<p>Phosphorus (P) deficiency and strong P fixation limit crop productivity in arid and semi-arid soils. This study evaluated waste-derived biochars from cow bones (BBC) and shrimp processing residues (SWB) as slow-release P sources compared with superphosphate (SP) for enhancing soil biochemical properties and sweet basil (<i>Ocimum basilicum</i> L.) productivity. Incubation and pot experiments were conducted in fluvial (Entisols) and calcareous (Aridisols) soils to evaluate the impact of BBC and SWB on P fractionation, release kinetics, soil enzyme activity, and plant growth. Biochars were characterized using SEM, FTIR, and EDX to assess surface morphology and chemical structure. SWB exhibited higher porosity and organic–mineral content than BBC. Both biochars significantly increased microbial biomass carbon (up to 126.1% in sandy soil) and alkaline phosphatase activity (up to 212.5%), whereas SP showed minimal effects. Phosphorus release followed power function kinetics, with SWB achieving higher early desorption and labile P increases (65.2–51.4%), while BBC favored moderately stable P fractions. Sweet basil grown in biochar-amended soils showed enhanced nutrient uptake, particularly P (185–196%), N (110–128.9%), K (228.6–236.4%), and Ca (141–180%), higher leaf area (51.4–65.2%), increased fresh and dry biomass (53.8–54.5%), and greater photosynthetic pigment concentrations compared with SP or control treatments. SWB and BBC function as effective slow-release P fertilizers, improving soil biochemical activity, nutrient availability, and crop productivity, while offering a sustainable alternative to soluble mineral P fertilizers in arid soils.</p> Graphical abstract <p></p>