Influence of soil pH‐sorption interactions on the carry‐over of fresh and aged soil residues of imazamox

Long‐term carry‐over of some imidazolinone herbicides has been observed in soil that is below pH 6.5, resulting in significant damage to sugarbeet. In a field study in south‐west Minnesota, imazamox concentration decreased rapidly in soil, regardless of pH. Despite similar amounts of aged soil residues of imazamox remaining at different pH levels, bioavailability differed with pH as a result of pH effects on sorption–desorption interactions. At low pH, more imazamox was sorbed than at high pH, but it was readily desorbed. At high pH, less imazamox was sorbed initially, but it did not readily desorb. Thus, after 3 months, the remaining imazamox in low‐pH soil was desorbable and bioavailable, resulting in injury to oilseed rape and sugarbeet. In aged, low‐pH soil, adding lime released bound imazamox, which was then degraded, resulting in less carry‐over.