This research was motivated by the impact of the diffuse P sources coming from agriculture on surface water quality and eutrophication. The goal of this Ph.D. thesis was to contribute to improving the management of drained agricultural areas to optimize the retention of Fe-associated P. Because of the legacy P in the soil, P-retention measures are needed in agricultural catchments to improve the surface water quality in the short- to middle-term. The first part of this thesis focused on monitoring the P transport at the farm scale. The second part of this thesis focused on investigating the mechanisms of P-retention by iron-coated sand (ICS), a Fe-rich by-product from drinking water production.