This study examined the effect of K (as K2SO4) supply on acid production under N2-fixing plants of lupin (Lupinus angustifolius L. cv. Gungurru) and clover (Trifolium subterraneum L. cv. Dalkeith) grown in a K-deficient soil with a low pH buffer capacity for 55 days in the glasshouse at 20/12 °C (day/night). Increasing K supply up to 240 mg K kg-1 soil markedly increased plant growth of both species but clover growth was more responsive than lupin. Growing plants for 55 days decreased soil pH by 0.65–0.85 units under lupin and 0.45–0.83 units under clover. The amounts of H+ produced per kg biomass (specific acid production) were the highest at the nil K supply, generally decreased with increasing K level up to 30 mg K kg-1 under lupin and up to 120 mg K kg-1 soil under clover and only slightly increased with further increasing K under lupin. Increasing K2SO4 supply proportionally increased plant uptake of K and SO 4 2- but generally decreased concentrations of Ca, Mg, Na, P and Cl. Specific acid production correlated well with concentrations of excess cations and ash alkalinity, and total acid production was strongly correlated with total excess cations and total ash alkalinity in plants. These relationships were not affected by K treatment and species. Specific acid production also correlated with plant Ca concentration but not with K concentration. In addition, lupin and clover extruded similar amounts of H+ per kg biomass produced. It is suggested that application of K2SO4 does not have a significant impact on acid production by lupin and clover.