Low molecular organic acids have been utilized in kinetic studies on potassium release from K-bearing minerals in soils. This study was undertaken to investigate the kinetics of potassium release from two soils of the state of Rio Grande do Sul, Brazil. A horizon samples (0-20 cm) of a Gleisol and a Chernosol were extracted with oxalic acid 0.01 mol L-1, in fifteen sequential extractions ranging from 1 to 864 h, through a period of 3,409 h. Oxalic acid induced K release from sand, silt and clay in both soils. The parabolic diffusion equation showed that potassium release occurred at different rates, in two phases for sand and silt, and in three phases for clay, at different rates. The amounts of potassium released from soil fractions were only 3.4% and 6.2% of total K, in the Gleisol and in the Chernosol, respectively.
available potassium; parabolic diffusion equation; oxalic acid