Sorption of the herbicide isoxaflutole and its main degradate, diketonitrile (DKN), to natural clays, SAz-1, SWy-2 and SHCa-1, and the organoclay derivatives (octadecylammonium (ODA) and hexadecyltrimethylammonium (HDTMA)) of these clays was investigated. Isoxaflutole hydrolysis to DKN was too rapid in aqueous solutions with organoclays to characterize sorption. No measurable DKN sorption was observed for the natural clays. Sorption of DKN was greater on organoclays with an interlayer paraffin-like complex that were prepared from the high-charge SAz-1 clay than on organoclays with a bilayer or monolayer interlayer complex prepared using lower-charge SWy-2 or SHCa-1 clays. Desorption isotherms indicated that sorption was irreversible. For SAz-1 with HDTMA at ∼100% of the clay CEC, the d001 values suggest that DKN enters the interlamellar space of the organoclay and dissociates into the anion. The DKN anion forms a very stable chelate complex with the residual cations and/or partially-coordinated structural cations. This strong interaction supports the irreversibility of the sorptive process.

The sorption of two sulfonylurea herbicides (SU), metsulfuron methyl and nicosulfuron, on pure clays and organoclays was investigated. Three clays (Arizona smectite, SAz-1, Wyoming smectite, SWy-2, and hectorite, SHCa-1), were treated with amounts of octadecylammonium (ODA) or dioctadecyldimethylammonium (DODMA) cations equal to ∼50 and 100% of the clays’ cation exchange capacity (CEC). Sorption isotherms were fitted to the Freundlich equation. While no measurable sorption was found on the pure clays (Kf = 0), organoclays prepared using both primary and quaternary amines were effective as SU sorbents. The metsulfuron methyl Kf values ranged between 196 and 1498 µmol1−1/n kg−1 L1/n, and Kf values for nicosulfuron, which were lower than those of metsulfuron methyl, ranged from 35 to 198 umol1−1/n kg−1 L1/n. As shown by sorption coefficients, Kd and KOC, SWy-2 treated with DODMA at ∼100% of the CEC was the most effective sorbent for metsulfuron, Kd = 684 L kg−1 and KOC = 2138 L kg−1. For nicosulfuron the most effective sorbent was SAz-1 with ODA at ∼ 50% of the CEC (Kd = 147 L kg−1 and KOC = 1233 L kg−1). In contrast to other weak-acid herbicides, such as phenoxy and picolinic acids, no clear relationships were found between sorption and layer charge, organic carbon content, and basal spacing of the organoclays for both sulfonylurea herbicides. Sorption of both herbicides on organoclays was assumed to involve hydrophobic and polar interactions for which the availability of interlayer room between organocations was a very important factor.

Interactions of water-soluble soil organic matter (WSSOM) with the herbicides bromacil, metribuzin, alachlor, diquat, and paraquat were examined to determine if these interactions can improve predictions of herbicide leaching potential. A high-performance liquid chromatography gel filtration column was used to separate WSSOM extracts from four mineral agricultural soils into fractions with approximate log molecular weights of 4.96 to 6.82. WSSOM fractions were predominantly anionic in nature with some hydrophobic character. The amount of paraquat bound by WSSOM ranged from 1.1 to 2.1 mmol g−1DOC−1 with KDOC (partition coefficient for dissolved organic carbon) values from 0.050 to 0.187 L kg−1. Diquat was bound at 0.9 to 1.5 mmol g−1DOC−1 by the extracts with KDOC values from 0.044 to 0.143 L kg−1. Bromacil, metribuzin, and alachlor did not bind to the extracts tested. WSSOM did not increase paraquat solubility in the presence of soil. Binding of these herbicides to WSSOM in the soils used in this study would not be a significant mechanism for increased mobility and groundwater contamination potential.

