Nitrification is

Options

A. performed only by plants.
B. the reduction of ammonium ions to nitrate ions.
C. the reduction of nitrate ions to nitrogen gas.
D. performed by certain bacteria in the soil.

Answer

**Step-by-Step Solution:**

1. **Understanding Nitrification**: Nitrification is a biological process that involves the conversion of ammonium ions (NH4+) into nitrates (NO3-) through the action of specific bacteria in the soil.

2. **Identifying the Bacteria Involved**: The process of nitrification is primarily carried out by certain types of bacteria. The first step of nitrification is performed by bacteria such as Nitrosomonas and Nitrococcus, which oxidize ammonium ions to nitrites (NO2-).

3. **Conversion of Nitrites to Nitrates**: The second step involves the oxidation of nitrites to nitrates, which is carried out by another group of bacteria known as Nitrobacter.

4. **Evaluating the Options**:
- The first option states that nitrification is performed only by plants, which is incorrect as it is a bacterial process.
- The second option suggests the reduction of ammonium ions to nitrate ions, which is also incorrect because nitrification involves oxidation, not reduction.
- The third option mentions the reduction of nitrate ions to nitrogen gas, which is a process known as denitrification, not nitrification.

5. **Conclusion**: The correct answer is that nitrification is performed by certain bacteria in the soil, specifically by Nitrosomonas, Nitrococcus, and Nitrobacter.

**Final Answer**: Nitrification is performed by certain bacteria in the soil.

---