Nodulation and nitrogen fixation by subterranean clover in acid soils as influenced by lime application, toxic aluminium, soil mineral N, and competition from annual ryegrass

The effects of low soil pH and lime addition on nodulation and nitrogen fixation in subterranean clover (Trifolium subterraneum L.) were examined in field and glasshouse experiments in south-west Australia. Field data from a number of broadacre pastures indicated that soil acidity combined with high soil aluminium reduced clover dependence on nitrogen fixation (% Ndfa) and that addition of lime, generally, promoted modulation and increased % Ndfa. In a detailed study at one site additions of lime increased soil pH, reduced extractable aluminium concentrations below phytotoxic concentrations and increased plant nodulation. Glasshouse experiments also demonstrated positive effects of liming in reducing extractable aluminium and increasing total N2 fixation and % Ndfa, but beneficial effects on N2 fixation were partly offset by counteractive effects on symbiosis through increased nitrogen mineralisation at high lime application rates. Where companion reegrass (Lolium rigidum Gaudin) was pot cultured with subterranean clover soil mineral N released following liming was absorbed preferentially by the grass, resulting in increases in % Ndfa of the associated clover.