Some continuing uncertainties in knowledge of herbicide behaviour in the soil

SUMMARY Recent developments concerned with the variability in the performance of soil‐ applied herbicides are reviewed with particular reference to phytotoxicity and to factors controlling persistence. It remains questionable that measurements of adsorption and herbicide concentration in the soil solution can adequately predict the phytoxicity of a herbicide in the soil. This is because such an approach ignores the spatial distribution of the herbicide and the parts of the plant through which a herbicide can be taken up. It also ignores the influence of climatic factors on persistence, uptake and plant response. In a general way persistence and the factors which control it are understood but detailed, reliable, quantitative analyses of the phenomena are not yet available. Much remains to be done before the interactions of the soil microflora with its environment and consequent effects on herbicides can be defined. There are still opportunities to improve the precision of use of soil‐applied herbicides based on better comprehension of the processes involved in the expression and duration of phytotoxicity.