Which one of the following elements is not an essential micronutrient for plant growth?
a) Ca
b) Mn

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Hint: It has an atomic number 20 and is mostly found in teeth and bones.Plant growth requires various nutrients for their proper growth and development. Some nutrients are required in very less quantity and some nutrients are required in large quantities by the plants. According to their need of nutrients the nutrients can be divided into micronutrients and macronutrients.

Complete answer:
 The micronutrients are the mineral elements required in very less quantity by the plants. Some of the essential micronutrients required by the plants are boron, zinc, manganese, iron, copper, molybdenum and chlorine. The elements which are required in large quantities by the plants are known as macronutrients. Some of the essential macronutrients needed by the plants are nitrogen, calcium, magnesium, sulphur, potassium and phosphorus. Calcium is considered as the essential macronutrient for the plant growth. Micronutrients present in the plants are necessary for their healthy development and they also help in preventing the diseases and maintaining the well-being of the plant. The plants get their major nutrients from the soil. There are also many nutrients which can be used up from the atmosphere also. Farmers use fertilizers and manures to enrich the soil with the nutrients. This helps the crops to grow in good condition and also helps them in keeping away from the diseases.
So, the correct answer is ‘Ca’.

Note: Calcium is the essential macronutrient. It is required in large quantities by the plants to carry out various life processes. It is not a micronutrient. The micronutrients are the nutrients which are required in very small quantities. Soil helps the plant to obtain the natural elements present in it.