Nitrification can best be explained as the conversion of_ to_by bacteria

Nitrification is a two-step biological process in the nitrogen cycle carried out by specialised soil bacteria. In the first step, ammonia (\(\text{NH}_3\)) or ammonium (\(\text{NH}_4^+\)) is oxidised to nitrites (\(\text{NO}_2^-\)) by bacteria such as Nitrosomonas. In the second step, nitrites are further oxidised to nitrates (\(\text{NO}_3^-\)) by bacteria such as Nitrobacter.

The overall process can be summarised as:

\[\text{NH}_4^+ \xrightarrow{Nitrosomonas} \text{NO}_2^- \xrightarrow{Nitrobacter} \text{NO}_3^-\]

Nitrification is therefore best described as the bacterial conversion of nitrites to nitrates (the second and defining step that produces the plant-available nitrogen form). Nitrates are the form of nitrogen most readily absorbed by plant roots.

The conversion of nitrates to gaseous nitrogen is called denitrification, not nitrification. The conversion of nitrates to ammonium compounds describes ammonification in reverse or assimilatory nitrate reduction. The conversion of nitrates to nitrites would be a reversal of the nitrification process and does not describe nitrification.