How to promote sustainable phosphorus fertilisation

Increasing phosphorus (P) content in agricultural soils stimulates crop yield but also increases phosphorus losses to surface waters and groundwater (Figure 1). Adequate P availability in the soil is necessary for crop growth but above a certain soil P content, yields do not increase. Below a certain threshold soil P content, P losses are limited, but losses can increase exponentially when the soil P content exceeds the threshold. Target soil P content A small ‘target zone’ soil P content with optimal crop yields and limited P losses can be defined (Tunney, 2002; McDowell, 2012). Soils with low soil P content (A in Figure 1) can receive net P inputs (= P fertilisation input minus crop P output) without large increases in P losses, whereas soils with large soil P contents (B in figure 1) can have several years of net P export without compromising crop yields. Soil P content should evolve in the next 10-30 years towards the target zone. This sustainable choice is optimal for both the environment and yields. Measures for reaching this goal are (1) an adapted fertilisation legislation and (2) adequate fertilisation recommendations (see below). Figure 1 presents a simplified picture that does not take hydrology, connectivity or soil characteristics into account. Moreover, the method to measure soil P content must be appropriate because results often depend upon soil characteristics. (1) Legislation regarding phosphorus fertilisation in European countries and regions varies from no direct regulation to strict maximum phosphorus application rates (Amery & Schoumans, 2014). By taking the risk for soil P losses into account for identifying the maximum phosphorus application rate, evolution towards the target zone can be promoted (Figure 1). Some European countries already have P application limits that depend upon the soil P content, stimulating evolution towards the target zone. Further progress could be made by using an environmental instead of an agricultural soil P measurement, and also by taking hydrology and connectivity into account. To optimise yields and minimise P losses, legislation regarding P fertilisation within and outside Europe should include maximum P application limits differentiated to soil P content and P loss risk. (2) The soil P content is generally taken into account when formulating P fertilisation recommendations. But even for similar soil-crop situations, fertilisation advice differs more than threefold in Europe (Jordan- Meille et al., 2012). New recommendation systems in some European countries are flowing from updated models and data, more appropriate safety margins and new insights into P availability measurements. The new P recommendations are generally lower than before (Albertsson, 2008; Krogstad et al., 2008; Bussink et al., 2011). New environmentally friendly recommendation systems can largely limit P fertilisation and costs while maintaining crop yields (Csathó et al., 2009). Additional research on and development of new P fertilisation advice that guarantees optimal yields while limiting P losses should therefore be encouraged.