Formation and distribution of 14C-atrazine degradation products in the top 120 cm of soil were determined over 16 mo under field conditions in an Estherville sandy loam. After 16 mo, 78% of applied 14C was still present in the soil. By 2 mo after treatment (MAT), 14C had moved to the 30- to 40-cm depth; however, movement to depths greater than 40 cm was not observed. Greater than 98% of the 14C remaining in the soil profile after 16 mo was in the top 20 cm. Twenty-seven percent of the 14C applied was atrazine 16 MAT. Atrazine was the predominant 14C-compound in soil below 10 cm. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil. The proportion of 14C as HA in the top 10 cm increased from 15% 2 MAT to 37% 16 MAT. Deethylatrazine (DEA) was the predominant degradation product at the 10- to 30-cm depth and accounted for up to 23% of the 14C present in the 10- to 20-cm depth. Deisopropylatrazine (DIA) accounted for less than 6% of the radioactivity recovered at any soil depth. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased, indicating that DEA and DIA are more mobile in soil than HA. Detection of HA at depths greater than 10 cm appears to be due to in situ degradation of atrazine previously moved to that soil depth. The large amount of 14C remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for an extended period following application and have the potential to contaminate ground water.

Formation of 14C-atrazine degradation products and their distribution in the top 90 cm of soil was determined over 16 mo in a Webster clay loam in the field. After 16 mo, 64% of the applied 14C could still be accounted for in the 90-cm soil profile. At 1 mo after treatment (MAT), 14C moved to the 70- to 80-cm depth. Rapid movement of radioactivity could be attributed in part to preferential movement through vertical macropores. Atrazine accounted for 32% of the 14C applied 16 MAT and was the predominant 14C-compound in soil below 10 cm through 12 MAT. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil accounting for 9% of the 14C present 1 MAT and increasing to 24% within 6 MAT. Deethylatrazine (BEA) was the predominant degradation product at depths greater than 10 cm, accounting for 26% of the 14C in the 10- to 20-cm depth 16 MAT. Deisopropylatrazine (DIA) accounted for less than 10% of the 14C recovered at any soil depth. Deethyldeisopropylatrazine (DEDIA) and an unidentified product were detected in soil extracts 1 MAT indicating further degradation past primary metabolites. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased indicating that DEA and DIA are more mobile in soil than HA. The large amount of radioactivity remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for a long period of time, having the potential to move deeper in the soil and ultimately contaminate ground water.

A significant limitation in using sorption coefficients (Kd) to predict solute transport through natural soils is the spatial variability of soil properties over large field areas. Spatial variability in Kd for imazethapyr was determined on representative samples from a 31.4-ha field, covering a pH range from 4.9 to 7.6 and an organic carbon (OC) range from 1.45 to 5.80 g kg−-1. Kd varied from 0.18 to 3.78 across the field, with an average value of 1.56. The analysis of Kd variability showed two distinct patterns in spatial distribution: areas in which pH > 6.25 and Kd < 1.5, where Kd variation is based primarily on pH, and areas in which pH < 6.25 and Kd > 1.5, where other soil properties, i.e., OC content, have a significant influence on Kd variation. Based on soil pH distribution, an easily measured property, the field was divided into two potential management areas. This separation allowed identification of portions of the field where herbicide sorption would be minimal, with a relatively higher potential for leaching (i.e., areas with Kd < 1.5), and provided a rationale for site-specific imazethapyr application.

The effects of treatment with twelve metabolites of trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl) benzenamine] on the gross morphology and ultrastructure of trifluralin-susceptible (S) and -resistant (R) biotypes of goosegrass [Eleusine indica (L.) Gaertner. # ELEIN] root meristems were examined. Unlike trifluralin, ten of the twelve metabolites including the major metabolites of trifluralin caused no root tip swelling even at water-saturated concentrations. Two of the minor metabolites, 3-methoxy-2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine (TR-36M) and N-(2,6-dinitro-4-(trifluoromethyl)phenyl)-N-propylpropanamide (TR-40), caused root swelling when applied as a saturated solution. The most effective metabolite, TR-36M, caused root swelling at 10 μM or greater concentrations. Because trifluralin caused swelling of the root tips even at a 0.01 μM concentration, trifluralin is 1000-fold more effective than the most herbicidally active metabolite. Both herbicidally active metabolites are also much less effective than trifluralin at increasing the mitotic index. Structural studies indicate that the two herbicidally active metabolites arrest mitosis at prometaphase due to the loss of spindle microtubules, similar to trifluralin. Cortical microtubules are also affected by the highest concentrations of these two metabolites. A resistant biotype, in which mitosis is unaffected by trifluralin, was unaffected by any of the trifluralin metabolites.

