A sound understanding of the factors affecting the mobility of pesticides in soil is an essential prerequisite to the development of functions which can accurately represent the array of processes which pesticides may undergo in soils. This requires detailed investigations on equilibrium and kinetic aspects of the sorption and desorption involved, the nature, extent and rate of any chemical transformation or biological degradation, and the roles of convection, diffusion and dispersion in the mobility of the pesticides in relevant soil structures. While a substantial amount of data is available on the interactions of pesticide chemicals with soil constituents, much of this has been obtained overseas. Also, the effects on these processes of soil properties, such as texture, clay mineral species, organic matter content, pH, salts and solvents present, need to be measured in detail for a much wider range of local soil-pesticide combinations in Australia. In particular, the significance of sorption time-dependency and long-term reversibility of sorption on the mobility of pesticides and the effects of soil environment on the rates of degradation under natural leaching conditions in the field remain poorly understood. The literature contains a multiplicity of predictive models varying greatly in terms of their complexity and claimed applicability. There is an urgent need to define the degree of complexity with which fundamental processes are treated in such models and the extent of characterisation of the range of physical, chemical and biological mechanisms required to avoid problems of site specificity and to provide a satisfactory database for predictive modelling. The development of practical methodologies, both for simulation modelling and compatible data collection, is essential.
