© 2010 19th World Congress of Soil Science, Soil Solutions for a Changing World

Phosphorus losses from agricultural land can be a major source of eutrophication of surface waters. To meet the objectives of the Water Framework Directive, measures should be taken in agricultural soils. The degree of phosphorus saturation (DPS) is used in some countries to evaluate levels soil P saturation and subsequent risks of losses. The adequacy of this parameter to the Walloon situations had to be assessed before large-scale surveys. As a prior stage, this study has shown that DPS=Pox/0.66*(Alox+Feox) was relevant to study the P saturation of studied soils. A mean DPS of 32 % was observed. Risks of P losses seem high in some areas and should be attributed to 2 factors: soil properties which drive the sorption capacity on the one hand and soil fertility management by farmers. A close relationship was found between DPS and available P and with sorption capacity and Pav:Pox ratio, which suggests that phosphorus availability is favoured when sorption sites are saturated, that is on soils with low PSC. However, as P transfers from soil to water may occur through particulate erosion, the total sorbed P should be taken into account too. DPS is thus of interest as environmental indicator but it shouldn't be used alone