Kinetics of Nonexchangeable Potassium Release from Soils by Sodium Tetraphenylboron method

Nonexchangeable soil-K release rate can significantly affect K dynamics and it's bioavailability during plant growth. In this study, twelve soil samples taken from different wheat cropping regions of Iran were incubated with NaBPh4 to (i) investigate the kinetics of nonexchangeable K release and (ii) to compare the ability of various kinetic equations (zero-, first- and second-order, parabolic diffusion, simplified Elovich and power function) to describe nonexchangeable K release. Results revealed that potassium release was initially rapid followed by a slower rate. Almost 80% of total released soil-K after a time passage of 192h occurred in the first 48hs. The portion of soil total K that was released by NaBPh4 during the 8 day incubation period varied between 12.8 and 55.8% in different soils. NaBPh4-extracted K in a 5min period was highly correlated with NH4OAc- extracted K (r2=0.78, p<0.0001). Results indicated that nonexchangeable K release from most soils has been approximately completed after 96h incubation time. Comparison of coefficients of determination (r2) and standard errors of estimate (SE) showed that the power function, parabolic diffusion and simplified Elovich equations would be of the capability to describe the nonexchangeable K release (r2=0.951 to 1.000). Rate index, as the product of two rate parameters of the power function equation, was highly correlated (r2=0.99) with illitic-K in soils where K release was approximately completed within 192h period. Fit of the data to the parabolic diffusion model yielded two linear relationships with different slopes, suggesting two different rates of K diffusion from soils.