An agri-environmental phosphorus saturation index for acid coarse-textured soils

Potato (Solanum tuberosum L.) is a high P demanding crop contributing to P saturation of acid coarse-textured soils. The P status is commonly assessed agonomically by soil testing of P (STP) and environmentally by soil P saturation or P solubility indexes. The aim of this work was to develop a P fertilizer recommendation model integrating P environmental risk and crop response probability, using P and Al extracted by the Mehlich-III soil testing method (mg P or Al L-1 of soil) and expressed as (P/Al)(M-III) percentage. The environmentally critical (P/Al)(M-III) percentage was 15%, corresponding to the critical degree of phosphate saturation (DPS) value of 25% (oxalate extraction procedure) proposed in the Netherlands. Using the Cate-Nelson procedure, the critical (P/Al)(M-III) percentage across 78 field experiments was 8.2% as determined by inductively coupled-argon plasma (ICP). Probability of potato response to P was close to 100% even above the critical STP values as shown by power tests on relative yields. The P recommendation model was computed from conditional expectations of 50 and 80% within each fertility group below 15% as (P/Al)(M-III) Above 15%, the model recommended 21 kg P ha(-1), the amount of P exported by a tuber harvest of approximately 42 Mg ha(-1) Using conversion equations with 114 soil samples for STP methods, fertility groups built for Quebec acid coarse-textured soils were found to be comparable with those currently used in the Netherlands and Lower Saxony (Germany). The (P/Al)(M-III) percentage provided a reliable and unifying criterion for making environmentally acceptable and agronomically efficient P recommendations for sustainable potato production.