All plants require nitrogen, phosphorus and potassium as their primary nutrient. Thus oxygen is not considered as primary nutrient for plants.

Usually, as we know that plants need carbon-dioxide (CO2) for their respiration and in contrary, plants do evolve oxygen (O2). So, the ans. is oxygen. hope it helps :)

Primary nutrients are essential elements that plants require in large quantities for their growth and development. These nutrients are necessary for various physiological and metabolic processes within the plant. Oxygen is not considered a primary nutrient for plants because it is primarily obtained from the air and water, and plants have evolved mechanisms to efficiently utilize the available oxygen.

Here is a detailed explanation of why oxygen is not considered a primary nutrient for plants:

1. Definition of primary nutrients:
- Primary nutrients are elements that are required in relatively large quantities by plants for their proper growth and development.
- These nutrients play crucial roles in various plant processes, such as photosynthesis, respiration, and nutrient absorption.

2. Role of oxygen in plants:
- Oxygen is involved in cellular respiration, which is the process by which plants convert stored energy into a usable form.
- During respiration, plants utilize oxygen to break down sugars and release energy for their metabolic activities.
- Oxygen is also necessary for the breakdown of nutrients in the soil, promoting nutrient availability for plant uptake.

3. Oxygen availability:
- Unlike other primary nutrients like nitrogen, phosphorus, and potassium, plants do not directly obtain oxygen from the soil.
- Oxygen is primarily obtained from the air through the plant's leaves and stems.
- Oxygen can also be absorbed by plant roots from the soil, but the concentration of oxygen in the soil is usually sufficient for plant growth.
- Oxygen is highly abundant in the air and water, and its availability is generally not a limiting factor for plant growth.

4. Oxygen as a byproduct of photosynthesis:
- Plants produce oxygen as a byproduct of photosynthesis, the process by which they convert sunlight into energy-rich carbohydrates.
- Oxygen produced during photosynthesis is released into the air, contributing to the oxygen content of the atmosphere.
- This oxygen is then available for other living organisms, including humans, to breathe.

In conclusion, oxygen is not considered a primary nutrient for plants because it is primarily obtained from the air and water, and its availability is usually not a limiting factor for plant growth.