Factors affecting the early growth of Leucaena leucocephala 1. Effects of nitrogen, phosphorus, lime and irrigation at three sites in south-east Queensland

Slow establishment has been identified as a major limitation to the more widespread adoption of the fodder tree Leucaena leucocephala in Queensland. Field experiments were conducted at Mt Cotton, Gayndah and Theodore 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 intervals 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 appeared to be the major limitation to plant growth at the inland sites of Theodore and Gayndah where post-establishment irrigation increased final dry weight by 250%. Waterlogging reduced plant height increase at the low lying Mt Cotton site following higher than normal rainfall. Increase in plant height at the other sites stopped when night temperatures fell below 15°C. Application of P at 75-1200 kg/ha tripled final dry weight of leucaena at Mt Cotton (5 mg/kg bicarbonate-extractable 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. Application of N (200 kg N/ha) increased dry weight of Rhizobium-inoculated leucaena 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 dry weight of leucaena at Mt Cotton despite the soil being slightly acidic (pH 5.9 in 1:5 HO suspension). Further work on the role of colonisation by arbuscular mycorrhiza fungi in determining uptake of P by leucaena is warranted in view of the large response to very high rates of P in young seedlings at Mt Cotton