The nitrification process is most active when the pH is between

The nitrification process is most active when the pH is between 6.5 and 7.5.
Nitrification is the biological oxidation of ammonia to nitrite followed by the oxidation of the nitrite to nitrate occurring through separate organisms or direct ammonia oxidation to nitrate in comammox bacteria.
Nitrification is pH-sensitive and rates of nitrification will decline significantly at pH values below 6.5. The resulting decrease in pH could lead to decreasing rates of nitrification.