Which of the following is not non-essential element for plants?

Options

A. potassium
B. sodium
C. aluminium
D. cobalt

Answer

To determine which of the following is not a non-essential element for plants, we need to analyze the options provided: potassium, sodium, aluminum, and cobalt.

### Step-by-Step Solution:

1. **Understanding Essential and Non-Essential Elements**:
- Plants require certain elements for their growth and development. These elements are categorized into essential and non-essential elements.
- Essential elements are those that are necessary for plants to complete their life cycle, while non-essential elements are not required for growth but can be beneficial.

2. **Identifying Essential Elements**:
- There are 17 essential elements for plants, which include macronutrients (required in larger quantities) and micronutrients (required in smaller quantities).
- Macronutrients include elements like nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur.
- Micronutrients include elements like iron, manganese, zinc, copper, molybdenum, boron, and chlorine.

3. **Analyzing the Options**:
- **Potassium**: This is a macronutrient and is essential for plants. It plays a crucial role as a cofactor for many enzymes and is important for cell turgidity and nutrient transport.
- **Sodium**: This is generally considered a non-essential element for most plants, although some plants may benefit from it.
- **Aluminum**: This is typically regarded as a non-essential element and can even be toxic to plants in high concentrations.
- **Cobalt**: This is also considered a non-essential element for most plants, although it can be beneficial in trace amounts.

4. **Conclusion**:
- Among the options provided, potassium is the only element that is essential for plants, while sodium, aluminum, and cobalt are non-essential.
- Therefore, the answer to the question "Which of the following is not a non-essential element for plants?" is **potassium**.

### Final Answer:
**Potassium**