The fixation of free nitrogen by bacteria in the soil is done by

Options

A. Azotobacter
B. Nitrosomonas
C. Nitrobacter
D. Thiobacillus.

Answer

### Step-by-Step Solution

1. **Understanding Nitrogen Fixation**: Nitrogen fixation is the process by which atmospheric nitrogen (N₂) is converted into a form that can be utilized by living organisms, primarily plants. This process is crucial for the nitrogen cycle and is primarily carried out by certain bacteria.

2. **Identifying the Bacteria**: Among the options provided:
- **Azotobacter**: A free-living bacterium that is known for its ability to fix atmospheric nitrogen into a usable form for plants.
- **Nitrosomonas**: This bacterium is involved in the nitrification process, converting ammonia (NH₃) into nitrite (NO₂⁻).
- **Nitrobacter**: This bacterium further converts nitrite (NO₂⁻) into nitrate (NO₃⁻), also part of the nitrification process.
- **Thiobacillus**: This bacterium is involved in the process of denitrification, which reduces nitrates to nitrogen gas, thus releasing nitrogen back into the atmosphere.

3. **Correct Answer**: Based on the roles of the bacteria listed, **Azotobacter** is the correct answer as it is specifically known for fixing free nitrogen in the soil.

4. **Additional Information**: Azotobacter not only aids in nitrogen fixation but also promotes seed germination and enhances crop growth, making it beneficial for agriculture.

5. **Conclusion**: Therefore, the fixation of free nitrogen by bacteria in the soil is primarily done by **Azotobacter**.