The green manure plant, Tithonia diversifolia (Tithonia) has been identified as having the potential to significantly enhance soil phosphorus (P) cycling in high P fixing, acid tropical soils, farmed by resource-poor smallholders. It has been hypothesized that Tithonia has the potential to exploit soil P pools that are largely unavailable to crop plants such as rice and maize. To support agronomic research, controlled environment experiments were conducted. Aspects of phosphorus uptake by Tithonia and the influence of Tithonia residues on crop P uptake were investigated. The kinetics of the P uptake transporters were determined for whole Tithonia root systems using a solution culture P depletion method. At a whole root system level the P uptake kinetic parameters were similar to those determined for a range of crop plants by the same method. Radiotracers were used to investigate P translocation in Tithonia. High concentrations of P (ca. 3.5g P kt1) have been reported in Tithonia shoots, of plants growing in low-available-P soils. The results of this study indicated that P accumulates in the vacuoles of green tissues, with P acquired by roots being supplemented by P from rapidly senescing lower leaves. The uptake of P by roots and associated mycorrhizal fungi was investigated using radiolabelled substrates. Tithonia plants were grown in rhizotrons with or without root access, but always with hyphal access, to radiolabel P placed in a subcompartment. The presence of Tithonia roots contributed to improved uptake of a relatively labile Po source, but hyphae were able to take up inorganic P, even when bound to iron or calcium in similar quantities to when roots were also present. Tithonia has been reported as possessing allelochemicals that inhibit seed germination and root development. To determine if this might constrain use of Tithonia as a green manure, allelopathic effects of Tithonia applications were determined. Root and shot growth of maize, but not wheat, was reduced 10 days after being placed in soil in which Tithonia shoots had been applied, at rates similar to those used by farmers growing crops with Tithonia as a green manure. Low plant recovery of mineral P fertilizer discourages farmers with limited cash resources from investing in P fertilisation. Small mineral P applications, applied with Tithonia residues have been reported to improve total P recovered by the subsequent crop. In an experiment in which wheat was grown in rhizotrons with subcompartment radiolabelled P applications, it was shown that to improve P uptake and plant growth, Tithonia and mineral P fertiliser had to be combined. This indicates that the effect is not strongly associated with improved general crop nutrition, but is due to microbial P immobilisation, followed by mineralisation at rates synchronous to crop P demand. Although these experiments and those of other workers suggest that expectations for Tithonia in enhancing P cycles in P limiting soils may be without solid foundation, the research has advanced understanding of the issues involved in the use of green manures to increase crop available P in tropical soils.