The phosphorus saturation degree as a universal agronomic and environmental soil P test
Maarten van Doorn, Debby van Rotterdam, Gerard H. Ros, G.F. Koopmans, Erik Smolders, W. de Vries
Abstract
Phosphorus (P) is an essential nutrient for crops and is applied to agricultural soil to bring or keep the soil at a certain target soil P status in view of an optimal crop yield. Environmental objectives, however, are rarely considered in current P fertilizer recommendations. In this review paper, we argue that current P fertilizer recommendations must be revised in order to balance crop yield, water quality and the use of finite P resources. This revision requires insights into the total pool of reversibly bound P and the capacity of the soil to bind P. Current soil P tests (SPTs) used in routine agronomic soil testing do not provide these insights. We identify the oxalate extraction method as a high-potential agri-environmental SPT as it measures the total pool of reversibly bound P acting as a reserve for plant-available P while it also quantifies the maximum soil P sorption capacity from the simultaneous measurement of amorphous iron- and aluminium-(hydr)oxides. From these results, the Phosphorus Saturation Degree (PSD) can be calculated. We show that those insights are pivotal for the combined assessment of crop response, the risk of P losses to the water system and the judicious use of finite P reserves. In practice, agronomic target P levels should be lowered in soils with a low P sorption capacity to decrease the risk of P leaching. Agronomic target levels should also be lowered in soils with a high P sorption capacity to ensure a judicious use of finite P reserves.