Astragalus nitidiflorus is an endangered legume endemic to the southeast of the Iberian Peninsula. This species develops symbiotic relationships with N-fixing bacteria. However, the problem of isolating its rhizobia has not been solved. Because poor N fixation in plants can be corrected by fertilization, the effect of N-P-K fertilizers on growth, leaf chlorophyll and mineral ions was studied. Plants of A. nitidiflorus were grown in 100%-substrate with different N-P-K fertilizer rates (mg l^–1): 1-1-8 (S0), 69-29-35 (SL), 144-43-131 (SM) and 245-58-235 (SH). A treatment with substrate plus soil from the natural habitat and no fertilizers (T0) was included. The reference foliar contents of N, P and K were 42.5, 3.5 and 36.5 mg g^–1, respectively. Although the species did not form root nodules when grown in substrate, T0 plants produced active nodules that allowed the plants to grow properly without fertilization. In the absence of nodules, both N fertilization (~144 mg l^–1) and Fe fertilization (>12 mg l^–1) are vital, as is, to a lesser extent, K fertilization (~75 mg l^–1 K₂O). The S0 and SL reduced leaf chlorophyll, while SM prevented its degradation.

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