In soils containing vermiculite, fixed NH+4(Fix NH+4) levels are affected by exchange reactions with other cations, which may directly impact both NH+4availability to nitrifiers and mobility through the soil. Some previous soil suspension studies have shown that in certain soils, K+is highly effective at displacing adsorbed NH+4, whereas in other studies, K+induced high affinity NH+4adsorption in soils exhibiting vermiculitic behavior. The purpose of this study was to demonstrate the influence of K+on NH+4mobility in soils in a heterovalent ternary cation exchange system (K-NH4-Ca). For this study, we selected two Kentucky soils of mixed mineralogy with varying quantities of vermiculite: a Maury silt loam (fine, mixed, mesic Typic Paleudalfs) topsoil (0-6 cm) and an Eden clay loam (fine, mixed, mesic, Typic Hapludalfs) subsoil (15-30 cm). Ammonium breakthrough curves (BTC) were obtained by leaching packed soil columns with either 5 mM NH+4plus 1 mM Ca2+solution or 5 mM NH+4, 5 mM K+, plus 1 mM Ca2+solution. Ammonium BTC with and without added K+for both soils indicated, as expected, that NH+4movement through the column was impeded by soil retention. For the Maury soil in the presence of K+, the NH+4BTC appeared approximately 1 pore volume to the left of the NH+4curve in the absence of K+, signifying that K+competed with NH+4for soil surface exchange sites, whereas for the Eden soil in the presence of K+, the NH+4BTC appeared approximately 5 pore volumes to the left of the NH+4curve without K+. This signified an apparently strong competitive effect by K+, with respect to NH+4, for soil surface exchange sites. However, extraction of soil subsamples from the columns revealed that adding K+to the Maury soil reduced ExNH4but did not change Fix NH4levels, whereas for the Eden soil, adding K+reduced ExNH4and greatly increased Fix NH+4. FTIR analysis of the soils revealed NH+4deformation bands at 1454 and 1430 cm−1, suggesting that NH+4was held in the Maury and Eden soils in two chemically distinct binding environments, possibly representing two different NH+4sinks, i.e., exchangeable and fixed. Furthermore, this distortion of the tetrahedral molecule, evidenced by the shifts of the NH+4deformation bands to wavenumbers beyond 1399 cm−1for free or uncomplexed NH+4, led us to propose that the vermiculite interlayer was more stereospecific for the NH+4ion than for the physically and chemically distinct spherical K+ion. That the IR spectra were identical for both soils in the absence and presence of added K+implied agreement with the extraction data that K+was not able to affect interlayer NH+4complexation directly. We propose that K+fixation collapsed the interlayer around NH+4ions, thus seeming to induce NH+4fixation.