The model considers transformation between three phases of K in soil, viz. solution, exchangeable and non-exchangeable. A kinetic type of reaction was assumed between each two subsequent phases, i.e. a forward and a backward reaction, describing in total four reactions and four rate constants viz. adsorption, desorption, fixation and release. Except adsorption reaction which was Freundlich type of fractional order, all reactions were of first order. A computer programme written in Personal Computer Continuous Simulation Modelling Programme (PCSMP) language has been used to simulate three curves viz. delta K, exchangeable K and total labile K versus solution K concentration and fitted with the experimental curves of an Ustochrept, a Chromustert and a Plinthaqualf by adjusting the parameters. Unlike the existing model, the experimental results showed that the fixation did not start immediately after the exchangeable K exceeds release threshold level, but showed a gap between release threshold level and fixation threshold level. Modification of the computer programme by simulating fixation to start at a higher value than the release threshold level and multiplying fixation rate constant by a factor termed as hysteresis factor, the model fits the data closely with variance of fit less than 0.002 for six parameters.
Threshold concept, potassium release and fixation, kinetic model, hysteresis
