Effects of surface application of lime and superphosphate to acid soils on growth and N2 fixation by subterranean clover in mixed pasture swards

Abstract Forage production in south-eastern Australia is well below potential. Poor productivity in the higher rainfall zones has often been attributed to effects of soil acidity and phosphorus deficiency on N 2 fixation by legumes in pasture swards. Difficult terrain and lack of suitable equipment commonly prevents the incorporation of lime or phosphatic fertilisers in permanently grazed pastures, and amelioration of soils in these situations must rely upon surface applications. A study was initiated in May 1990 at field sites at Bungendore and Braidwood in N.S.W. and at Beechworth in Victoria to investigate the effect of surface applied lime (0 or 2500 kg ha −1 ) and superphosphate (0, 10 or 20 kg P ha −1 ) on N 2 fixation and growth of subterranean clover ( Trifolium subterraneum ). By November 1991 (18 months after treatment application) liming had increased surface soil pH (from 4.2–4.5 to 4.9) and significantly decreased the concentrations of CaCl 2 extractable aluminium and manganese at all sites. However, pasture growth and subterranean clover content was increased by lime or superphosphate applications only at Bungendore. Plant analyses indicated that phosphorus availability may have limited pasture response, despite the inclusion of two rates of superphosphate. Additional superphosphate (50 kg P ha −1 ) was applied to the original 10 kg P ha −1 plots at all sites in the autumn of 1992. Amounts of N accumulated in pastures at the end of spring were increased by 2–3-fold with the additional superphosphate at Braidwood and by 24–61% at Beechworth and Bungendore. These effects on productivity were principally determined by changes in the subterranean clover component. Estimates of the proportion of subterranean clover N derived from symbiotic N 2 fixation (P NFix ) using the 15 N natural abundance procedure, indicated that clover reliance on N 2 fixation for growth was high at Bungendore and Beechworth (87–96%), regardless of season, rate of superphosphate application, or lime treatment. At Braidwood, P NFix ranged from 50–73% in 1991 to 81–97% in 1992. Amounts of N 2 fixed at the three experimental sites ranged from −1 to 119 kg N ha −1 , and were determined by pasture composition, or by seasonal and treatment-induced differences in growth of subterranean clover. The implications of the findings are discussed in terms of management to alleviate nutritional stress and improve the contribution of subterranean clover in permanent pastures on acid and phosphorus deficient soils.