Release kinetics of nonexchangeable potassium in soils using sodium tetraphenylboron

Sodium tetraphenylboron (NaBPh4) can extract both exchangeable K+ and nonexchangeable K+ held in the interlayer of phyllosilicate minerals in soils and is a potential soil test method for plant-available K+. Eleven midwestern soils were used to (i) compare the ability of various kinetic equations to model nonexchangeable K+ release by NaBPh4, (ii) determine the contribution of particle size fractions to nonexchangeable K+ release, and (iii) measure changes in mineralogy associated with K+ release by NaBPh4. Nonexchangeable K+ release in most soils was almost complete in 96 h and was well described by Elovich, simplified Elovich, parabolic diffusion, and power function equations (r(2) = 0.962 to 0.997). However, neither form of the Elovich model described the early phase of K+ release adequately. A rate index defined as the product of the two rate parameters of the power function equation was related to illite K+ (r(2) = 0.995). Fit of the data to the parabolic diffusion model yielded two separate straight lines, suggesting two different rates of K+ diffusion from illite. Potassium release rates increased as particle size decreased, indicating that in whole soils, fast early release is primarily from the fine and coarse clay fractions, and slower late release is mainly from the coarse clay and fine silt fractions. X-ray diffraction analysis showed that expansion of the 1.01-nm basal spacing to 1.23 nm was nearly complete in the 96-h NaBPh4 treatment. These results show that mathematical models can be used to infer the mechanism(s) of nonexchangeable K+ release by NaBPh4 and estimate the relative K+-supplying power of soils.