Summary.The use of legumes to improve the productivity
of pastures has usually been associated with an increase in soil
acidification. The present study compared the acidification potential of 12
N2-fixing pasture legume species/genotypes using a
solution culture assay. The legumes were chosen because of their use or
potential use in farming systems in the mediterranean zones of southern
Australia. The species grown were Trifolium subterraneum
L. (vars. Dalkeith, Yarloop and Clare),
Trifolium glomeratum L.,
Trifolium tomentosum L.,
Medicago murex Willd.,
Medicago polymorpha L.,
Medicago truncatulaGaertn.,
Ornithopus compressus L.,
Ornithopus sativusBrot.,
Trifolium vesiculosum and
Trifolium balansae. Proton excretion was measured over a
period of 21 days from days 40 to 61 after germination. The amount of protons
excreted into solution varied between 143 and 265 cmol
H+ /kg dry matter for the different species and
genotypes. In general, T. tomentosum and
T. glomeratum excreted greater amounts of protons per
unit biomass than Medicago species and
T. subterraneum, which, in turn, excreted more protons
than Ornithopus species. The amount of proton excreted
per kilogram biomass was well correlated with the concentrations of excess
cations over anions and ash alkalinity in plants across all the species. Total
acid production was highly correlated with accumulation of excess cations and
ash alkalinity, total dry matter production and total nitrogen fixation during
the study period. It is suggested that the potential proton excretion by new
pasture legume species should be considered when they are introduced into
farming systems.