Investigations into the causes of residue variability on carrots in the UK

A pilot study was established to determine whether the preferential flow of water and thus insecticide in carrot beds might be responsible for the hig h residues of organophosphorus insecticides detected in individual carrot r oots grown in the UK. A field site typical of UK carrot growing conditions was selected on a sandy soil with a low organic carbon content. Brilliant b lue dye was applied in water to a small number of field plots located in th e carrot beds to trace water movement through the bed and not to simulate i nsecticide application or irrigation. The plots were excavated following su fficient time for infiltration and drainage. Horizontal and vertical cross- sections through the soil profile were cut and descriptions of the dye pres ence in relation to soil features and the position of the carrot roots were made. Dye tracing and soil analyses showed there was a clear mechanical cu ltivation effect which generated a preferential movement of dye and water w ithin the bed. The subsequent growth of carrots also created additional pat hways of preferential movement due to stem-flow or canopy drip. A second st udy which increased replication of samples and allowed analysis of triazoph os and chlorfenvinphos residues in the carrots could not identify any singl e factor which was conclusively responsible for initiating high residues in individual roots.