With the increasing participation of biomass in the world energy matrix, large amounts of ash are produced through combustion, resulting in the need to dispose of this waste to minimize the environmental impact. An alternative is to use ashes as phosphorus supplements in microalgae cultures. The present work describes the development and use of a balanced culture medium based on the minimal processing of oil palm biomass ash to cultivate Arthrospira platensis Paracas, Neochloris oleoabundans UTEX 1185, and Dunaliella salina SAG 184. The acid extraction process of phosphorus (P) was defined by evaluating the following parameters: temperature (20 to 70 °C), acid load (0.01 to 0.03 mols/g of ash) of HNO3, and liquid/solid ratio (50 to 150 mLg−1). The best efficiency of the extraction process was 97%. The use of HNO3 allowed for the production of an extract containing balanced amounts of N and P sources, the BAX medium (Biomass Ash Extract). This medium was efficient for cultivating the three microorganisms studied, reaching biomass concentrations of 2.03, 0.902, and 0.69 g/L or 84%, 82%, and 99% of the control concentrations for A. platensis, N. Oleoabundans, and D. salina, respectively. In a final scaling-up test, A. platensis showed productivity of 0.047 g L−1d−1 in a 120 L tank in a greenhouse. BAX can be an alternative nutrient medium for microalgae cultivation, especially in integration with biomass-fueled biorefineries.