Without the rotation of crop by a pulse in between cereals, the farmer observed the increases in natural fertility due to

Options

A. Rhizobium
B. Azotobacter
C. Acetobacter
D. Clostridium

Answer

### Step-by-Step Solution

1. **Understanding the Context**: The farmer observed an increase in natural fertility without rotating crops with pulses. This suggests that some natural processes or organisms in the soil contributed to this increase.

2. **Identifying Key Organisms**: The transcript mentions several bacteria: Rhizobium, Azotobacter, Acetobacter, and Clostridium. We need to identify which of these is responsible for the increase in soil fertility.

3. **Analyzing Azotobacter**: Azotobacter is a free-living nitrogen-fixing bacterium. It converts atmospheric nitrogen into ammonia, which is a form of nitrogen that plants can utilize. This process enriches the soil with nutrients, thereby increasing its fertility.

4. **Comparing Other Bacteria**:
- **Rhizobium**: This is also a nitrogen-fixing bacterium but is symbiotic, meaning it forms a relationship with plant roots. It is not directly responsible for the observed increase in fertility without crop rotation.
- **Acetobacter**: This bacterium converts ethanol to acetic acid and does not play a role in nitrogen fixation or soil fertility.
- **Clostridium**: This is a pathogenic bacterium that can cause diseases and is not beneficial for soil fertility.

5. **Conclusion**: The increase in natural fertility observed by the farmer is primarily due to the presence of Azotobacter in the soil, which is facilitating the conversion of atmospheric nitrogen into ammonia.

### Final Answer
The increase in natural fertility observed by the farmer is due to the activity of **Azotobacter**, a nitrogen-fixing bacterium that converts atmospheric nitrogen into ammonia.