Jerome B. Weber

Mechanisms of adsorption of s-triazines by clay colloids and factors affecting plant availability.

Many new organic agricultural chemicals have been developed which offer great potential in the production and preservation of food and fiber (Weber 1969). In the process of developing and utilizing these chemicals we must come to understand how they affect the target organisms and what becomes of them after they have done their job. The purpose of this review is to discuss1 the behavior of one family of these organic compounds, the s-triazines, at the molecular level, in systems in which the chemicals are associated with clay minerals. Factors affecting the adsorption and release of s-triazines by clay colloids and the availability of the compounds to plant roots will also be included. The literature citations are limited to those studies in which s-triazines and clays were directly associated and situations in which other compounds behaved in a manner similar to the s-triazines.

Fate of polychlorinated biphenyls (PCBs) in soil-plant systems.

Polychlorinated biphenyls (PCBs) are a class of chlorinated aromatic hydrocarbons which are thermally and chemically very stable. The PCBs represent a mixture of specific biphenyl hydrocarbons with varying degrees of chlorination. Substitution for hydrogen on the ring structure of biphenyl by chlorine gives rise to a number of compounds and isomers with 209 possible combinations (HUTZINGER et al.1974). The nomenclature of chlorobiphenyls is based on the position and extent of substitution on the biphenyl ring structure as shown in Figure 1. The available sites for chlorination are 2 to 6 in ring A and 2′ to 6′ in ring B. Thus, chlorobiphenyls may carry 1 to 10 chlorine atoms, depending on the degree of chlorination. Usually in commercial preparations, different mixtures of chlorobiphenyls are produced rather than a single pure compound. Typical characteristics of polychlorinated biphenyls (PCBs) are high thermal and chemical stability, low vapor pressure, high dielectric constant, high electric resistivity, high density, substantially hydrophobic, and high lipophilicity. With increasing chlorination from 18.6% to 80% these properties are accentuated. Molecular weights of PCBs range from 188 for monochlorobiphenyl to 494 for decachlorobiphenyl. Their melting points range from 34° to 198°C and boiling points are usually 267°C. Appearances of PCBs range from clear mobile oils to light yellow, sticky, solid resins. PCBs are quite soluble in nonionic surfactants such as ethylene oxide, Tween 201 and Tween 80.

A proposal to standardize soil/solution herbicide distribution coefficients

Abstract Herbicide soil/solution distribution coefficients (Kd) are used in mathematical models to predict the movement of herbicides in soil and groundwater. Herbicides bind to various soil constituents to differing degrees. The universal soil colloid that binds most herbicides is organic matter (OM), however clay minerals (CM) and metallic hydrous oxides are more retentive for cationic, phosphoric, and arsenic acid compounds. Weakly basic herbicides bind to both organic and inorganic soil colloids. The soil organic carbon (OC) affinity coefficient (Koc) has become a common parameter for comparing herbicide binding in soil; however, because OM and OC determinations vary greatly between methods and laboratories, Koc values may vary greatly. This proposal discusses this issue and offers suggestions for obtaining the most accurate Kd, Freundlich constant (Kf), and Koc values for herbicides listed in the WSSA Herbicide Handbook and Supplement. Nomenclature: Readers are referred to the WSSA Herbicide Handbook and Supplement for the chemical names of the herbicides.

Biological activities of 2,4-dinitrophenol in plant-soil systems

2,4-Dinitrophenol (DNP)is a phenol prepared by alkaline hydrolysis of 2,4-dinitro-1-chlorobenzene which in turn is prepared from the nitration of monochlorobenzene (Hartford 1973). Alternative routes of preparation are by nitration of monochlorobenzene (Hartford 1973). Alternative routes of preparation are by nitration of benzene with NO2 and mercurous nitrate or by the oxidation of m-dinitrobenzene. Pure DNP is a solid of yellowish to yellow orthorhombic crystals, with molecular weight 184.11, density 1.683 g/ml, and melting point of 115° to 116°C (Windholz 1976). It has a water solubility of 6.0 g/L at 25°C (Morrison and Boyd 1973). DNP is soluble in most organic solvents and essentially nonvolatile, but does sublime at temperatures above its melting point (Windholz 1976). The compound is moderately acidic, with a pKA of 4.09 and ionizes as shown in Figure 1 (Pearce and Simpkins 1968). At pH 2.6, DNP is colorless but becomes yellow in solution at pH 4.4 and hence has been used as an indicator (Windholz 1976). DNP can also be used as a reagent to detect potassium and ammonium ions.

