An agricultural soil was treated with dairy-waste compost, ammonium-sulfate fertilizer or no added nitrogen (control) and planted to silage corn for 6 years. The kinetics of nitrification were detemined in laboratory-shaken slurry assays with a range of substrate concentrations (0–20 mM NH4+) over a 24-h period for soils from the three treatments. Determined concentrations of substrate and product were fit to Michaelis–Menten and Haldane models. For all the treatments, the Haldane model was a better fit, suggesting that significant nitrification inhibition may occur in soils under high ammonium conditions similar to those found immediately after fertilization or waste applications. The maximum rate of nitrification (Vmax) was significantly higher for the fertilized and compost-treated soils (1.74 and 1.50 mmol N kg−1 soil day−1) vs. control soil (0.98 mmol kg−1 soil day−1). The Km and Ki values were not significantly different, with average values of 0.02 and 27 mM NH4+, respectively. Our results suggest that both N sources increased nitrifier community size, but did not shift the nitrifier community structure in ways that influenced enzyme affinity or sensitivity to ammonium. The Km values are comparable to those determined directly in other soils, but are substantially lower than those from most pure cultures of ammonia-oxidizing bacteria.