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Soil Sorption and Alfalfa Uptake of Strychnine¹

ABSTRACT

Strychnine (C₂₁H₂₂N₂O₂) sorption on soils and uptake in alfalfa (Medicago sativa L.) were investigated. Using application rates of 52–1040 mg/L strychnine, sorption isotherms were developed on three Nevada soils. Strychnine was found to sorb strongly to the three soils studied; however, isotherms were all nonlinear and the extent of sorption varied with each soil. Desorption characteristics paralleled sorption isotherms, indicating reversibility. Strychnine extraction with ammonium chloride (NH₄Cl) solutions was greater than that obtained with water extraction on all soils. An apparent relationship between specific surface area, cation exchange capacity (CEC), and total strychnine sorption was noted. Strychnine movement through soil columns under saturated flow was also investigated for the three soils. These results were similar to the sorption studies in that the soil with the lowest sorptive capacity also required the fewest pore volumes for the eluting strychnine concentration to equal that of the starting concentration. The binding mechanism of strychnine to soil is suggested to be a combination of cation exchange and surface absorption to the organic and inorganic fractions of soil. Potential uptake of strychnine in alfalfa was investigated by applying treated milo (Sorghum vulgare var. subglabrescens) bait to the base of alfalfa plants at rates that exceeded normal rodent treatments. Strychnine was not detected (detection limit 0.03 mg/kg) in whole alfalfa samples taken 4 and 11 d following treatment. The data indicate that strychnine is relatively immobile in soil systems, particularly those of moderate-to-high CEC, and is not expected to leach rapidly nor be significantly taken up by plants.

Key Words: strychnine mobility • cationic pesticide • agricultural rodenticide

¹ This paper is a contribution of the Nevada Experiment Station, Max C. Fleischman College of Agriculture, University of Nevada, Reno, NV 89557. Journal Series no. 615.

² Associate Professor of Integrated Pest Management, Associate Professor of Soil & Water Science, Graduate Fellow, and Laboratory Technician, respectively.

Received for publication September 2, 1982.