Arbuscular mycorrhizal fungi (AMF) are generally involved in the adaptation of native plants to ultramafic soils, especially in New Caledonia. These soils are deficient in major elements, particularly in phosphorus (P), and are rich in potentially toxic metals such as nickel, chromium, cobalt, and manganese. We aimed to test the effects of increasing P doses on mycorrhizal functions of plants grown on P-deficient ultramafic soil. We analyzed the effects of soil P fertilization on growth, mineral nutrition and potentially toxic metal absorption of plants inoculated or not with AMF native isolates. Three endemic plants frequently used in the ecological restoration were tested: Metrosideros laurifolia (Myrtaceae), Alphitonia neocaledonica (Rhamnaceae), and Tetraria comosa (Cyperaceae). They were grown in pots supplied with different doses of P, after being inoculated or not with AMF. P fertilization increased greatly the growth rate of all three species. In pots non-supplied with P, only M. laurifolia showed a higher growth rate when inoculated with AMF, but all plant species showed different positive effects of mycorrhizal symbiosis, such as better mineral nutrition, particularly for potassium (K) and calcium and a higher calcium/magnesium values (Ca/Mg). Mycorrhizal colonization was not reduced by P supply, but the specific positive effects of AMF on growth and mineral nutrition were reduced or suppressed. Negative effects of P fertilization on mycorrhizal functions were induced without reduction of mycorrhizal colonization. As the adaptive traits of the three plants to ultramafic soils were obtained by a reduction of their growth rate, we hypothesized that the high increase of this growth rate induced by P fertilization could have altered this adaptive structure.