The term esential mineral nutrient or element was proposed by Aron and Stout (1939).  They concluded that for a nutrient to be considered to be "essential" three criteria must be met:

	A plant must be unable to complete its life cycle in the absence of the mineral nutrient.
	The function of the plant nutrient must not be replaceable by another mineral nutrient.
	The nutrient must be directly involved in plant metabolism.  For example, as a component of essential plant constituent or required for a distinct metabolic process, such as an enzyme reaction.