Using the phosphorus saturation degree as a guide for sustainable phosphorus management balancing crop production and water quality objectives

Incorporating environmental boundaries into P fertilizer recommendations is key to reconcile agronomic objectives and P leaching risks to ground- and surface waters. Current agronomic soil P quantity tests, used as the basis for fertilizer recommendations, are poorly suited for this purpose as they provide no information on the ortho-P concentration in soil solution which is prone to leach. Therefore, we converted agronomic soil P test values to the equilibrium ortho-P concentration in soil solution through the corresponding P saturation degree (PSD), using information on the P sorption capacity and the affinity of ortho-P to bind to soil. We derived an environmental PSD threshold and compared this with the current PSD and the agronomic target values. In Dutch agricultural soils, current PSD values exceed the agronomic target and environmental threshold for 84 % and 94 % of the agricultural land area, respectively. Decreasing the current PSD to the environmental threshold through P mining showed limited adverse effects on crop yields, except in areas being vulnerable to P losses because of a low P sorption capacity and high hydrological connectivity. Here, the cultivation of less P-sensitive crops or the provision of other ecosystem services than food production may be more appropriate. Limited adverse effects on crop yield result from high agronomic soil P targets in Dutch fertilizer recommendations, based on achieving 99 % of the maximum yield for the P-sensitive potato crop. Given the high livestock density and excess manure in the Netherlands, reducing the current PSD to the environmental threshold poses a significant challenge.