Sorption of Atrazine, Terbutryn, and GS-14254 on Natural and Lime-Amended Soils

Adsorption of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)- s -triazine], terbutryn [2-( tert -butylamino)-4-(ethylamino)-6-(methylthio)- s -triazine], and GS-14254 [2-( sec -butylamino)-4-(ethylamino)-6-methoxy- s -triazine] decreased on natural and limed soils as the soil pH increased to pH 8. More reduction in adsorption occurred with GS-14254 than with the other two herbicides. Increased toxicity to barley from GS-14254 was proportional to decreased adsorption by soils. Adsorption-desorption studies with Chehalis sandy loam (Cumulic Ultic Haploxeroll) indicated that extent and strength of adsorption were equivalent for GS-14254 (pKa 4.36) and terbutryn (pKa 4.32) at a soil pH of 5.20. Two soils, Boyce silt loam (pH 9.6, Cumulic Haplaquoll) and Crooked silt loam (pH 8.5, Xerolic Durorthid), adsorbed the s -triazines readily in contrast to the soils below pH 8. The high adsorption values were substantiated by the GR 5 0 values on barley for atrazine and terbutryn, 10.2 and 13.2 kg ai/ha, respectively.