Kinetics studies of potassium release from soils can improve our knowledge about its availability to plants. The present study, carried out in 1993, was undertaken to investigate the kinetics of potassium release and to compare four kinetics equations to describe the K-release from three soils of the State of Rio Grande do Sul, Brazil. Samples of A horizons of a Vertisol, of an Ultisol and of an Oxisol, were equilibrated with a cation exchange resin (IR-120) from one minute up to 2,400 hours. Exchangeable potassium was totally released to the H-saturated resin in 12 minutes in all soils. The cumulative amounts of non-exchangeable-K released from soils decreased in the sequence Vertisol (436 mg kg-1 K) > Ultisol (64 mg kg-1 K) > Oxisol (46 mg kg-1 K). The parabolic diffusion equation described the data satisfactorily as shown by the highest correlation coefficient and the lowest standard error of the estimate. Coefficients for non-exchangeable-K release rate were 2.09 x 10-2 h-1 for the Vertisol, 1.89 x 10-2 h-1 for the Ultisol and 0.97 x 10-2 h-1 for the Oxisol.
non-exchangeable potassium; diffusion equation; kinetics; potassium