Which one of the following is not an essential micronutrient for plants ?

Correct option: 3) Sodium. Widely accepted lists of essential plant micronutrients include boron, zinc and copper; these elements are required in trace amounts for normal growth and metabolism [1]. Textbook lists commonly name iron, zinc, manganese, copper, boron, chlorine and molybdenum as core micronutrients and emphasize their very low concentrations in plant tissues [2]. Sodium is not part of these core micronutrient lists and is generally regarded as non-essential for most plants; it may be beneficial or considered essential only in certain species or under special circumstances, but it is not included among the standard essential micronutrients used in plant nutrition frameworks [2].

Sources

1. [1] Indian Economy, Nitin Singhania .(ed 2nd 2021-22) > Chapter 9: Agriculture > FERTILISERS > p. 302
2. [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 25: Agriculture > z. Micronutrientsl > p. 363