Step 1: Placing nitrification inside the nitrogen cycle.
In soil, nitrogen continuously changes its chemical form through the nitrogen cycle.
Plants mainly absorb nitrogen in the form of nitrate \( \text{NO}_3^- \), not ammonia directly.
Therefore, ammonia must first be converted into nitrate before plants can use it.
Step 2: Identifying the required chemical changes.
Nitrification involves two oxidation reactions:
\[ \text{NH}_3 \rightarrow \text{NO}_2^- \rightarrow \text{NO}_3^- \]
Since oxidation reactions are involved, aerobic bacteria are required for this conversion.
Step 3: Matching bacteria with each step.
– The conversion of ammonia to nitrite is carried out by bacteria that oxidize ammonia.
– The conversion of nitrite to nitrate is carried out by bacteria that oxidize nitrite.
Step 4: Identifying the correct bacterial pair.
Nitrosomonas → oxidizes ammonia to nitrite.
Nitrobacter → oxidizes nitrite to nitrate.
Together, they complete the nitrification process in soil.
Step 5: Eliminating other options.
(B) Azotobacter and Rhizobium → fix atmospheric nitrogen, not oxidize ammonia.
(C) Pseudomonas and Bacillus → mainly involved in decomposition and denitrification.
(D) Clostridium and Thiobacillus → associated with anaerobic metabolism and sulfur cycle.
Final Conclusion:
The bacteria responsible for nitrification in soil are:
\[ \boxed{\text{Nitrosomonas and Nitrobacter}} \]