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+4 availability 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+4 adsorption in soils exhibiting vermiculitic behavior. The purpose of this study was to demonstrate the influence of K+ on NH+4 mobility 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+4 plus 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+4 movement through the column was impeded by soil retention. For the Maury soil in the presence of K+, the NH+4 BTC appeared approximately 1 pore volume to the left of the NH+4 curve in the absence of K+, signifying that K+ competed with NH+4 for soil surface exchange sites, whereas for the Eden soil in the presence of K+, the NH+4 BTC appeared approximately 5 pore volumes to the left of the NH+4 curve 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 ExNH4 but did not change Fix NH4 levels, whereas for the Eden soil, adding K+ reduced ExNH4 and greatly increased Fix NH+4. FTIR analysis of the soils revealed NH+4 deformation bands at 1454 and 1430 cm−1, suggesting that NH+4 was held in the Maury and Eden soils in two chemically distinct binding environments, possibly representing two different NH+4 sinks, i.e., exchangeable and fixed. Furthermore, this distortion of the tetrahedral molecule, evidenced by the shifts of the NH+4 deformation bands to wavenumbers beyond 1399 cm−1 for free or uncomplexed NH+4, led us to propose that the vermiculite interlayer was more stereospecific for the NH+4 ion 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+4 complexation directly. We propose that K+ fixation collapsed the interlayer around NH+4 ions, thus seeming to induce NH+4 fixation.