The Access Code says:
Land managers often need to apply fertilisers or other materials, such as slurry or lime, to fields of crops. The duration of the hazard depends on the material used but can extend from a few hours to four days in the case of sulphuric acid. As these can be dangerous to public health, land managers are required to ensure that people do not enter land on which pesticides have been used. Follow any advice asking you to avoid using particular routes or areas at these times.