A study was carried out to investigate changes in the soil plant-available P, the P nutrition and the growth of Pinus radiata seedlings grown in association with understory, broom (Cytisus scoparius L.) or ryegrass (Lolium multiflorum) on Orthic Allophanic Soil, following the application of three rates of triple superphosphate (TSP) (0, 50, and 100 mg·kg-1P) under a glasshouse condition. The application of P fertilizer enhanced P availability in the rhizospheric of radiata seedlings and the bulk soils in a P-deficient site. P availability in the rhizospheric soils of ryegrass and broom, grown in association with radiata, were also increased by the presence of radiata roots. P concentrations in new shoot needles, old shoot needles, stem and roots of radiata pine increased with increase rates of TSP application, but the effects of ryegrass and broom on P nutrition of radiata seedlings depended on the soil P status. In the absence of P fertilizer addition (control treatment), P concentrations in new shoot needles, old shoot needles, stem, and roots of radiata grown in association with broom were higher than those with ryegrass, whereas, when P fertilizer was added (50 and 100 mg·kg-1) the P concentration was lower. This is probably related to the growth of broom that may have removed much of the plant-available P in the soil as indicated by the consistently lower Bray-2 P concentration in the rhizosphere soil of radiata in association with broom than that in the rhizosphere soil of radiata in association with grass at the two high P rates. Furthermore, in the high P fertile soil (application rate of 100 mg·kg-1), the dry matter yield of radiata was lower when it was grown with broom than with ryegrass. This result suggests that in moderate to high P fertile soils, P. radiata seedlings grow better with ryegrass than with broom, because broom grows vigorously in high P fertile soil and competes with P. radiata for P and perhaps for other nutrients as well.