Sorption Characteristics of Atrazine and Imazethapyr in Soils of New Zealand: Importance of Independently Determined Sorption Data

We investigated sorption characteristics of two commonly used herbicides, atrazine and imazethapyr, in 101 soils with allophanic and non-allophanic clays of New Zealand using the batch equilibration technique. Soil properties, such as organic carbon (OC) content, texture, pH, amount and type of clay, and cation-exchange capacity (CEC), were tested against the sorption coefficients (K(d)) of these herbicides. There was a wide variation in the sorption affinities of the soils, as the K(d) values of atrazine and imazethapyr ranged from 0.7 to 52.1 and from 0.1 to 11.3 L kg(-1), respectively. For atrazine, the sorption affinities for the allophanic set of soils (mean K(d) of 8.5 L kg(-1)) were greater than for the non-allophanic set of soils (mean K(d) of 7.5 L kg(-1)). However, no effect of allophanic status was found for imazethapyr sorption (mean K(d) of 0.82 and 0.76 L kg(-1) for allophanic and non-allophanic, respectively). None of the measured soil properties could alone explain adequately the sorption behavior of the herbicides. The variation of OC soil sorption coefficients, K(oc), was also larger for atrazine (mean K(oc) of 126.9 L kg(-1)) than for imazethapyr (mean K(oc) of 13.2 L kg(-1)). The prediction equations for atrazine and imazethapyr developed overseas failed to provide the acceptable values of sorption coefficients for the soils of New Zealand. The study highlights the danger of using sorption coefficient data from the literature for practical assessments of the herbicide leaching in New Zealand soils.