Select a best fertilizer.

Options

A. Bacillus thuringiensis and lichens
B. synthetic fertilizers
C. Azolla and cyanobacteria
D. all of the above

Answer

To select the best fertilizer from the options provided, we will analyze each option step by step:

### Step 1: Understand the options
The options given are:
1. Bacillus thuringiensis
2. Lichens
3. Synthetic fertilizers
4. Azolla and cyanobacteria
5. All of the above

### Step 2: Analyze Bacillus thuringiensis
- Bacillus thuringiensis (Bt) is a gram-positive bacterium known for producing a toxin that is effective against certain insect pests.
- It is primarily used in genetically modified organisms (GMOs) like Bt cotton, which is resistant to insects.
- **Conclusion**: It is not a fertilizer.

### Step 3: Analyze Lichens
- Lichens are symbiotic associations between fungi and algae or cyanobacteria.
- They are not used as fertilizers but can be found on the roots of higher plants and contribute to soil formation.
- **Conclusion**: They are not a fertilizer.

### Step 4: Analyze Synthetic Fertilizers
- Synthetic fertilizers are chemical products designed to enhance soil fertility.
- While they can increase crop yields, they may have negative effects on soil health and the environment.
- **Conclusion**: They are not considered the best option for sustainable agriculture.

### Step 5: Analyze Azolla and Cyanobacteria
- Azolla is a free-floating aquatic fern that has a symbiotic relationship with cyanobacteria (e.g., Anabaena).
- This association is known for its ability to fix atmospheric nitrogen, converting it into ammonia, which is beneficial for plant growth.
- **Conclusion**: Azolla and cyanobacteria are considered biofertilizers and are effective in enhancing soil fertility sustainably.

### Step 6: Final Conclusion
- The best fertilizer among the options provided is **Azolla and cyanobacteria** due to their ability to fix nitrogen and improve soil fertility without the negative impacts associated with synthetic fertilizers.

### Answer:
**Azolla and cyanobacteria are the best fertilizers.**

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