Phosphate saturation degree and accumulation of phosphate in various soil types in The Netherlands

Abstract Objective 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. Methods 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 (P act ) in relation to the maximum phosphate sorption capacity (PSC m ) 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 (PSD crit ) of the soil type. The critical PSD is determined for characteristic soil types based on phosphate sorption and desorption characteristics, and a reference depth (L ref ) where a defined P concentration (C p ) in solution may not be exceeded. A stratified soil survey is used to determine the actual PSD of the soils. Results 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 P 2 O 5  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. Practice 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. Implementation 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.