Availability of a cationic herbicide adsorbed on clay minerals to cucumber seedlings.

Montmorillonitic and kaolinitic clays are effective in decreasing the toxicity of paraquat, an organic cation, to cucumber plants. The cation was adsorbed on the surface of the kaolinite clay particles and slowly became available to the plants. When it was adsorbed in the interlayer spacings of the montmorillonite clay, however, it was not available to the plants.

Sorption of diniconazole and metolachlor by four soils, calcium-organic matter and calcium-montmorillonite

Diniconazole was sorbed by four soils in amounts 7 to 20 times that for metolachlor, by Ca-organic matter (Ca-OM) in amounts twice that for metolachlor, and by Camontmorillonite (Ca-Mont.) in amounts similar to metolachlor. Sorption of diniconazole greatly increased and desorption decreased from Ca-OM and Ca-Mont as solution pH decreased. Sorption by the soils, as indicated by Freundlich K values, were highly correlated with the organic carbon content of the soils for diniconazole and with organic carbon and clay contents of soils for metolachlor. Sorption of diniconazole was through physical sorption forces at neutral pH levels and was by way of cation exchange forces at low pH levels.

Sorption of Simazine and S-Metolachlor to Soils from a Chronosequence of Turfgrass Systems

Abstract Pesticide sorption by soil is among the most sensitive input parameters in many pesticide-leaching models. For many pesticides, organic matter is the most important soil constituent influencing pesticide sorption. Increased fertility, irrigation, and mowing associated with highly maintained turfgrass areas result in constant deposition of organic material, creating a soil system that can change drastically with time. Changes in soil characteristics could affect the environmental fate of pesticides applied to turfgrass systems of varying ages. Sorption characteristics of simazine and S-metolachlor were determined on five soils from bermudagrass systems of increasing ages (1, 4, 10, 21, and 99 yr) and compared to adjacent native pine and bare-ground areas. Surface soil (0 to 5 cm) and subsurface soil (5 to 15 cm) from all sites were air-dried and passed through a 4-mm sieve for separation from plant material. Using a batch-equilibrium method, sorption isotherms were determined for each soil. Data were fit to the Freundlich equation, and Kd (soil sorption coefficient) and Koc (organic carbon sorption coefficient) values were determined. Sorption and soil system age were directly related to organic matter content in the soil. Sorption of both herbicides increased with age of the soil system and was greatest on the surface soil from the oldest bermudagrass soil system. Herbicide sorption decreased at greater soil depths with lower organic matter. Greater amount of 14C–simazine sorbed to subsurface soil of the oldest turfgrass system compared to 14C–S-metolachlor. Results indicate that as bermudagrass systems age and accumulate higher organic matter levels increased herbicide sorption may decrease the leaching potential and bioavailability of simazine and S-metolachlor. Nomenclature: Simazine; S-metolachlor; bermudagrass; Cynodon dactylon [(L.) Pers.].

Evaporative effects on mobility of 14C-labeled triasulfuron and chlorsulfuron in soils

A laboratory soil column experiment was conducted to determine the mobility of 14 C-triasulfuron in A horizon material of Farnum loam, fine-loamy, mixed, thermic Pachic Argiustoll from Kansas, Norfolk loamy sand, fine-loamy, siliceous, thermic Typic Hapludult, and Rion sandy loam, fine-loamy, mixed, thermic Typic Hapludult from North Carolina, and Webster silt loam, fine-loamy, mixed, mesic Typic Haplaquoll from Iowa as well as 14 C-chlorsulfuron in Rion sandy loam and to measure the effects of evaporation on capillary transport of the leached herbicides upward when soil columns were in contact with free water and not over free water. Triasulfuron mobility was in the order Norfolk (R f = 0.52) = Rion (R f = 0.48) > Farnum (R f = 0.40) > Webster (R f = 0.26), and R f was inversely related to the organic and humic matter contents of the soils. Evaporation of water from the soil surface of leached triasulfuron-treated Rion and Norfolk soils over a 2-week period had pronounced effects on herbicide transported upward, particularly when soil columns were in contact with free water simulating a shallow water table. Evaporation had no effects on capillary transport upward in Farnum and Webster soils, probably because of their higher contents of organic and humic matter. Chlorsulfuron was 18% more mobile than triasulfuron in Rion sandy loam, and both herbicides were distributed in a similar pattern in the soil when water was applied to the surface and leached or applied to the base of the columns and transported upward by capillary action. Redistribution of the herbicides upward in capillary water will likely influence persistence of the chemicals, particularly in soils over shallow water tables.

