The paper analyses how farmers take water pollution by nitrates into account. The author proposes a geographical approach to farmer practices aimed at confronting the farmers' values and spatial practices with the spatial delimitations of other actors concerned by a local pollution issue. The research methodology involved surveys of twenty farmers in a French rural area (Yonne department) who were compelled to take a local problem of nitrogen pollution into account in their farming practice. Two spatial entities were combined in studying changes in their practices: the farm and the catchment area. The perception of space by the farmers relates to "the farm lands" and to "individual fields or groups of fields" on their farms. These spatial categories differ from the ones used by agricultural advisers and regulations to circumscribe vulnerable areas and land units. The study of field by field adjustment of nitrogen inputs on wheat shows a diversity of space delimitations among farmers. Mapping these results allows the identification of the spatial adjustments and change of practice imposed by the consideration of pollution risks. In conclusion, these results allow a reflection on the relationships between environmental values and environment-related practices.