Symbiotic nitrogen fixation (SNF) is the main natural source of nitrogen (N) in terrestrial ecosystems worldwide. Previous studies have shown that fixation of N by plants can be limited by the availability of phosphorus (P) in soils. We used global meta-analysis to investigate how P availability controls SNF. In experiments in which plants were grown in an artificial medium, severe P deficiencies in the nutritive solution ([PO4]<5-42M) depressed SNF flux through both a direct decrease in the plant fixation rate (i.e., decreased N fixed per unit of plant biomass) and an indirect effect (i.e., through plant biomass). In most experiments with plants grown in soils, SNF was proportional to plant biomass and was consequently only indirectly limited by P. Some cases using unfertilized and weathered soils (ultisols or oxisols), where plants were particularly P stressed, were an exception with both direct and indirect P limitations. Our global analysis of the P-SNF relationship indicated that P bioavailability commonly limited SNF flux. We conclude that the main driver of in situ P limitation is indirect via limitation of plant growth, except in certain cases where both indirect and direct constraints may play a role. These cases of severe P deficiency may be mainly found in weathered tropical soils of Africa and South America, probably in unfertilized croplands which are depleted in P due to repeated biomass harvests.