Soil Adsorption of Neutral and Anionic Forms of a Sulfonamide Herbicide, Flumetsulam

Abstract

The binding of a weakly acidic herbicide (N‐2, 6‐difluorophenyl‐5‐methyl‐1,2,4‐triazolo(1,5a)pyrimidine‐2‐sulfonamide) to soil is presented as an ionization of the weak acid in solution, and the partitioning of both the anionic and neutral forms of the compound into the soil organic matter. This theoretical description is combined with experimentally determined bulk adsorption constants to give the organic C adsorption constants (Koc) for neutral (650 ± 40 L kg–1) and anionic (12 ± 1 L kg–1) forms of the molecule. Additional experiments with four soils adjusted to different pH levels gave an average Koc of 750 L kg±1 for the neutral form. Describing the soil sorption as a combination of anionic and neutral forms provided an adequate description of the measured sorption.

Sorption of isoxaflutole by five different soils varying in physical and chemical properties

“…16,27 Experiments were performed with South Deer®eld, MA soil to study the effect of soil pH on the sorption of isoxa¯utole. The soil had a normal pH of 6.5 in a 1:1 ratio with water and, hence, the soil was incubated with 0.01 M calcium chloride and 200 mg litre Isotherms were constructed at pH 4.5, 5.5, 6.5, 7.5, and 8.5 (Fig 2).…”

Yield and injury effects on vegetable crops planted in flumetsulam-treated soil

“…The soil pH at the site (pH 6.1) was considerably lower than at the sites where flumetsulam was applied in 1993 (pH 6.6) and 1994 (pH 6.8). Flumetsulam is more persistent under acid soil conditions (Lehmann et al 1992), as adsorption is greater in low pH soil and this reduces microbial degradation (Fontaine et al 1991). Increased persistence at the 1992 application site may have resulted in more injury and greater yield reductions in sensitive crops planted subsequently.…”

“…The estimated half-life of flumetsulam in the soil ranges from 2 wk to 4 mo, depending on soil OM content and soil pH (Lehmann et al 1992). Soil OM and pH affect adsorption of flumetsulam and so are important factors governing its behaviour in soil (Fontaine et al 1991). Flumetsulam adsorption to soil is highest at low soil pH (pH < 6.3) and high OM (>2.5%) (Lehmann et al 1992).…”

Sensibilidade de cultivares de soja (Glycine max) aos herbicidas diclosulam e flumetsulam

“…Esta poderá ser maior naquelas cultivares de ciclo mais longo, e menor para aquelas de ciclo precoce, pois as primeiras terão um tempo maior para recuperação no caso de um estresse químico. Devido a natureza ácido-fraco do flumetsulam, sua adsorção aos coloídes do solo é fortemente dependente do pH do meio, além dos teores de argila e matéria orgânica (Fontaine, 1991). Por causa de seu pKa (constante de dissociação) de 4,6 o composto existe nas formas neutras e aniônicas na maioria dos solos agrícolas, com mais alta proporção de ânions em pH mais elevado.…”

“…Baseado nestes dados o autor afirma que isto pode ser aplicado a outros compostos com valores de pKa na faixa de pH do solo, como por exemplo o diclosulam. A adsorção dependente do pH para outros herbicidas ácidos foram relatadas por Walker et al (1989), Stougaard et al (1990), no caso das imidazolinonas, e no caso do pentaclorofenol Lee et al (1990), citados por Fontaine (1991), descrevem que a adsorção do pentaclorofenol é dependente das proporções da forma neutra e aniônica.…”