State one reason for adding blue-green algae to the agricultural soil.

Explanation

**Step-by-Step Solution:**

1. **Identify the Organism**: Blue-green algae, also known as cyanobacteria, are microorganisms that play a significant role in agriculture.

2. **Understand Nitrogen Fixation**: One of the primary functions of blue-green algae is nitrogen fixation. This is a natural process where atmospheric nitrogen (N₂) is converted into ammonia (NH₃) by certain bacteria.

3. **Role in Soil Fertility**: The ammonia produced through nitrogen fixation is then added to the soil. This ammonia is a vital nutrient that plants can absorb through their roots.

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