1. Phosphorus (P) transfer from agricultural land to freshwater systems has been studied across many scales and environmental compartments that range from understanding biogeochemical processes in soils and fields, to assessment of localised in-stream biotic and ecological impacts. 2. This study tackles the challenges of scale when moving from soil hillslope to headwater catchment scale. The focus is on process rules derived from reductionist approaches at the relatively fine scale, and exploring the signal and evidence thereof at the headwater catchment scale. 3. The methodology uses new data of P dynamics in agricultural grassland headwater catchments in south-west England. 4. We found the following: (i) it was not possible to disaggregate an influence of soil (Olsen) P concentration on P export at the larger scale; (ii) there was no clear temporally dynamic relationship between P additions of fertiliser and recycled manure and the resulting P transferred to the headwater scale; however, (iii) ploughing, digging of stream channel and leakage from farm storage all affected the temporal concentration dynamics; and (iv) overall P loss was influenced by higher long-term history of P inputs, livestock and the domination of hydrologic processes. 5. It is concluded that process rules derived at the finer soil or plot scale cannot always produce a clearly discernable signal when studied at the larger headwater catchment scale.