Developing more sustainable forage systems requires efficient decision support tools for fertilization management. Soil phosphorus (P) tests have long been used as decision support tools for fertilization management but, more recently, plant nutrition indices using the P concentration of shoot biomass were developed to assess the P nutrition status of grasslands. The objectives of this study were to (i) evaluate the relationship between the phosphorus nutrition index (PNI) and the yield response to P fertilization (ii) analyze relationships between PNI and soil plant-available P (SPAP) indicators, and (iii) evaluate PNI assets for P diagnosis in forage system. Five long-term (≥ 9 years) grassland P fertilization experiments under different soil and climate environments in Canada, Switzerland, France and Romania were used. Three SPAP indicators were tested: CP, the soil solution orthophosphate ions (oPion) concentration (mg P L−1), Olsen P (mg P kg−1), and, a process-based assessment (Qw + Pr) from the sum of oPion in the soil solution (Qw, mg P kg−1) and diffusive oPion with time and CP (Pr, mg P kg−1). PNI was calculated as sward P concentration divided by the critical P concentration.The cumulative effect of P fertilization resulted in a wide range of SPAP values. Overall, CP varied from 0.03 to 3.6 mg P L−1, (Qw + Pr60 min) from 6−52 mg kg−1, and Olsen P from 4−40 mg kg−1. The PNI varied from 48–94% in plots with no applied P, and from 83–121% in P-fertilized plots. A generally positive relationship between relative forage dry matter yield and PNI was established, with a critical PNI value of 92% that distinguishes P-limited and non-P-limited grassland nutrition. Positive relationships between PNI and the three SPAP indicators confirmed that the soil P status influenced the grassland P nutrition status. Critical values on a stock basis for a target PNI value of 92% were similar for Olsen P (12.9 kg P ha−1) and (Qw + Pr60 min) (13.5 kg P ha−1). This study opens perspectives for P diagnosis improvement in forage systems.