Leucaena (Leucaena leucocephala) is relatively slow to establish on acidic soils of low nitrogen status, partly due to its inherent character of slow nodulation. Relatively few studies have examined the
use of combined nitrogen and lime to promote seedling and subsequent growth of leucaena on such soils.
Two experiments, one in the field and another in glasshouse, were conducted on an acidic red-yellow podzolic (Ultisol) soil in south- east Queensland to examine these aspects.
The field experiment was conducted for one year from December, 1982. The long-term effect of lime (2.5 t ha-1) and combined nitrogen (25 kg N ha-1), applied at establishment., on shoot growth was estimated 8 and 12 months after planting.
The percentage increase in shoot dry weight to combined nitrogen application after 8 and 12 months was almost the same (31.8 and 32.9 respectively). But, for lime the shoot dry weight increased from 43 to 55 per cent. The response to the combined application of
nitrogen and lime was additive.
In the glasshouse, using large pots, the effect of rates of application of combined nitrogen (0, 50, 100, 200 and 400 kg N ha-1) on leucaena seedling growth in the absence or presence (3 t ha-1) of lime at 25, 50 and 75 days was investigated.
At day 25 and day 50 lime significantly increased plant height but had no effect on other plant characters. Combined nitrogen significantly increased shoot dry weight and plant height at both harvests.
At day 75, lime significantly increase shoot, leaf, root and nodule dry weights, plant height and leaf number in the absence of applied nitrogen. It had no effect
at N50 or N100 and depressed shoot, leaf and root dry weights at N200 and N400. Possible reasons for the negative interaction at N200 and N400 were discussed.
Combined nitrogen at day 75 significantly increased plant height and leaf, shoot and root dry weights. Nodule number and nodule dry weight were significantly increased up to Nl00, severely reduced at N200 and completely suppressed at N400. Lime and combined nitrogen up to N100 significantly increased
Lime significantly increased root : shoot ratio at N0 only while combined nitrogen progressively decreased root :shoot ratio up to N40 .
The nitrogen status of the youngest fully expanded leaves was significantly increased by combined nitrogen at rates above N50 at day 25, N200 at day 50 and N100 at day 75.
It was concluded that on an acidic soil of low nitrogen status, application of combined nitrogen up to 100 kg N ha-1 or lime in the absence of combined nitrogen would improve leucaena growth at establishment stages and lime, in addition, would have a long-term beneficial effect in promoting subsequent leucaena growth. Practical implications of the results were mentioned.