This study investigated the effects of applying modified bone char by sulfur (MBC) with humic acid and co-applying bone char (BC) with sulfur (S) or humic acid (HA) on chemical properties, phosphorus (P) availability, and spinach growth in calcareous sandy soil. This pot experiment has twelve treatments: Control (CK), bone + S (BS), bone + HA (BHA), BC + S (BCS), BC + HA (BCHA), MBC, MBC + HA (MBCHA), acidified BC with 0.1 N H2SO4 (0.1ABC), acidified BC with 1 N H2SO4 (1ABC), rock phosphate (RP), RP + S (RPS), and RP + HA (RPHA). The B, BC, MBC, 0.1ABC, 1ABC, and RP were added at 300 mg P kg− 1 soil doses. Spinach was grown in this experiment. Applying all treatments significantly increased soil phosphorus availability. Available phosphorus increased from 11.61 mg kg− 1 (CK) to 19.70, 19.76, 21.82, 22.25, 22.45, 26.09, 19.58, 21.01, 15.26, 18.95, and 17.77 mg kg− 1 for BS, BHA, BCS, BCHA, MBC, MBCHA, 0.1ABC, 1ABC, RP, RPS, and RPHA, respectively. The effectiveness of the treatments in this study on the available phosphorus improvement was in the order of MBCHA > MBC > BCHA > BCS > 1ABC > BHA > BS > 0.1ABC > RPS > RPHA > RP > control. Compared to the control treatment, applying BHA, BCS, BCHA, MBC, MBCHA, 1ABC, RPS, and RPHA to the soil significantly increased the fresh shoot of the spinach plant. Fresh shoot of spinach increased from 46.02 g pot− 1 for CK to 54.41, 54.36, 56.94, 50.39, 51.91, 48.83, 54.24, and 49.52 g pot− 1 for BHA, BCS, BCHA, MBC, MBCHA, 1ABC, RPS, and RPHA, respectively. The effectiveness of treatments in improving the fresh weight of spinach was in the order of BCHA > BHA ≈ BCS > RPS > MBCHA > MBC > RPHA > 1ABC > control > RP > BS > 0.1ABC. Our results concluded that co-applying bone char with sulfur is optimal for enhancing soil quality indicators and improving fresh and dry shoots of spinach. Due to its cheaper price, it is preferable to add sulfur with bone char rather than humic acid.