Early growth and N2-fixation of leucaena and gliricidia at different levels of phosphorus application

The phosphorus (P) uptake and use efficiency in relation to N₂ fixation and growth of three provenances of gliricidia (Gliricidia sepium) two cultivars of leucaena (Leucaena leucocephala) and one ofL. diversifolia, all inoculated withRhizobium, were determined at five levels of P application. Increasing the P application rate resulted in a 33% increase in dry matter and total N ofL. diversifolia and an 18% dry matter increase of gliricidia provenance 13/84 over the control without P. Leucaena K8, which did not respond to P application, yielded as well with low P asL. diversifolia at the high P indicating the lower P requirement of cultivar K8. Leucaena cultivar Singapore did not yield well at either low or high P. Correlation analyses showed that differences between species and cultivars/provenances for P uptake per plant were largely related to differences in shoot growth rate and the distribution of P between roots and tops. P use efficiency was not related to dry-matter production. Genetic control of P and dry matter distribution of is probably more important than P availability in the growth media for gliricidia and leucaena provenances or cultivars. Increasing the P application rate significantly increased the number and mass of nodules in leucaena cultivars and gliricidia provenances. Significant increases in the proportion of N derived from atmospheric N₂ (% Ndfa) due to low P application (20 mg P kg^–1 soil) were observed within leucaena but not for gliricidia provenances. No significant increases in % Ndfa occurred with higher P application rates suggesting that symbiotic N₂ fixationper se is stimulated only with low rates of P.