Nitrification in soil is characterized by a sigmoidal curve with a delay, maximum rate and retarded phase. A model based on the Verhulst equation describes all three phases of the nitrification process excellent. Equations to calculate the maximum nitrification rate (Kmx) and the delay period (t') exist already. Equations to calculate the approximate duration (?t) and termination (ts) of the maximal rate phase were also derived from the Verhulst equation and are reported. The duration of maximal nitrification when NH4+is oxidized at a maximal rate is the period from the end of the delay period until the retarded phase starts. The termination of the maximal rate phase gives an approximate time of when the retarded phase, due to a depletion of HH4+, starts. At this stage in the nitrification process additional NH4+must be applied to prevent the minimum changes in the maximum nitrification rate of a soil. When first‐order kinetics is used, the course of nitrification can in future consequently be described quantitatively by four parameters Kmx, t’, ?t and ts. Similar parameters can also be calculated from experimental data of other biological reactions which are characterized by sigmoidal curves.