from all soils evaluated. The Prentiss soil retained 6% of the applied flumetsulam in the upper 5 cm and the Okolona soil retained 22% when irrigation immediately followed flumetsulam application. When the irrigation interval was delayed at least 3 d, the Okolona soil retained 40% in the upper 5 cm, whereas the Prentiss soil retained 10%. Flumetsulam mobility was dependent on irrigation timing and soil type.

A bioassay using red beet root length indicated an increase in imazethapyr bioactivity in Berryland sand (BLS) as soil pH increased from 3.7 to 6.5. Increasing pH above 6.5 had no effect on imazethapyr bioactivity. The lowest imazethapyr concentrations detected by the bioassay at pH 6.5 were 0.5, 1, 2.5, and 5 to 10 μg kg−1 in acidwashed quartz sand, BLS, Aura loamy sand, and muck soil, respectively. After application of 0.07 kg ae ha−1 with 12.5 mm of simulated rainfall at 3-d intervals, imazethapyr remained in the 0 to 15 cm soil in BLS soil columns. Under this rainfall regime, the herbicide moved 30 cm deep 5 mo after application, but residues decreased significantly in surface soil. Imazethapyr bioactivity was highest and mobility was lowest at low soil temperature (10 C) and low moisture (4 to 6% w/w). Imazethapyr movement was monitored successfully using the red beet root length bioassay, and an herbicide residue map was developed to show the pattern of imazethapyr bioactivity in soil columns.

Widespread groundwater contamination has prompted studies on the fate and transport of solute through soil. Large quantities of atrazine and metribuzin are applied annually in the Humid Pampas of Argentina, creating the need to study the fate of these herbicides in soils of the region. The objective of this work was to study the vertical transport of atrazine and metribuzin in packed soil columns for three loam soils representative of the Humid Pampas of Argentina. Bromide was used as a nonreactive tracer. The convection dispersion equation was fitted to chemical breakthrough data to obtain a parameter characterizing chemical transport. Bromide breakthrough curves (BTCs) were similar among soils. BTCs for atrazine and metribuzin revealed significant interaction among soils and herbicides. The average values for the organic carbon (OC) partition coefficients (Koc) derived from column flow experiments were 119 and 48 ml/g for atrazine and metribuzin, respectively. Metribuzin in leachate was 97.3% of the total recovered, whereas atrazine was 3.5%. This behavior can be explained by their different affinities to OC. The OC contents of the Balcarce, Necochea, and Nueve de Julio soils were 4.1, 3.4, and 1.9%, respectively. The lowest leaching values of herbicides were found in the Balcarce soil, suggesting that OC content was the main factor in controlling herbicide transport in these soils.