Symbiotic nitrogen fixation is the main natural source ofnitrogen in ecosystems. Previous studies have shown that this process can be limited by the availability of phosphorus (P) in soils. However, it is still unclear whether or not phosphorus controls symbiotic nitrogen fixation. Through a compilation of a global dataset, our analyses showed that worldwide in the case of nutritive solution experiments, the specific nitrogen fixation rate increased until a very low P concentration threshold value but remained constant above this value. Conversely, in field experiments, the specific fixation rate remained constant at any P supply, highlighting that plants can obtain sufficient P from the soil to maintain an optimal nitrogen fixation rate. Those results wereeconfirmed by an extensive analysis of soil P status throughout the world, showing always adequate quantities of bioavailable soil P, despite common low soil solution phosphorus values. Our results demonstrate that the in situ symbiotic nitrogen fixation rate is not dependent on P availability and that the fixation flux is only proportional to plant biomass increment. By identifying the mechanistic basis for the phosphorus control of terrestrial nitrogen fixation, this study provides a theoretical framework easily implementable into further models that couple nutrient cycling and global changes.