ADSORPTION OF ATRAZINE, SIMAZINE, AND GLYPHOSATE IN SOILS OF THE GNANGARA-MOUND, WESTERN-AUSTRALIA

Sandy soils were sampled from second rotation sites of Pinus pinaster Ait. on the Gnangara Mound in Western Australia. Adsorption isotherms were measured for atrazine 2-ethyl-N-4-isopropyl-1,3,5-triazine-2,4-di amine], simazine 6-chloro-N,N'-diethyl-1,3,5-triazine-2,4-diamine], an d glyphosate [N-phosphonomethyl-aminoaceticacid]. Adsorption isotherms were also measured for degradation products of atrazine: hydroxyatraz ine -2-ethyl-N-4-isopropyl-1,3,5-triazine-2,4-diamine] (HA), desethyla trazine [6-chloro-N-isopropyl-1,3,5-triazine-2,4-diamine] (DEA); and o f glyphosate: aminomethylphosphonic acid (AMPA). The adsorption of the 2 triazines was proportional to soil organic carbon content and was n ot affected significantly by other soil parameters. The affinity for s oil organic carbon was in the order atrazine = simazine = DEA > HA. Af finity of atrazine for the type of organic matter in the Gnangara Moun d soils (expressed as K-oc) was significantly greater than is commonly reported for other soils. The adsorption of glyphosate and AMPA incre ased strongly with iron and aluminium content of soils and decreased w ith increasing soil organic carbon content. This would indicate that g lyphosate and AMPA are mainly adsorbed by clay minerals, while soil or ganic matter competes for adsorption sites and inhibits adsorption. Co ntrary to what is usually reported for batch adsorption of pesticides in soils, significant increases in adsorption of the triazines and gly phosate were measured after 1 day of equilibration.