This paper reports a soil-based bioassay of rice husk bio-ash impacts on redox properties and rice production. Bio-ash was produced during the pyrolytic process of rice husk as a boiler fuel, after which it was applied as an amendment to a flooded soil. Bio-ash was amended before rice transplanting at different mass rates including 0%, 0.1%, 1% and 5%, which was defined as T1, T2, T3, and T4 respectively. Results show that rice husk bio-ash in flooded soil could increase soil Eh value, pH value

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... and organic matter content (OM), and also reduce reductant amount and activity. The numbers of rice white roots of T2 and T3 increased by 30% and 8% compared to that of T1 in the tillering stage. Increased grain yields due to bio-ash amendments at the rate of 0.1% and 1% were 3.43% and 11.8% higher than that of T1.