EFFECTS OF SURFACE APPLICATION OF LIME AND SUPERPHOSPHATE TO ACID SOILS ON GROWTH AND N-2 FIXATION BY SUBTERRANEAN CLOVER IN MIXED PASTURE SWARDS

Forage production in south-eastern Australia is well below potential. Poor productivity in the higher rainfall zones has often been attribut ed to effects of soil acidity and phosphorus deficiency on N-2 fixatio n by legumes in pasture swards. Difficult terrain and lack of suitable equipment commonly prevents the incorporation of lime or phosphatic f ertilisers in permanently grazed pastures, and amelioration of soils i n these situations must rely upon surface applications. A study was in itiated 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-') on N-2 fixation and growth of subterranean clover (Trifolium s ubterraneum). By November 1991 (18 months after treatment application) liming had increased surface soil pH (from 4.2-4.5 to 4.9) and signif icantly decreased the concentrations of CaCl2 extractable aluminium an d manganese at all sites. However, pasture growth and subterranean clo ver content was increased by lime or superphosphate applications only at Bungendore. Plant analyses indicated that phosphorus availability m ay have limited pasture response, despite the inclusion of two rates o f superphosphate. Additional superphosphate (50 kg P ha(-1)) was appli ed 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 i ncreased by 2-3-fold with the additional superphosphate at Braidwood a nd by 24-61% at Beechworth and Bungendore. These effects on productivi ty were principally determined by changes in the subterranean clover c omponent. Estimates of the proportion of subterranean clover N derived from symbiotic N-2 fixation (P-NFix) using the N-15 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 fix ed at the three experimental sites ranged from <2 kg N ha(-1) to 119 k g N ha(-1), and were determined by pasture composition, or by seasonal and treatment-induced differences in growth of subterranean clover. T he implications of the findings are discussed in terms of management t o alleviate nutritional stress and improve the contribution of subterr anean clover in permanent pastures on acid and phosphorus deficient so ils.