In this question, we are asked to identify the nutrient that is not a structural component of plants. Let`s analyze each option to better understand:

Option 1: Nitrogen - Nitrogen is an essential nutrient for plant growth and is required in relatively large amounts. It is a vital component of amino acids, proteins, and chlorophyll, which are important for plant structure and function. Therefore, nitrogen is a structural component of plants.

Option 2: Calcium - Calcium plays a critical role in plant cell wall development and strength. It is necessary for root, shoot, and leaf growth, as well as in the regulation of various plant processes. Calcium is indeed a structural component of plants. Therefore, option 2 is incorrect.

Option 3: Phosphorus - Phosphorus is involved in several key plant processes, including energy transfer, photosynthesis, and DNA synthesis. It is an essential component of nucleic acids, sugars, and ATP, all of which are vital for plant structure and function. Therefore, phosphorus is a structural component of plants.

Option 4: Potassium - Potassium is necessary for various plant functions, such as water uptake, enzyme activation, and carbohydrate metabolism. It also contributes to plant growth, but it is not a primary structural