Temporal distribution of 14C in soil water from field lysimeters treated with 14C-metolachlor

In a previous study utilizing fallow field lysimeters of an undisturbed, loamy sand soil treated with 14 C-metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide], 2 to 5% of the applied 14 C was mobile to soil depths of 56 to 96 cm. The objective of this 120-day study was to determine the temporal distribution of 14 C-metolachlor and/or metabolite(s) in soil water from similar field lysimeters and their possible contribution to groundwater contamination. Undisturbed soil column field lysimeters (20.3-cm i.d. × 101-cm long; 16 gauge steel) were driven into a conventionally tilled Dothan loamy sand (fine-loamy, siliceous, thermic Plinthic Kandiudult) and treated with 14 C-metolachlor and tritiated water ( 3 H 2 O) and subjected to natural rainfall or irrigation. Percent recovery of metolachlor and/or metabolite(s) in the soil, as based on 14 C measurement, was 62% at 30 days, 63% at 60 days, 51% at 90 days, and 49% at 120 days. Recovery of 3 H 2 O was 36, 24, 6 and 0.25% of the applied for the same time periods. By 30 and 60 days after application (DAA), 3 H 2 O had distributed symmetrically in the soil profile, whereas, a large percentage of the 14 C was retained in the upper 24 cm. No 14 C and <1% of the applied 3 H 2 O was recovered in leachate the first 30 days. Cumulative recovery of 14 C in leachate was <1% of that applied at 60 days, 3% at 90 days, and 7% at 120 days. Cumulative recovery of 3 H 2 O in leachate for the same time periods was 22, 39, and 39% of that applied. The symmetrical breakthrough curve for 3 H 2 O indicated no preferential flow or immobile water, whereas the breakthrough curve for 14 C was asymmetrical as a result of the sorption-desorption processes. Peak concentrations of 14 C and 3 H 2 O in the leachate occurred at 94 and 63 DAA, respectively. The sorptive tendencies of both radiolabeled species distinguished the magnitude of movement, with 3 H 2 O much more mobile than 14 C-metolachlor and/or metabolite(s). Assuming that all 14 C in leachate was parent, average metolachlor concentrations in leachate were less than the National Health Advisory level, which may indicate that metolachlor should be considered a low risk chemical because of its potential to contaminate groundwater in soils with low organic matter and high clay content in the subsoil.

Soil Properties Influence Saflufenacil Phytotoxicity

Abstract Saflufenacil, a pyrimidinedione herbicide, is used for contact and residual broadleaf weed control in various crops. Bioactivity of saflufenacil in soil was tested in greenhouse and laboratory studies on 29 soils representing a wide range of soil properties and geographic areas across the United States. A greenhouse bioassay method was developed using various concentrations of saflufenacil applied PPI to each soil. Whole canola plants were harvested 14 d after treatment, and fresh and dry weights were recorded. Nonlinear regression analysis was used to determine the effective saflufenacil doses for 50% (ED50,), 80% (ED80), and 90% (ED90) inhibition of total plant fresh weight. Bioactivity of saflufenacil in soil was strongly correlated to soil organic (R  =  0.85) and humic matter (R  =  0.81), and less correlated to cation exchange capacity (R  =  0.49) and sand content (R  =  −0.32). Stepwise regression analysis indicated that organic matter was the major soil constituent controlling bioactivity in soil and could be used to predict the bioactivity of saflufenacil. Saflufenacil phytotoxicity was found to be dependent on soil property; therefore, efficacy and crop tolerance from PRE and PPI applications may vary based on soil organic matter content and texture classification. Nomenclature: Saflufenacil; canola, Brassica napus L.