An oxygen sensor was inserted directly into the soil for the determination of oxygen content in paddy soils.The rate of chemical consumption of oxygen in the soil was related to the reduction intensity of the reductant.The rate was faster for organic reducing substances and sulfide,requiring 20-30 minutes for the attainment of a steady state.The reaction for ferrous iron required 2-3 hours.The reaction rate with manganous ions was the slowest, even when a large amount of Mn²⁺ was added to the soil.The change in oxidation-reduction potential followed that of the oxygen content,with an abrupt fall to lower than 100 mV when the oxygen content decreased to less than 1-2 mg/1.