The impact of de-oiled and dried two-phase olive mill waste (DW) amendment on the sorption,
degradation, leaching, and persistence of the herbicide terbuthylazine in a Mediterranean agricultural soil were studied under laboratory and long-term field conditions. The results are compared with those of a previous study, where the intermediary by-product of the olive-oil processing technology (OW) was applied to the same soil. The adsorption of terbuthylazine by the soil significantly increased with the increase of the amount of DW in the laboratory and field-amended soils. The amount of terbuthylazine leached was significantly decreased or even not detected in the field-amended soil columns, but leaching losses of the herbicide increased by up to 52.7% in the laboratory amended-soils. The DW soil application significantly increased the half-life of terbuthylazine from 6.7 days for the original soil to 21 and 104 days in the field and laboratory amended soils, respectively. In the field study, DW addition decreased the vertical movement of terbuthylazine through the amended soils and increased the persistence of the herbicide in the upper 5 cm of the soils. This study suggests that DW amendment can play a positive role in preventing contamination of groundwater by terbuthylazine, especially after the transformation of the organic matter by aging processes. The results were quantitatively different from those previously obtained with the intermediary by-product OW, showing the importance of the specific characteristics of the amendment in determining its effect on pesticide environmental fate and behaviour.