SUMMARY An analysis of published data suggests that the N2‐fixing symbiotic vascular plants extrude more H+ per unit N fixed than would be expected from data on the same genotypes growing on NH4+ if the plants had the same chemical composition when grown on the two N sources. The H+/N ratio with urea as the N source is similar to that with N2. The higher H+/N ratio and higher organic acid/N ratio with N2 or urea as N source implies higher whole‐plant energy and water costs per unit of biomass and, ultimately, inclusive fitness, produced. The rhizosphere acidification resulting from H+ extrusion may serve to change rhizosphere pH to some ‘optimal’ value, and to increase the availability of such limiting resources as P, Mo and Fe which are especially needed in diazotrophy. Data in the literature are consistent with these possibilities in the few cases examined. Within the plant, data on xylem and phloem sap composition in conjunction with shoot composition, of diazotrophically‐growing legumes suggest that shoot acid‐base homoiostasis can be maintained via the import of appropriate solutes in the xylem and the export of appropriate solutes in the phloem. Acid‐base regulation of the nodules in the absence of any H+ exchange with their environment can also probably be explained in terms of the solutes supplied in the phloem and exported in the xylem. This conclusion is based on data in the literature on the composition of stem phloem sap and of xylem sap exuding from detached nodules of diazotrophic vascular plants. These considerations do not exclude the possibility of net H+ efflux from nodules fixing N2 in contact with an aqueous medium. The limited data available are consistent with extrusion of some of the H+ generated in nodules as an alternative to their neutralization by metabolism of organic anions entering in the phloem. Such H+ extrusion by nodules could aid in their acquisition of Fe from the medium, albeit not always at a phase in the life or the nodule when there is a net requirement for Fe.

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