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 nutrition of the tropical tree legume leucaena (Leucaena leucocephala), grown in pots and in the field. Soils chosen were typical of those used for leucaena production in Queensland and included sites at which previous 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. Results 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 shoot weight 42 days after sowing (P<0.05). However, results of the bioassay were not positively correlated with plant growth of leucaena 63 days after sowing in pots or at any time in the field. This was because colonisation levels in most soils had reached levels that were adequate to meet the phosphorus requirements of the plant. Low population levels of AM fungi, therefore, do not appear to be a major long-term limitation to growth of leucaena in most soils. However, at 2 sites where previous commercial leucaena plantings had failed, other factors became relatively more important in limiting first season yield. These included 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, chemical and biological factors, therefore, all need to be considered in selecting suitable sites for leucaena establishment.