Shang, C. and Arshad, M. A. 1998. Sorption of clopyralid, dicamba and MCPA by two soils with conventional and no-till management. Can. J. Soil Sci. 78: 181–186. Sorption is one of the key processes controlling the fate of agrochemicals in soil sys-tems. The sorption of agrochemicals on a soil matrix can be affected by changes in chemical and physical properties caused by different tillage practices. Clopyralid (3,6-dichloro-2-pyridinecarboxylic 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 sorption of the three herbicides by surface soils (0–5 cm) from different tillage systems was studied by batch equilibrium. Of the herbicides used, MCPA was sorbed on soil in the greatest quantity, dicamba was the least sorbed, and the sorp-tion was dependent on the pKa values of herbicides. In a sandy loam soil, more herbicide was sorbed under no-tillage (NT) than conventional tillage (CT). However, in a clay soil the difference in herbicide sorption between the two tillage systems was not consistent. The results can be explained in terms of changes in soil pH and organic matter content. The presence of clopyralid and MCPA lowered the sorption of dicamba, and MCPA was more effective than clopyralid in competing for sorption sites with dicamba.
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