FACTORS AFFECTING THE EARLY GROWTH OF LEUCAENA-LEUCOCEPHALA .2. IMPORTANCE OF ARBUSCULAR MYCORRHIZAL FUNGI, GRASS COMPETITION AND PHOSPHORUS APPLICATION ON YIELD AND NODULATION OF LEUCAENA IN POTS

Slow seedling growth is a Limitation to the more widespread adoption o f the tree legume, leucaena (Leucaena leucocephala). Three glasshouse trials examined the role of arbuscular mycorrhiza and phosphorus (P) n utrition in determining early growth and nodulation of leucaena. Treat ments included soil types, inoculation with arbuscular mycorrhizal (AM ) fungi, P application, grass competition and fumigation with methyl b romide, an anti-fungal agent. Plant measurements included colonisation by AM fungi, nodule weight, tissue nitrogen (N) and P concentrations. Slower early growth of leucaena in a soil from Mt Cotton than in soil s from Gayndah or Theodore was due to slow colonisation of roots by AM fungi. Sequential harvests of plants revealed that rate of colonisati on in the Mt Cotton soil was only half that in the Theodore soil prior to 28 days after planting resulting in subcritical P concentrations 2 1 days after sowing and an approximate halving of top dry weight 41 da ys after sowing. However, following increased infection, tissue P conc entration and final plant growth 98 days after sowing were similar in both soils. Early seedling growth in the Mt Cotton soil was increased by inoculating the soil with mulch containing AM fungi but not with so il collected from beneath established leucaena added at a lower rate. Phosphorus application significantly increased growth of leucaena seed lings, but only the highest rate of 1200 kg P/ha was able to prevent e arly P deficiency. Final growth was reduced by 50% in the presence of Panicum maximum as a result of increased competition for N and P and b y 90% in fumigated soil as a result of P deficiency. The results of th ese experiments confirm the important role of AM fungi on early seedli ng growth of leucaena. However, the potential to increase early growth using a soil or mulch inoculum containing AM fungi or P fertiliser ma y be limited by the high rates of application needed. More work is nee ded to determine whether slow rate of infection is a significant limit ation in soils other than the Mt Cotton soil in the field.