Laboratory studies were conducted to determine the influence of degradation and sorption processes on the dissipation of alachlor in one Colorado soil (Kim clay loam) and three Minnesota soils (Port Byron silt loam, Webster silty clay loam, and Estherville sandy loam) as a function of soil depth. Persistence and movement of alachlor in an irrigated corn production system also were determined on the Kim soil. Laboratory degradation data fit first-order kinetics, and rate constants ranged from 0.0094 to 0.0251 d-1 and varied with soil type and depth. For instance, in 60- to 75-cm-depth Kim soil, alachlor degraded at a slower rate (k = 0.011 d-1) than in surface soil samples (k = 0.022 d-1). Alachlor sorption to the four soils was moderate (Kf = 0.7 to 7.4; Kf,oc = 71 to 470) and concentration dependent (1/n < 1.0). Significant hysteretic desorption of alachlor from soils also was observed (1/n desorption < 1/n sorption). The combined effect of degradation and sorption processes has been used to classify a chemical's potential to leach to groundwater. Based on Kf,oc and dissipation half-life, alachlor would be classified as a “leacher” in Kim, Port Byron, and Estherville soils and classified as transitional between “leacher” and “nonleacher” in the Webster soil. The dissipation first-order rate constant (k) of alachlor in Kim soil in the field was 0.036 α 0.012 d-1. Dissipation was apparently not due to leaching since bromide applied at the same time remained in the top 15 cm during the first 28 d. It appears that laboratory-derived leaching indices may overestimate actual leaching and should be used with caution for predictive or regulatory purposes.

Intact soil columns were evaluated as a screening technique to determine the effect of tillage on herbicide movement through soil. Alachlor was applied at 3.3 kg ai ha-1 to intact surface 0- to 10-cm and subsurface 10- to 20-cm soil columns (15-cm diam) taken from long-term no-till and conventional tillage plots and leached with 11.6 pore volumes (7 L; 39 cm) of water at a rate that did not create ponding. Leachate was collected in 0.07 pore volume fractions. Twice as much alachlor leached from surface no-till than from surface conventional tillage columns. The differences in leaching patterns from the surface soil can be attributed to the effect of tillage on soil physical and chemical properties. Using intact soil columns in the laboratory can be a useful rapid screening technique to evaluate tillage impacts on herbicide movement.

The adsorption of naphthalene by a Bosket loam soil was studied at 6.5, 15, 25, and 37C. Because the solubility and vapor pressure of naphthalene are precisely known over this temperature range, a complete thermodynamic description of the differences between standard vapor, crystal, solution, and soil-adsorbed states is possible. The results indicate that enthalpies and entropies of the solution and surface standard states are quite similar (though very different from the vapor state) resulting in a near-zero standard enthalpy difference and a small standard entropy increase for adsorption from solution. Considering naphthalene as a model for herbicides and as a probe into the nature of the soil surface, the results emphasize the importance of the unique structured nature of water acting as a solvent, and the competition of water for adsorption sites.

Because adsorption is a major factor controlling herbicide persistence, activity, and mobility in soils, an extensive literature exists on the measurement of this process. The adsorption data are usually fitted to the Freundlich equation, but attempts to interpret this equation theoretically have had only limited success. A meaningful thermodynamic interpretation of the Freundlich equation can be developed using a proposed standard state for adsorbed herbicide, which assumes that soil organic matter forms a solid solution with the herbicide. When herbicide vapor pressures and aqueous solubilities are taken into account, the losses in free energy of different herbicides, on adsorption onto wetted soil surfaces, are shown to be similar to each other and are slightly less than the loss on absorption to the solid herbicides themselves. The Freundlich exponent is related in a simple way to the decreasing energy of available soil sites as the amount of herbicide adsorbed increases, but this change is relatively small over the range of concentrations used in adsorption experiments. Sample results are given for six triazine, three urea, and one uracil herbicides.

