An incubation experiment was conducted on a clay loam soil using a factorial design to study the effects of temperature, water content, and applied NH4+-N on the change of NO3−-N and NH4+-N concentrations over time. Each of the factors and their interactions were highly significant, indicating the importance of the often neglected moisture–temperature interaction. Due to this interaction, optimum moisture contents for the activity of the nitrifying population appeared to be dependent upon temperature. Approximately one half of the added NH4+ was fixed within 2 days after (NH4)2SO4 was added to the soil. A concurrent study showed that soluble NH4+, exchangeable NH4+ and clay-fixed NH4+ existed in equilibrium upon adding NH4+. A Langmuir kinetic model might describe the relationship between exchangeable and fixed NH4+ whereas a nonlinear Freundlich equilibrium model yielded the best agreement between the soluble and exchangeable NH4+.