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Greenhouse and field studies demonstrated that plants compete for soil-applied atrazine (2-chloro4-(ethylamino)-6-(isopropyl- amino)-s-triazinel. Oats (Avena sativa L. 'Neal') and soybeans (Glycine max (L.) Merr. 'Clark 63') were grown in greenhouse bioassay studies to study the effects of plant population on bioassay sensitivity. High plant populations were not as effective as low plant populations in detecting low levels of atrazine in soil. Conversely, by increasing plant populations or decreasing soil volumes quantitative measurement of higher atrazine concentrations can be determined. A lC-labeled atrazine study showed that by increasing soybean populations from one to six per pot, atrazine uptake per plant was decreased 50%. When soybeans were planted at a rate of six plants per 100 cm2 in a field treated with atrazine at 1.1 kg/ha, soybean dry weight production was 97% of the untreated control; when planted at a rate of one plant per 100 cm2, dry weight was only 26% of the comparable untreated check. Thus, herbicide rates for optimum weed control may have to be increased with increasing weed populations or increased crop seeding rates or both. 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 ppmw except where noted on specific experiments. The bioassay crops were grown in 10 cm tall polyethylene pots in a fiberglass greenhouse maintained between 22 and 31 C. Supplementary lighting provided a day length of 16 h in the range of 0.4 to 0.6 microeinsteins intensity. Pots were watered by alternate surface watering and subirrigation. The plant yields were determined after harvesting the above ground portion and oven drying 48 h at 80 C. The experimental designs, except where noted, were ran- domized complete block with four replications. The data were

The adsorption of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] was measured on soil at water:soil ratio of 5:1 and 0.4:1. The adsorption isotherms for atrazine were described by the Freundlich equation. A decrease in water:soil ratio and in soil moisture content led to an increased adsorption of the herbicide. Increasing the concentration of electrolytes in the soil solution by adding solutions up to 0.1M CaCl2increased atrazine adsorption. Greater amounts of atrazine were adsorbed at 30 than at 5 C on four soils at 0.1 bar moisture content. This indicates an endothermic reaction which was observed both before and after correction for differential atrazine solubility due to temperature. Thermodynamic quantities associated with the adsorption reaction were calculated to characterize the adsorption of atrazine on soil.

Microbial degradation of imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} was monitored by measuring14CO2evolution for 7 months under controlled laboratory conditions. Up to 10% of the14C chain-labeled imazaquin that was applied to a Crowley silt loam was evolved as14CO2in 7 months. Less evolution of14CO2occurred on a Sharkey silty clay, a soil with higher clay and organic matter content, than on silt loam soils. The loss of 66 to 100% of the imazaquin applied to a Crowley silt loam incubated for 8 months at 18 C or 35 C, respectively, suggested that metabolic changes in addition to CO2evolution were occurring. Rapid loss of imazaquin phytotoxicity occurred when soils were held at warm-moist (35 C and −33 kPa) conditions conducive to microbial growth. Imazaquin was more persistent in soils stored under cool, dry (18 C and −100 kPa) conditions. Imazaquin on a soil surface dissipated rapidly when exposed to ultraviolet light or sunlight. Photodecomposition could be a major mode of imazaquin dissipation if this herbicide is allowed to remain on the soil surface.

Root and shoot extracts of 3-week-old sudangrass [Sorghum sudanense(Piper) Stapf, var. Piper], grain sorghum [Sorghum bicolor(L.) Moench], and corn (Zea maysL.) plants degraded 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) in order: shoot > root and corn ≫ sorghum = sudangrass. In 3-week-old detopped plants, the rate of atrazine exudation was 14 times greater in sudangrass and sorghum than in corn when grown in Keith sandy loam containing 0.5 ppmw14C-atrazine. Extraction and analysis of plant shoots revealed that 7 to 8% of the14C was present as atrazine in sudangrass and sorghum whereas no atrazine was found in corn. In14C tracer studies, thin-layer chromatography showed that sudangrass and sorghum metabolized atrazine by a pathway which differed from the pathway in corn. Sudangrass and sorghum metabolized atrazine primarily to 2-chloro-4-amino-6-(isopropylamino)-s-triazine and 2-chloro-4-amino-6-(ethylamino)-s-triazine which are only partially detoxified compounds. Corn metabolized atrazine to 2-hydroxy-4-(ethylamino)-6-(isopropylamino)-s-triazine (hydroxyatrazine) which is non-phytotoxic.

Degradation of 2-chloro-4-ethylamino-6-isopropyl-s-triazine (atrazine) was studied in the 0 to 9, 14 to 24, and 36 to 48-inch depths of Sharpsburg silty clay loam and Keith silt loam soils. The effects of moisture, temperature, and microorganisms on degradation were observed. Atrazine was degraded two to three times faster in the topsoils than in the subsoils. Each 10 C temperature increase from 15 to 35 C caused the degradation rate to increase two to three times in these soils. Increasing the moisture content of the soils from 0.4 to 0.8 field capacity caused 14CO2 evolution from chain-labeled atrazine-14C treated soils to increase zero to six times. At 25 C and 0.8 field capacity, 100 to 194 atrazine molecules were detoxified for each molecule evolved as 14CO2 from chain-labeled atrazine. Solution culture studies showed cleavage of the side chain and ring of atrazine-14C except in the 14 to 24 and 36 to 48-inch depths of Keith silt loam.

