Behaviour of herbicides in soil: simulation and experimental assessment.

The mathematical models of the transport and the transformation rate of herbicides in soil that are available in the literature and the tests done on them are reviewed.A simulation model of the transport of herbicides in field soil, based on the best model available in the literature, was developed. The detailed field experiments carried out on a bare loamy sand soil with two soil-applied herbicides (cyanazine and metribuzin) in spring and summer to test this model are described.A new sub-model for the evaporation of water from bare soil was developed and tested in the field, with acceptable results. Testing the herbicide transport model against the concentration profiles measured in the field, showed that calculated penetration of the two herbicides in soil at a few months after application was much deeper than that measured.To elucidate the cause of the discrepancy between calculations and measurements, the sorption of the two herbicides onto soil collected from the experimental field was studied in detail in laboratory experiments. Based on these studies a new model for the sorption of the herbicides was developed and incorporated into the transport model. The main new element in this model was a sorption process that equilibrates at a time scale of months. A comparison between concentration profiles calculated with the new model and those measured in the field showed that the new model successfully explained the field measurements