Define biofertilizers.

Explanation

**Step-by-Step Solution:**

1. **Definition of Biofertilizers**: Biofertilizers are defined as living microorganisms that enhance the nutrient quality of the soil.

2. **Function of Biofertilizers**: They work by increasing the availability of essential nutrients to plants, thereby promoting plant growth and soil health.

3. **Main Sources**: The primary sources of biofertilizers include:
- **Bacteria**: Certain bacteria can fix atmospheric nitrogen, making it available to plants.
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