Fertility of paddy fields is improved by addition of

Options

A. Rhizobia
B. Gypsum
C. Sodium sulphate
D. Blue- Green Algae

Answer

### Step-by-Step Solution:

1. **Understand the Question**: The question asks which addition improves the fertility of paddy fields. Fertility can be enhanced by increasing the availability of essential nutrients in the soil.

2. **Identify Key Nutrients**: The primary nutrients that contribute to soil fertility are nitrogen (N), phosphorus (P), and potassium (K). These are often referred to as NPK fertilizers.

3. **Evaluate Options**: The question provides several options. We need to evaluate each one to determine if it contributes to the fertility of paddy fields.

4. **Analyze Each Option**:
- **Option 1: Rhizobia**: Rhizobia are nitrogen-fixing bacteria that form symbiotic relationships with leguminous plants. Since paddy (rice) is not a legume, Rhizobia would not enhance its fertility.
- **Option 2: Gypsum**: Gypsum (calcium sulfate) is used to improve soil structure but does not significantly increase nutrient content, particularly nitrogen.
- **Option 3: Sodium Sulfate**: Sodium sulfate provides sodium and sulfate ions but does not enhance the fertility of paddy fields.
- **Option 4: Blue-green algae (Cyanobacteria)**: These organisms, such as Azospirillum and Anabaena, can fix atmospheric nitrogen and increase nitrogen content in the soil without needing a symbiotic relationship with plants. This makes them beneficial for paddy fields.

5. **Conclusion**: Based on the analysis, the addition of blue-green algae (cyanobacteria) is the best choice to improve the fertility of paddy fields due to their ability to fix nitrogen.

### Final Answer:
The fertility of paddy fields is improved by the addition of **blue-green algae (cyanobacteria)**.

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