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Poor soil fertility is a major constraint for crop production that is commonly corrected with inorganic fertilizers, but their high cost or environmental and health effects have necessitated alternative management strategies. Hence, the dry biomass of Mexican sunflower (Tithonia diversifolia) was evaluated as a sustainable alternative resource to improve soil fertility and production of African nightshade (Solanum nigrum L.) A field experiment was laid out as randomized complete block design with four treatments (Control – no fertilizer, urea, NPK and Tithonia) and four replications. Soil available phosphorus ranged from 10.3–16.3 mg kg-1 and differed significantly (P ˂ 0.05) across treatments, with the highest in Tithonia and NPK as compared to urea and control. In comparison to the baseline soil, residual soil phosphorus increased by 7.7% for Tithonia, but decreased by 1.7% for NPK, 27.7% for control, and 46.3% for urea. The yield of African nightshade correlated significantly (P < 0.05) with the content of soil available phosphorus, and ranged from 8.6–14.9 t ha-1 fresh and 1.5–2.5 t ha-1 dry biomass that differed significantly (P ˂ 0.05) across treatments, with the highest in NPK as compared to control. Plant height ranged from 24.6–33.2 cm and differed significantly (P ˂ 0.05) across treatments, with the highest in NPK. The number of leaves ranged from 80–117 per plant and differed significantly (P < 0.05) across treatments, with the highest in NPK as compared to urea. Both Tithonia and inorganic fertilizers increased the yield of African nightshade comparatively, which demonstrates the potential of Tithonia biomass as a sustainable alternative resource for soil fertility management in vegetable production systems.