We investigated the sorption of five pesticides (metsulfuron methyl, atrazine, 2,4-D, phorate, and terbufos) in a range of allophanic and non-allophanic soils of New Zealand, using surface and subsoil horizons from 10 soil series. Sorption of pesticides was measured by a batch equilibrium technique using C-14-labelled pesticides. The effect of soil properties on pesticide sorption was also examined. Sorption of pesticides was adequately described by the Freundlich equation with an R(2) value > 0.97. The value of the exponent in the fitted Freundlich equation for the pesticide sorption varied from 0.79 to 0.98. The pesticide sorption, as measured by the distribution coefficient (K-d, sorption per unit concentration), was as follows: terbufos > phorate > 2,4-D > atrazine > metsulfuron methyl (K-d = 20.7, 18.1, 4.88, 3.74, and 0.54 L/kg, respectively). Sorption of pesticides was higher for allophanic than for non-allophanic soils and in general decreased with depth. Multiple regression analysis between K-d and soil properties indicated that the important properties which explained most of the variation in pesticide sorption include: organic carbon (C) and pH for metsulfuron methyl (R(2) = 0.81***); organic C and oxalate-extractable aluminium (Al) for atrazine (R(2) = 0.86***), phorate (R(2) = 0.93***), and terbufos (R(2) = 0.95***); and organic C, pH, and oxalate-extractable Al for 2,4-D (R(2) = 0.87***). The distribution coefficient of the pesticides, based on organic C (K-oc = K-d/organic C) content, varied less between the soils than did the K-d values. The variation in K-oc was much less for non-ionic pesticides (phorate and terbufos) than for ionic pesticides (metsulfuron methyl, atrazine, and 2,4-D). Removal of organic C decreased the K-d values of all pesticides but it caused an increase in K-oc, values of ionic pesticides (metsulfuron methyl, atrazine, and 2,4-D). Empirical equations based on the octanol-water partition coefficient (K-ow of pesticides were used to predict the sorption of non-ionic pesticides in New Zealand soils. Most of the equations over-predicted the sorption of non-ionic pesticides in these soils. This illustrates the limitation of using empirical equations for predicting K-oc values.
