An Agri‐Environmental Phosphorus Saturation Index for Acid Coarse‐Textured Soils

Abstract

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 agronomically 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.