The kinetics of potassium release to sodium tetraphenylboron solution from the clay fraction of highly weathered soils

Thirty six soil clay samples from surface and subsurface horizons of 18 Thai upland Oxisols and Ultisols under a tropical monsoonal environment were characterized for chemical composition, mineralogy and K release to 0.3 M sodium tetraphenylboron (NaTPB) solution for periods up to 168 h. The clay fraction was dominated by kaolinite with various amounts of accessory minerals and analytical TEM showed that most clays contained small amounts of illite. The amounts of K released to the NaTPB solution from illite-containing clays were higher than from clays containing kaolinite and inhibited vermiculite with no detectable illite. Potassium release kinetics for all samples are adequately described by the parabolic diffusion (r = 0.91–0.99), power function (r = 0.87–0.99) and Elovich equations (r = 0.86–0.99). The intercept constants of the equations are strongly positively related to the ratio of illite to kaolinite and total K and Mg contents of the clays. These relationships indicate that minor amounts of illite in these clays strongly affect the kinetics of K release. The slope constants of the parabolic and Elovich equations are also positively related to these clay properties. Some kaolinite particles contained K which may be present in residual micaceous layers interleaved in kaolinite crystals. XRD patterns from samples extracted with NaTPB show a decrease in illite peak intensity with a concomitant increase in vermiculite peak intensity due to K removal from illite by NaTPB.