The capacity of subterranean clover (Trifolium subterraneum L. cv. Clare), medic (Medicago murex Willd. cv. Zodiac), serradella (Ornithopus sativus Brot. line SP1/13), biserrula (Biserrula pelecinus L. line Mor99), and woolly clover (Trifolium tomentosum L.) to acidify soil under N2 fixation was compared in a pot experiment using a poorly buffered sandy soil. The amount of acid produced per kg shoot dry matter (specific acid production) varied betweefin species and with growth stages, ranging from 44 to 128 cmol/kg shoot. Subterranean clover and serradella acidied soil to a greater extent than woolly clover and medic, whereas biserrula acidified soil least. Irrespective of pasture species and growth stage, specific acid production correlated well with concentrations of excess cations and calcium in shoots. Furthermore, total excess cation, ash alkalinity, and calcium in shoots were all good indicators of total acid production across all of the species.