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 i n soils and is a potential soil test method for plant-available K+. El even 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 nonexchangeab le K+ release, and (iii) measure changes in mineralogy associated with K+ release by NaBPh4. Nonexchangeable K+ release in most soils was al most complete in 96 h and was well described by Elovich, simplified El ovich, parabolic diffusion, and power function equations (r(2) = 0.962 to 0.997). However, neither form of the Elovich model described the e arly phase of K+ release adequately. A rate index defined as the produ ct of the two rate parameters of the power function equation was relat ed to illite K+ (r(2) = 0.995). Fit of the data to the parabolic diffu sion model yielded two separate straight lines, suggesting two differe nt rates of K+ diffusion from illite. Potassium release rates increase d as particle size decreased, indicating that in whole soils, fast ear ly release is primarily from the fine and coarse clay fractions, and s lower late release is mainly from the coarse clay and fine silt fracti ons. X-ray diffraction analysis showed that expansion of the 1.01-nm b asal spacing to 1.23 nm was nearly complete in the 96-h NaBPh4 treatme nt. These results show that mathematical models can be used to infer t he mechanism(s) of nonexchangeable K+ release by NaBPh4 and estimate t he relative K+-supplying power of soils.