SORPTION OF CLOPYRALID, DICAMBA AND MCPA BY 2 SOILS WITH CONVENTIONALAND NO-TILL MANAGEMENT

Sorption is one of the key processes controlling the fate of agrochemi cals in soil systems. The sorption of agrochemicals on a soil matrix c an be affected by changes in chemical and physical properties caused b y different tillage practices. Clopyralid (3,6-dichloro-2-pyridinecarb oxylic acid), dicamba (3,6-dichloro-2-methoxybenzoic acid), and MCPA ( (4-chloro-2-methylphenoxy)acetic acid), weakly acidic herbicides, are commonly used to control broadleaf weeds in crop production. The sorpt ion of the three herbicides by surface soils (0-5 cm) from different t illage systems was studied by batch equilibrium. Of the herbicides use d, MCPA was sorbed on soil in the greatest quantity, dicamba was the l east sorbed, and the sorption was dependent on the pK(a) values of her bicides. In a sandy loam soil, more herbicide was sorbed under no-till age (NT) than conventional tillage (CT). However, in a clay soil the d ifference in herbicide sorption between the two tillage systems was no t consistent. The results can be explained in terms of changes in soil pH and organic matter content. The presence of clopyralid and MCPA lo wered the sorption of dicamba, and MCPA was more effective than clopyr alid in competing for sorption sites with dicamba.