Which of the following is not an essential nutrient for plants?

Sodium is not considered an essential nutrient for plants. While it can play a role in certain physiological functions, such as ion balance and the synthesis of specific compounds, it is not necessary for all plants to complete their life cycle. Essential nutrients are those that are required for plant growth and development, and a deficiency would inhibit growth or lead to reproductive failure.

Calcium, nitrogen, and phosphorus, on the other hand, are classified as essential nutrients. Calcium contributes to cell wall structure and stability, nitrogen is crucial for protein synthesis and overall growth, and phosphorus is key for energy transfer and photosynthesis. These nutrients are vital for proper function and development, making sodium's role more of a supporting one rather than essential.