The future agricultural crop phosphorus (P) demand will increase due to the projected growth in world population. This may cause an increased risk for surface water eutrophication, due to enhanced P losses to freshwater bodies from agricultural fields, unless the P use efficiency is increased.

The proposed PhD project targets three knowledge gaps. Firstly, there is a lack of a single P indicator with a related critical (target) value for both crop yield and surface water eutrophication risks. Secondly, when studying phosphorus at the regional to global scale, current P models lack a detailed spatially explicit parametrization of P adsorption and desorption. Thirdly, long-term sustainable future agricultural P demand at the regional and global scale balancing crop P requirements against losses to surface water has received little attention.

Firstly, we will derive critical soil Phosphorus Saturation Degree (PSD) levels for crop yield and construct a framework to back-calculate critical PSD levels in view of water quality, intending the use of a single soil P test including both agronomic and environmental optima. Secondly, we will improve the parameterization of P adsorption/desorption in spatially explicit models at the regional scale (INITIATOR, up to parcel level) and global scale (IMAGE-DPPS, 0.5 · 0.5 degrees) by deriving pedo-transfer functions between P sorption constants and soil properties controlling P availability. Finally , we will apply those improved models to assess target P inputs at the short-term (2021-2050) and long-term (2050-2100) that are required to raise or maintain P at agronomically optimal levels, and to assess related water quality impacts. This will provide insight in sustainable soil P levels at the regional and global scale, defined as a balance between an optimal soil P level for crop yield and minimal environmental P losses.