The objective of this study is to quantify the potential risk of P leaching to groundwater in characteristic soil types in The Netherlands. In areas with shallow groundwater P losses via groundwater to surface waters is an important pathway.
The risk of P leaching to groundwater can be assessed by means of the phosphate saturation degree (PSD) methodology. The PSD is an index of the actual phosphate accumulation in the soil (Pact) in relation to the maximum phosphate sorption capacity (PSCm) of the soil to a reference depth. To assess the risk of P leaching the actual PSD of soils has to be compared to the critical PSD (PSDcrit) of the soil type. The critical PSD is determined for characteristic soil types based on phosphate sorption and desorption characteristics, and a reference depth (Lref) where a defined P concentration (Cp) in solution may not be exceeded. A stratified soil survey is used to determine the actual PSD of the soils.
The critical phosphate saturation degree we determined for the main Dutch soil types varies from 5%–78%. The average P accumulation in agricultural soils in The Netherlands is about 2050 kg P ha− 1 (4700 kg P2O5 ha− 1) and the 5‰ and 95‰ are approximately 850 and 4500 kg P ha− 1. Since maize can tolerate high manure application rates, more phosphate has been applied on maize in the past, and high P accumulations are measured. In about 43% of the agricultural land in The Netherlands the critical PSD value for the given soil type is exceeded.
Consequently, a large area of agricultural land contributes, or is expected to contribute to the P pollution of surface water in the nearby future. Especially from fields with a high PSD severe P losses can occur due to the convex relationship between the PSD and the P concentrations in soil solution.
Strategies and additional measures are needed for fields with a high PSD in order to substantially reduce the P losses from agricultural land to surface waters within a catchment.