Which element is not considered as macronutrient?
A. Mg
B. Ca
C. Mn
D. P

Hint: There are certain elements which are necessary for the plants growth and development and they are known as essential elements. The essential elements can be divided into - macronutrients and micronutrients.

Complete answer:
All elements are not essential for plant nutrition, only a few are essential for the growth and development of plants. These are called essential elements. A total of 17 elements are known to be essential nutrients for plants. In relatively large amounts, the soil supplies nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur, these are often called the macronutrients.

Let us analyze all the given options to find out the correct answer.
Option A: Magnesium (Mg) is an essential plant nutrient. It has a wide range of key roles in many plant functions. One of magnesium’s well-known roles is in the photosynthesis process, as it is a building block of the Chlorophyll, which makes leaves appear green. It acts as a phosphorus carrier in plants and is essential for phosphate metabolism. Magnesium is the central core of the chlorophyll molecule in plant tissue. It is required for the overall growth of the plant. Therefore, this is an incorrect option.

Option B: The role of Ca is to hold together the cell walls of plants. It is an essential plant nutrient. It is needed in large amounts by all plants for the formation of cell walls and cell membranes. Therefore, this is an incorrect option.

Option C: Elements like iron, boron, manganese, copper, zinc, and molybdenum are required for plants only in very small amounts but these are indispensable for the normal growth and development of plants. Manganese is an important micronutrient for plant growth and development and sustains metabolic roles within different plant cell compartments. Therefore, this is the correct option.

Option D: Phosphorus is an essential micronutrient. Its deficiency is a major consideration in cropping systems. It is an essential part of the components of DNA and RNA and is involved in cell membrane function and integrity. It is also a component of the ATP system of plants and animals. Therefore, this is an incorrect option.

Thus, the correct option is C) Mn.

Note: The macronutrients nitrogen (N), phosphorus (P), and potassium (K) or NPK for short is a complex fertilizer composed primarily of the three primary nutrients required for healthy plant growth.