Agricultural land use is a key driver of water quality through its impact on nutrient flows across landscapes. Phosphorus (P) is commonly applied to croplands in fertilizers and manure to improve soil fertility, but excess P can accumulate in soils and may accelerate eutrophication of waterbodies. Balancing agricultural P inputs with crop requirements is therefore crucial for water quality. A historical study of agricultural land use and water quality in the Saint Lawrence River sub-basin (574,000 km super(2)) is presented here, focusing on agricultural P budgets. These budgets, calculated each decade from 1901 to 2001 for the sub-basin and its tributary watersheds, reveal that while P management has improved in recent decades, large [...]