FACTORS AFFECTING THE EARLY GROWTH OF LEUCAENA-LEUCOCEPHALA .1. EFFECTS OF NITROGEN, PHOSPHORUS, LIME AND IRRIGATION AT 3 SITES IN SOUTH-EAST QUEENSLAND

Slow establishment has been identified as a major limitation to the mo re widespread adoption of the fodder tree Leucaena leucocephala in Que ensland. Field experiments were conducted at Mt Cotton, Gayndah and Th eodore in south-east Queensland during the 1987-88 summer to identify the major factors limiting first year yield of leucaena. Treatments at each site included irrigation (applied at sowing only, or at interval s throughout the trial), and rates of phosphorus (P), nitrogen (N) and lime. Attack by psyllid insects, weed competition and defoliation by wildlife were noted and minimised where possible. Soil moisture appear ed to be the major limitation to plant growth at the inland sites of T heodore and Gayndah where post-establishment irrigation increased fina l dry weight by 250%. Waterlogging reduced plant height increase at th e low lying Mt Cotton site following higher than normal rainfall. Incr ease in plant height at the other sites stopped when night temperature s fell below 15 degrees C. Application of P at 75-1200 kg/ha tripled f inal dry weight of leucaena at Mt Cotton (5 mg/kg bicarbonate-extracta ble P) but had no effect at Theodore (10 mg P/kg) or Gayndah (35 mg P/ kg). Larger than normal responses to P during early growth may be due to slow colonisation of the roots by arbuscular mycorrhiza. Applicatio n of N (200 kg N/ha) increased dry weight of Rhizobium-inoculated leuc aena by 27% at Theodore but had little or no effect at the other sites . Application of lime had no significant effects on plant height or dr y weight of leucaena at Mt Cotton despite the soil being slightly acid ic (pH 5.9 in 1 : 5 H2O suspension). Further work on the role of colon isation by arbuscular mycorrhiza fungi in determining uptake of P by l eucaena is warranted in view of the large response to very high rates of P in young seedlings at Mt Cotton.