Many flat, poorly drained soils, such as the Delmarva Peninsula, the upper Midwest, and certain areas of Europe such as Denmark and Netherlands, have been extensively drained through the construction of artificial drainage ditches and tiles to allow agriculture and other human activities.  In addition, many soils in these areas are saturated with regard to phosphorus (P) as a result of historic nutrient imbalances associated with intensive animal production. It would require many decades to draw down soil P saturation below commonly accepted environmental thresholds through crop production. Phosphorus loads delivered to the Chesapeake Bay through these systems can be quite high, with measured loads in excess of 25 kg ha^-1 year^-1. Research conducted on the Delmarva indicates that in excess of 85% of the P flowing through these ditches arrived there through subsurface pathways.  Few, if any, best practices can address the legacy P loads leaving artificially drained fields due to the subsurface transport pathways. Therefore, P filters that utilize industrial materials to sorb P from ditch flow have been developed and extensively tested. Many P sorbing materials and filter designs have been demonstrated to be effective. Filters can be designed for almost any target P removal rate. Filter designs, P sorbing material selection, and approaches to implementation will be discussed.