The limiting factor in nitrogen fixation of soil is

Options

A. soil nature (pH)
B. light
C. temperature
D. air

Explanation

**Step-by-Step Solution:**

1. **Understanding Nitrogen Fixation**:
- Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted into a form that plants can absorb, primarily ammonium (NH4+). This process is crucial for plant growth as nitrogen is an essential nutrient.

2. **Role of Soil pH**:
- The pH of the soil is a critical factor influencing nitrogen fixation. It affects the activity of nitrogen-fixing bacteria and the availability of nutrients in the soil.

3. **Optimum pH for Nitrogen Fixation**:
- The optimum pH for effective nitrogen fixation is around 4.0. At this pH, the conditions are favorable for the bacteria that facilitate this process.

4. **Effects of pH Variations**:
- If the soil pH is too low (acidic), it can lead to increased toxicity in the soil, which hampers nitrogen fixation.
- Conversely, if the pH is too high (alkaline), it can lead to salinity issues, which also negatively impacts nitrogen fixation.

5. **Conclusion**:
- Given the importance of soil pH in the nitrogen fixation process, it can be concluded that the limiting factor in nitrogen fixation of soil is the soil pH.

**Final Answer**: The limiting factor in nitrogen fixation of soil is **soil pH**.

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