The effects of twelve soil metabolites of trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] on soybean [Glycine max (L.) Merr.] growth, yield, and seed oil and protein content were determined. The metabolites included oxidative dealkylated, reduced nitro group, benzimidazole, azoxy, azo, oxidized, and hydroxylated derivatives of trifluralin. When applied at 0.22 kg ai/ha and incorporated 7.5 cm deep, no individual metabolite had a significant adverse effect on growth or yield. Three of the metabolites significantly decreased seed oil content and two of the metabolites significantly increased seed protein content. These effects are not considered important in that the differences, while statistically significant, are very small. Also, the amounts of metabolites applied greatly exceeded those expected to be found after many years of continuous use of trifluralin. Potential accumulation of individual metabolites from long-term use of trifluralin does not appear to adversely affect soybean growth, yield, or quality.

Research was conducted to determine the effect of starch encapsulation on soil dissipation and weed control with alachlor, metolachlor, and atrazine on an Estherville sandy loam in the field. Starch encapsulation increased persistence of alachlor in the surface 15 cm of soil compared to the emulsifiable concentrate formulation during the first 60 d after treatment More alachlor was detected 30 to 75 cm deep with emulsifiable concentrate than the starch-encapsulated formulation 30 d after treatment. Little alachlor was detected below 15 cm thereafter. Starch encapsulation also increased persistence of metolachlor in the surface 15 cm, but reduced concentrations at 15 to 30 cm compared to the emulsifiable concentrate 30 d after treatment. By 120 and 340 d after treatment, metolachlor concentrations at 15 to 30 cm were greater with starch-encapsulated than emulsifiable concentrate formulation. Starch encapsulation greatly increased atrazine persistence in the surface 15 cm compared to the dry flowable formulation. Although encapsulation increased atrazine concentration in the surface 15 cm, it reduced the concentration below 15 cm compared to the dry flowable formulation 30 and 60 d after treatment However, by 120 and 340 d after treatment, starch encapsulation often resulted in greater atrazine concentrations below 15 cm than the dry flowable formulation. Control of green foxtail and redroot pigweed with starch-encapsulated herbicides was similar or superior to the commercial formulations.

Irrigated sandy soils have a high potential for leaching of agricultural chemicals to ground water. Objectives of this study were to determine the movement and persistence of metolachlor applied annually to a Hubbard loamy sand soil (Udorthentic Haploboroll) under irrigated potato production. Metolachlor was applied to the plots at rates of 1.7 and 3.4 kg ai ha-1. A KBr tracer was applied to the same plots at the rate of 130 kg ha-1. Intact soil cores were collected before and after herbicide application during the cropping season. Water samples were collected weekly from suction samplers, 90 and 150 cm deep in each plot, during the growing season. Potato yields of 60 and 59 Mg ha-1 in 1989 and 1990, respectively, were above Minnesota state averages. Bromide tracer data confirmed the movement of water through the soil profile to at least a 150-cm depth. No detectable quantities of metolachlor were found below 30 cm in soil during either year. Time for dissipation of 50% of metolachlor ranged from 85 to > 106 d. Dissipation in 1990 appeared slower than in 1989 and residues remaining at the end of the season, averaged across application rates, were 44 and 64% in 1989 and 1990, respectively. No water samples contained metolachlor in 1989. Metolachlor was detected in 18 of 409 water samples collected during and after the growing season in 1990. Median detectable concentration of metolachlor was 0.4 μg L-1 in water samples from suction samplers at 90- and 150-cm depths in eight of 16 plots. Metolachlor Kd and Koc values, 3 and 220, respectively, were similar to reported values. Metolachlor sorption in the soil profile was correlated, in order of contribution, to organic carbon content, cation exchange capacity, percent sand, and percent clay.

A column leaching study was conducted in a greenhouse to determine the movement of atrazine and alachlor through soil as affected by alfalfa roots in three stages of decay. Alfalfa was grown in soil columns for 6 months and then killed with glyphosate. The 14C-atrazine and 14C-alachlor were applied to soil columns with bromide following 2, 13, or 26 weeks of root decomposition. Most of the herbicide remained in the top 9 cm of soil. However, atrazine and alachlor soil distribution profiles indicated greater preferential movement in columns with roots than in columns without roots. Higher levels of atrazine and alachlor were bound to soil at lower depths in the presence of roots than without roots. Adsorption of atrazine and alachlor on soil with and without alfalfa roots was not significantly different as determined by Freundlich adsorption isotherms. Degradation rates for atrazine and alachlor were not substantially different between soil with and without roots. Although only small amounts of the applied herbicide leached through the columns, preferential flow of herbicides through root-mediated soil macropores and cracks could be a mechanism of herbicide transport through soil under appropriate conditions.

