Optimizing the use of phosphorus (P) fertilizers is essential considering the environmental issues linked to over fertilization and the limited rock phosphate reserve. Diagnosing the crop P status by calculating a phosphorus nutrition index (PNI) based on the plant critical P concentration (Pc) could help determine the minimum amount of P fertilizer to ensure maximum crop yield. This study investigated (i) the effect of P fertilization on winter wheat (Triticum hybernum L.) shoot biomass and grain yield after several decades of different annual P fertilization rates, (ii) the stability over several site-years of the Pc expressed either as a function of shoot biomass or shoot nitrogen (N) concentration, and (iii) the possibility of using the PNI or the shoot N-to-Pc ratio as nutritional indicators to diagnose P deficiency and predict the expected response to P fertilization. Shoot biomass and its P and N concentrations were measured weekly at five site-years along with grain yield after several decades of P fertilization treatments that ranged between 0 and 5/3 of the theoretical P crop uptake. The P fertilization did not affect the grain yield, but it generally increased shoot biomass especially at the CD 27- -37 developmental stages. The Pc expressed either as a function of shoot biomass or shoot N concentration differed among site-years and this was attributed to differences in crop N status. When N was not deficient, we developed a Pc dilution curve as a function of shoot biomass (SB) (4.56 × SB−0.279) along with a linear relationship (Pc = 1.10 + 0.061 × N) and a power function (Pc = 0.34 × N0.632) between Pc and shoot N concentration. The N-to-Pc ratio was related to shoot biomass accumulation according to a power function, but the relationship also differed among site-years. The relative shoot biomass responded positively to the PNI up to different thresholds for limiting and non-limiting N conditions. The relative grain yield, however, was not related to the PNI. The PNI, based on the Pc dilution curve, has potential as a nutritional indicator to diagnose P deficiency and the expected response to P fertilization, but more research is needed to clarify the effect of N deficiencies on the Pc dilution curve.