Potassium-release mechanism on drying soils: nonexchangeable to exchangeable potassium by protonation of micas1

Experiments were designed to determine the mechanism by which drying soil causes an appreciable increment of exchangeable K (not exchangeable from the wet soil with M NH4OAc) to become exchangeable with M NH4OAc after the soil is dried. Earlier, this release was identified with the presence of fine micas. The hypothesis that the release mechanism involved proton dehydration from hydronium (OH3) ion could be tested because the dissociation of HOH leaves one OH ion in solution for each proton consumed in the mica-K release process. The moles H consumed g soil were measured by the rise in pH (OH in solution = H consumed = K released). Soil materials containing clay-sized micas were washed with pH 5 M NaOAc and CaCl2; subsamples were dried for 16 h at 24, 60, or 110°C. The moles exchangeable K g soil extracted with M NH4OAc from different subsamples were determined by flame emission spectrometry. The respective moles OH g soil produced were measured by the pH of an unbuffered M KCI supernatant solution from a duplicate sample. Undried control samples were treated in parallel. The pH increase (φ) in the unbuffered supernatant M KCI solution served as a direct measure of the total proton uptake by the sample from HOH, some of which is still present even at 110°C, as shown by DTA. For ease of comparison with φ, the increase on drying of moles K L g soil was calculated similarly, as ΔpK. A high correlation (24 to 110°C; r = 0.99; p < 0.001; slope of regression equation, 0.985) was obtained between φ and ΔpK. These expressions are the increased activity ratio of K and of OH as moles g soil, for the dried samples in excess of wet samples. The mechanism proposed is that hydrated protons are progressively dehydrated by the drying, so that they can escape the steric hindrance characteristic of hydrated or larger cations to penetration into the structural site of mica K, and form -SiOH. The protons (about 10 times the size of a water molecule) are able to penetrate into the interlayer of mica and exchange for structural K of -SiOK, forming -SiOH as an isolated crystal disorder. A similar mechanism could possibly fit K release from other potassium-bearing minerals.