Laboratory batch equilibrium studies were conducted to evaluate the sorption-desorption behavior of imazethapyr and its major plant metabolite, 5-hydroxyimazethapyr, in three Minnesota soils. Sorption of both compounds on all soils was low, and pH did not significantly influence sorption in the range of 4,8 to 7.1. Less 5-hydroxyimazethapyr was sorbed than imazethapyr on the same soil. Once sorbed, both compounds were only partially desorbable from all soils. Significant hysteresis and formation of nonextractable residues indicate that the small amount of chemical sorbed is bound to selective surfaces with strong bonds. Webster clay loam had greater irreversibility than Waukegen silt loam and Estherville sandy loam soil. The hysteresis observed in desorption may be responsible for the difference between mobility estimations made from laboratory sorption studies and the limited mobility observed in field experiments.

Field research was conducted at Rosemount, MN in 1990 and 1991 to determine the effect of starch encapsulation on weed control in corn with alachlor, metolachlor, and atrazine under two weed densities. At low weed densities (122 total plants/m2), giant foxtail and Pennsylvania smartweed control were not different with full rates of herbicide applied in starch encapsulated or commercial formulations. Velvetleaf control was less with starch-encapsulated than commercial formulations regardless of velvetleaf density. At high giant foxtail densities (740 plants/m2), control was not affected by formulation with full rates of alachlor plus atrazine. However, giant foxtail control was less with starch-encapsulated formulations than commercial formulations with 75% rates of alachlor plus atrazine and both 75 and 100% rates of metolachlor plus atrazine. At low weed densities, corn yield was not different with any treatment, except starch-encapsulated metolachlor plus atrazine at 75% rates, compared to the weed-free control. Alachlor plus atrazine at full rates was the only starch-encapsulated treatment to result in corn yield not different from the weed-free control at high weed densities. Conversely, all commercial formulation treatments resulted in corn yields not different from the weed-free control at high weed densities.

Adsorption and desorption isotherms for atrazine and two metabolites, hydroxyatrazine (HA) and S-glutathione atrazine (GSHA), were determined by batch equilibration on Plano and Waukegan silt loam soils at two soil pH levels (Plano, 6.1 and 4.5; Waukegan, 6.1 and 4.0). Freundlich adsorption isotherms were not affected by soil type except for GSHA at pH 4.0 to 4.5. When averaged over both soils, the order of adsorption at pH 6.1 was atrazine (Kf = 3.7) < GSHA (Kf = 7.3) << HA (Kf = 25) and at pH 4.0–4.5 was atrazine (Kf = 6.1) << HA (Kf = 58) ≤ GSHA (Kf: Plano = 35; Waukegan = 78). The average slope of the adsorption isotherms (1/nads) was 0.81. The slopes of all desorption isotherms (1/ndes) were less than their respective 1/nads, indicating hysteresis. Atrazine desorbed into soil solution (1/ndes > 0.0). With the exception of GSHA which desorbed from the pH 6.1 Plano silt loam (1/ndes = 0.15), desorption of HA and GSHA from other treatments was negligible (1/ndes = 0.0). Consequently, leaching of HA and GSHA in these and similar soils is not likely, due to high adsorption and low desorption.

Herbicide desorption isotherms may be affected by the amount of nondesorbable herbicide present in soil. Nondesorbable alachlor (as determined after methanol extraction) generally increased on a Waukegan silt loam (Typic Hapludolls) and a Ves clay loam (Udic Haplustolls) during five 0.01 M CaCl2 desorptions. Atrazine was totally extracted with methanol from the Waukegan soil after one desorption using 0.01 M CaCl2. However, after five desorptions with 0.01 M CaCl2 an average of 5.5 and 15.5% of the total recovered atrazine from two atrazine application rates was methanol nondesorbable from the Waukegan and Ves soils, respectively. Freundlich desorption isotherms adjusted for nondesorbable herbicide accounted for as much as 71 % of the difference between adsorption and desorption isotherms. Only a portion of the hysteresis observed can be attributable to nondesorbable herbicide.