High concentrations of potassium salts added to soil have been shown to inhibit nitrification as a result of high osmotic pressures. However, low levels of added K+have also been shown to inhibit nitrification in vermiculite-clay suspensions. The latter result led to the hypothesis that K+rendered NH+4unavailable to nitrifiers by inducing NH+4fixation. Although there is evidence that K+promotes NH+4fixation in soils containing vermiculite, the potential for this mechanism to directly control nitrification in soils has not been demonstrated experimentally. The purpose of this study was to determine if adding K+inhibited nitrification in soils containing vermiculite by inducing NH+4fixation. The soils used, a Maury silt loam (fine-silty, mesic, Typic Paleudalf) and an Eden silt loam (fine, mixed, mesic, Typic Hapludalf) were first partially loaded with NH+4by equilibrating them with a 5 mM(NH4)2SO4plus 1 mMCaSO4solution. Nitrification was then studied in suspensions by adding a quantity of pretreated soil sample (equivalent to 11.45 mg adsorbed NH4-N) to flasks containing a combined solution of 1 mMCaSO4and 0, 150, or 300 ppm K+(0, 1580, or 3160 μg K+g−1soil) as K2SO4. Flasks were bubbled continuously with atmospheric air and shaken at room temperature for 15 days. Nitrification seemed to be promoted initially by the addition of K+to both soil suspension systems. Nitrification continued after solution NH+4was apparently depleted in both soil systems. Total NO2-N and NO3-N produced was highest in the Maury soil, with 300 ppm K+, whereas the most nitrification occurred in the Eden soil with 0 ppm K+. Extractions of soil suspension subsamples after the 15-day incubation period revealed a significant increase in fixed NH+4(FixNH4), with added K+in the Eden soil but not in the Maury soil. This study provided evidence that nitrification in the Eden soil was inhibited in the presence of K+because of enhanced NH+4fixation.