Which of the following is not one of the three plants macronutrietns included in most fertilisers?

Options

A. O
B. N
C. P
D. K

Explanation

To solve the question "Which of the following is not one of the three plant macronutrients included in most fertilizers?", we can follow these steps:

### Step-by-Step Solution:

1. **Understand Macronutrients**: Begin by defining what macronutrients are. Macronutrients are essential nutrients that plants need in large quantities for growth and development. They are typically required in amounts greater than 10 millimoles per kg of dry matter.

2. **Identify the Main Macronutrients**: The primary macronutrients for plants include:
- Nitrogen (N)
- Phosphorus (P)
- Potassium (K)
- Additionally, Calcium (Ca), Magnesium (Mg), and Sulfur (S) are also considered macronutrients.

3. **Recognize Common Fertilizers**: Most fertilizers are formulated to provide the three key macronutrients that plants require in significant amounts. These are:
- Nitrogen (N)
- Phosphorus (P)
- Potassium (K)

4. **Evaluate the Options**: The question likely provides a list of options. Identify which of the options is not one of the three primary macronutrients commonly found in fertilizers.

5. **Identify the Correct Answer**: Based on the common knowledge of fertilizers, if the option includes Oxygen (O), it is the correct answer because while Oxygen is a macronutrient, it is not typically included in fertilizers as a nutrient source.

### Conclusion:
The answer to the question is **Oxygen**. It is not one of the three primary macronutrients included in most fertilizers.

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