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Corn, grain sorghum, and soybean are grown on about six million acres in eastern South Dakota each year. Two herbicides used routinely for weed control are atrazine(6-chloro-N-ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4-diamine) in corn and grain sorghum and alachlor (2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide) in all three crops. Six benchmark soil series that include a majority of the cropped acres treated with these herbicides are the Egan, Moody, Nora, and Brandt silty clay loams, and Clarno and Enet loams. Batch adsorption studies determined atrazine and alachlor binding characteristics to these soils and aids in assessing the amount of herbicide available for movement. These data also provide a basis for future use and management decisions for these and other related herbicides in similar soils. Soils from three horizons (A, B, and C) for each soil type were treated with atrazine or alachlor at four herbicide concentrations. Atrazine and alachlor sorption partition coefficients differed most in A horizon soils and ranged from 2.16 to 5.35 µmol1-1/n L1/nKg-1 for atrazine and 1.95 to 5.78 µmol1-1/n L1/nKg-1 for alachlor. Atrazine binding to A horizon soils ranked as Brandt >Egan = Moody > Enet = Clarno > Nora. Alachlor binding to A horizon soils ranked as Brandt >Moody > Nora > Enet > Clarno. B and C horizon soils had lower binding for both herbicides; the sorption partition coefficient for atrazine ranged from 0.12 to 1.9 µmol1-1/n L1/nKg-1 while alachlor ranged from 0.43 to 1.64 µmol1-1/n L1/nKg-1. These data indicate that some soil types would be more susceptible to herbicide leaching than others. Once the herbicide moves through the A horizon, it may move rapidly through the lower soil profile (because of the decrease in binding capacity), and therefore, increase the vulnerability of the aquifer to contamination. Best management practices for these herbicides are being investigated to limit their movement through soil.

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Proceedings of the South Dakota Academy of Science



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South Dakota Academy of Science