The uptake of 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) was studied in sudangrass [Sorghum sudanense(Piper) Stapf, var. Piper], grain sorghum [Sorghum bicolor(L.) Moench], and corn (Zea maysL.) to find whether differences in atrazine uptake exist among these species. The uptake of atrazine followed closely the growth patterns of corn, sorghum, and sudangrass during the first 5 weeks of growth. Concentration of14C from ring-labeled14C-atrazine in the soil reached a peak in corn, sorghum, and sudangrass plants after 2 weeks of growth and then declined. The14C concentrations were two to three times greater in sorghum and sudangrass than in corn throughout the 5-week period. Atrazine uptake per gram of plant growth by these crops was directly proportional to the concentration of atrazine in the soil and the proportionality factors were in the order: sudangrass < sorghum ≫ corn. Total uptake and the total growth were in order: corn ≫ sorghum = sudangrass. In a study where relative humidity was a variable, the amount of atrazine absorbed per ml of water was inversely related to total water usage.

In laboratory studies with 3-amino-2,5-dichlorobenzoic acid (amiben), adsorption of 14C-amiben was less than 14C-amiben methyl ester with all adsorbents tested. Adsorption of 14C-amiben was closely associated with soil organic matter. Also, considerable adsorption of 14C-amiben and 14C-methyl ester was observed on illite clay and on Sassafras loamy sand which contains clay minerals composed primarily of illite clay. Root uptake of amiben caused more injury than shoot uptake to greenhouse-grown corn (Zea mays L., var. Nebraska 202). The relatively insoluble butoxy ethyl ester of amiben was the most toxic amiben derivative to greenhouse-grown corn. Subirrigation moved the more soluble amiben derivatives to the soil surface where they had less effect on the root system. In field studies comparing four derivatives of amiben applied preemergence to corn in 1966, best weed control was from the ammonium salt of amiben at 2 and 4 lb/A and the methyl and butoxy ethyl ester at 4 lb/A; the amide was ineffective. The ammonium salt of amiben caused early stunting of corn but did not reduce yield. Selective weed control in soybeans [Glycine max (L.) Merr., var. Clark] was obtained in 1966 with the ammonium salt, 80% wettable powder, and monomethyl amine salt of amiben at 1 1/2 lb/A, and the methyl and butoxy ethyl ester at 6 lb/A. In 1967, the best amiben derivatives for weed control in soybeans were the amide and esters. Amiben derivatives more susceptible to leaching were less effective in controlling weeds in 1967 due to excessive rainfall after amiben applications.

A liquid scintillation counting solution was developed which optimized the direct assay efficiencies of14C-labeled 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), 3,6-dichloro-o-anisic acid (dicamba), and α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) in soil. In this counting technique, 3-g soil samples containing14C-labeled herbicide were placed directly into a liquid scintillation vial containing a phosphorescent counting solution. Samples were shaken and allowed to settle before assaying using a liquid scintillation spectrometer. This direct radioassay was as efficient or more efficient than double or single extraction processes. Automatic external standardization in conjunction with the liquid scintillation counter was an effective tool for estimating the14C present in soil samples.

Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triaazine] diffused at a rate of 15.2 × 10−8sq cm/sec at 25 C which was faster than the diffusion rate of 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) and 2-chloro-4,6-bis(isopropylamino)-s-triazine (propazine) in the same soils. Diffusion rates were highly correlated with total surface area of the soil. Increasing soil moisture content, soil pH, and soil temperature resulted in greater diffusion rates. A relative indication of herbicide mobility may be obtained from some of the physical and chemical properties of the soils and herbicides involved; however, rate of diffusion appears to be governed by a combination of soil and herbicide properties.

Phytotoxicity of α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin), incubated for 10 months in Sharpsburg silty clay loam at 0.8 field capacity, remained a 6.4, 0, and 1.6 ppmw of the original 8 ppmw at 15, 25, and 35 C, respectively. In Anselmo sandy loam after 10-months incubation, trifluralin phytotoxicity remained at 3.0, 3.2, and 0.6 ppmw of the original 4 ppmw at 15, 25, and 35 C, respectively. Breakdown of 14C-trifluralin to 14CO2 in both soils was more rapid at 1.6 field capacity than at 0.8 field capacity, and at 1 ppmw than at 100 ppmw of trifluralin. Breakdown of 14C-trifluralin to 14CO2 accounted for 5 and 3% of the decrease of 14C-activity in Sharpsburg silty clay loam and Anselmo sandy loam, respectively.

Dissipation of 2-methoxy-3,6-dichlorobenzoic acid (dicamba) was greater in Sharpsburg silty clay loam soil than in Anselmo sandy loam soil and was faster in the topsoil than in the subsoil. Breakdown increased with increasing soil incubation temperatures. Phytotoxicity of dicamba and 3-amino-2,5-dichlorobenzoic acid (amiben) in aqueous solutions decreased upon exposure to sunlight. Amiben was more susceptible to photodecomposition than was dicamba. Aqueous dicamba solutions exposed to sunlight for 16 days decreased growth of cucumber (Cucumis sativus L.) seedlings, while amiben solutions caused no growth reduction after 2 days' sunlight exposure. Dicamba solutions exposed to 60 C showed increased phytotoxicity while amiben solutions were unaffected. Dicamba was most phytotoxic to corn (Zea mays L.) seedlings at pH 4 and to cucumber seedlings at pH 5. Dicamba solutions were taken up by both shoots and roots of corn and cucumber seedlings; however, root uptake resulted in greater growth reductions.