The role of phosphorus in eutrophication of surface water has long been recognized. Negative effects of eutrophication include: reduced biodiversity of aquatic ecosystems and surface water quality, and algal blooms that restrict the use of surface waters for recreation and drinking water production. Algal toxic substances have caused fish kills and animal and human diseases in the past. Reduction of phosphorus input to surface waters is thus necessary, mostly starting with diminishing industrial losses, precipitating P in sewage water treatment plants and increasing the number of households that are connected with sewerage systems. Due to these reductions the relative contribution of agriculture to total P input to aquatic systems increases to 50 or even 90 % in many watersheds.

In Europe, the Water Framework Directive will force authorities of watersheds to improve the ecological status of the water. For many waterways this implicates a reduction of nutrient inputs. It is foreseen that this has especially a large impact for phosphorus.

The need for reducing P loss from agriculture is thus clear, and studies to find the main locations where P loss starts and on transfer routes to rivers and lakes have received much attention recently. Research on reducing P loss is also expanding, and many different options are proposed and tested (and sometimes rejected) under different soil, crop and environmental conditions. Thus, there is a strong need for a critical evaluation of these options, in order to reduce further research where little benefit is likely and to expand research on options that have proven to be successful.

In the past, International Phosphorus Workshops (1995 Wexford, 1998 Antrim, 2001 Plymouth) have greatly contributed to increase our knowledge about the relation between agriculture and P losses, and about transfer of P from soil to water. The 4^th International Phosphorus Workshop was a platform for evaluating the effectiveness, under varying conditions, of different types of options for reducing P losses from fields, and the transport of P in, or effects of P on surface waters. Discussions included aspects like the effectiveness of options for different forms of P (inorganic, organic, and particulate), economic aspects, and interaction with the nitrogen cycle.

Source orientated options, like a (drastic) reduction of P input to agriculture, or prevention of P transport from agricultural fields, may be considered to be most effective for avoiding unwanted effects. However, when considerable amounts of P have accumulated in the soil in the past, a reduction in P loss may take (much) time. Therefore, also ("end-of-pipe") options for reducing P transport in surface water and for abating consequences of eutrophication in surface water were discussed.