Search
Search Results
-
The physical and chemical properties of soils greatly affect the toxicity of bromacil and picloram. Consequently, a knowledge of the parameters which influence the soil inactivation of these herbicides is ...
Citation Citation
- Title:
- Surfactant effects on bromacil and picloram adsorption by Oregon soils
- Author:
- Gaynor, John Donald
The physical and chemical properties of soils greatly affect the toxicity of bromacil and picloram. Consequently, a knowledge of the parameters which influence the soil inactivation of these herbicides is necessary to determine proper application rates for good weed control. Enhancement in phytotoxicity with the addition of surfactants or surface active agents in the spray volume has been observed for these chemicals; however, little is known concerning the effects of these additives on picloram and bromacil adsorption. The objectives of this research are: 1) to evaluate the effects of cation saturation on bromacil adsorption by montmorillonite, 2) to determine the effects of pH and organic matter on the adsorption of bromacil and picloram by soils and 3) to determine the effects of dilute solutions of four surfactants on the adsorption of bromacil and picloram by soils. The adsorption of 2-¹⁴C-labeled bromacil on Ca²⁺, Cu²⁺ and Al³⁺ saturated montmorillonite was determined after equilibration a. 0° and 25°C. Seven concentrations of 2-¹⁴C-labeled bromacil and ¹⁴C-carboxyl labeled picloram were equilibrated without surfactant with nine Oregon soils at 25°C. Labeled bromacil and picloram at 5 ppm and 10 ppm concentration in 1% and 10% surfactant solution were also equilibrated at 25°C with eight of the above soils. After equilibrium the montmorillonite and soil samples were centrifuged and analyzed on a liquid scintillation counter for bromacil and picloram remaining in solution. The adsorption of bromacil at 25° and 0°C for each cationic saturated montmorillonite and at 25°C by the nine Oregon soils increased with concentration according to the Freundlich equation, Bromacil adsorbed on the montmorillonite increased with cation saturation in the order Ca²⁺ < Cu²⁺ < Al³⁺. The overall adsorption of bromacil was reduced when the temperature was increased to 25°C. Bromacil adsorbed on the cation saturated rnontmorillonite may occur by complexation of the Cu²⁺ or Al³⁺ cations with the ring N at the one position of the bromacil or by induction through a pH effect. The increased adsorption at the lower temperature may be due to a change in water solubility or adsorption energy. The equilibration of bromacil with the nine Oregon soils indicated organic matter was the most important parameter affecting adsorption. A highly significant correlation coefficient of 0.913 was calculated between the Freundlich K value for bromacil adsorption and organic matter. Bromacil adsorption in the low organic matter soils appeared to be directly correlated with an increase in exchangeable aluminum or pH as observed in the montmorillonite study. However, the wide range in organic matter at the intermediate level limited the opportunity to correlate bromacil adsorption with aluminum or pH. The adsorption of bromacil increased in the presence of the 1% cationic and anionic surfactants and decreased in the presence of the 1% nonionic surfactant solution compared to that without surfactant. Regardless of surfactant type the 1% surfactant solutions greatly decreased the variable effects of organic matter on bromacil adsorption in the Minam and Kinney soil series but not the Woodcock soil series. Bromacil adsorption from the 10% concentration of each surfactant type was greatly reduced compared to adsorption from the 1% surfactant solution. Strangely, the adsorption of bromacil from surfactant solution was always less in the Woodcock soil series than in the Minam and Kinney soil series. Picloram adsorption increased with concentration on the nine soils according to the Freundlich equation. A significant negative correlation coefficient of 0.667 was calculated between the Freundlich K value for picloram adsorption and pH; however, the interaction between pH and exchangeable aluminum could not be isolated. Complexation of picloram with exchangeable aluminum and adsorption of the molecular species by organic matter appeared to be the principle modes of adsorption. The adsorption of picloram by soils increased in the presence of the 1% cationic and nonionic surfactant solutions and decreased in the 1% anionic surfactant solution compared to that without surfactant. Adsorption in the presence of 1% cationic surfactant solution occurred from the molecular and ionic species as indicated by equal adsorption in the Minam (pH = 7) and Kinney (pH = 5) soil series. The 10% cationic surfactant solution decreased adsorption from that in the 1% surfactant solution; however, an inverse relationship between organic matter and adsorption was evident in the Minam and Kinney soil series. Adsorption from the nonionic surfactant solution was independent of surfactant concentration but decreased as pH increased. The 1% anionic surfactant solution effectively competed with picloram for adsorption sites. However, the 10% surfactant solution increased adsorption from that in the 1% surfactant solution possibly due to the creation of adsorption sites for picloram by the adsorbed surfactant. Increased phytotoxicity of bromacil and picloram has been observed by others when a nonionic surfactant was introduced into the spray solution. The results of this study indicate that part of the increased toxicity may be due to reduced soil adsorption of the chemicals in anionic and nonionic surfactant solutions. The addition of a cationic surfactant, on the other hand, may increase adsorption and provide poor weed control at recommended rates. Further investigation is required to determine if the herbicides adsorbed by the surfactant are available for root uptake and if the soil applied surfactant is phytotoxic.