FACTORS AFFECTING THE EARLY GROWTH OF LEUCAENA-LEUCOCEPHALA .3. ROLE OF INDIGENOUS ARBUSCULAR MYCORRHIZAL FUNGI AND ITS IMPORTANCE IN DETERMINING YIELD OF LEUCAENA IN POTS AND IN THE FIELD

Previous work has shown that slow colonisation of roots by arbuscular mycorrhizal (AM) fungi may be responsible for the slow early seedling growth of leucaena in some soils. The aim of the following experiments was to determine relative population levels of AM fungi in a range of Australian soils and their effect on the growth and phosphorus nutrit ion of the tropical tree legume leucaena (Leucaena leucocephala), grow n in pots and in the field. Soils chosen were typical of those used fo r leucaena production in Queensland and included sites at which previo us commercial plantings of leucaena had failed. Large differences were found in mycorrhizal population levels between soils and results of a bioassay ranged from 12 to 52% of leucaena root length infected. Resu lts of the bioassay were significantly correlated with plant height of leucaena grown in pots at 28 and 42 days after sowing (P<0.05) and sh oot weight 42 days after sowing (P<0.05). However, results of the bioa ssay were not positively correlated with plant growth of leucaena 63 d ays after sowing in pots or at any time in the field. This was because colonisation levels in most soils had reached levels that were adequa te to meet the phosphorus requirements of the plant. Low population le vels of AM fungi, therefore, do not appear to be a major long-term lim itation to growth of leucaena in most soils. However, at 2 sites where previous commercial leucaena plantings had failed, other factors beca me relatively more important in limiting first season yield. These inc luded poor soil physical structure, low phosphorus fertility and at 1 site, colonisation of leucaena roots with root knot nematode, a factor which had not previously been reported in Australia. Soil physical, c hemical and biological factors, therefore, all need to be considered i n selecting suitable sites for leucaena establishment.