Impact of oiled and de-oiled olive mill waste amendments on the sorption, leaching, and persistence ofS-metolachlor in a calcareous clay soil

Agricultural practices based on periodic inputs of organic amendments are strongly recommended for Mediterranean agro-ecosystems. Such amendments can change the soil's properties and transport characteristics, and hence affect the behaviour and fate of pesticides. S-metolachlor is an herbicide commonly used in intensive crops. The aim of this study was to investigate the influence of fresh oiled (OW) and de-oiled two-phase olive mill waste (DW) amendments on the sorption, leaching, and persistence of the herbicide S-metolachlor in a calcareous clay soil. The soil was amended in the laboratory with OW and DW at the rates of 2.5% and 5% (w/w). Significant increases in S-metolachlor sorption were observed in all amended soils. The addition of OW and DW increased the herbicide half-life from 27 d for the original soil to 41 and 47 d at the higher application rate of OW and DW, respectively. There was a significantly greater retention of the herbicide at the higher OW and DW loading rate. However, whereas the amount of S-metolachlor in the leachate was reduced by increasing the amount of OW, it was unaffected by increasing the amount of DW. The results lend support to the potential of OW and DW amendments as an effective management practice to increase S-metolachlor persistence in soils. This increase does not necessarily ensure decreased leaching of the herbicide but it could also increase the risk of surface water contamination at higher application rate.