How is it decided that the given element is a macronutrient or micronutrient?

If an element found in plant is larger amounts i.e., in excess of 10 m mole kg-1 of dry matter, it is considered macronutrient.
If an element found in plant is very small amounts i.e., less than 10 m mole kg-1 of dry matter, it is considered as micronutrient.