Acid sulfate soils iface major constraints, including high soil acidity, metal toxicity (Al and Fe), and low phosphorus (P) availability due to strong fixation, resulting in low agricultural productivity on these lands. This study aims to analyze the interaction effect of applying rice husk biochar and chicken manure compost ameliorants on increasing phosphorus availability (available P) by improving their supporting chemical properties, namely pH and redox potenstial (Eh), in acid sulfate soils. The iment was conducted using a two-factor Completely Randomized Design (CRD). The results showed that there wa a significant interaction between the combination of these two materials on all observed parameters. The combination of biochar and compost created a dual amelioration mechanism, in which the biochar’s alkalinity neutralized acidity in contrast, the compost’s organic acids chelated toxic metal ions and released phosphorus bound to the biochar. The combination treatment of 5 ts/ha of rice husk biochar and  2 ts/ha of chicken manure compost (B5A2) proved to be the best interaction. This treatment effectively increased soil pH from 4,62 to 5,20, suppressed oxidative conditions by decreasing the Eh value from 310 mV to 224 mV, thereby successfully optimizing phosphorus availabilitiy (available P) from 3,27 ppm to 6,19 ppm. In conclusion, the B5A2 dose combination is recommended as an effective and sustainable acid sulfate soil management strategy to increase phosphorus nutrient availability.