Plants require different kinds of elements for their maintenance, growth and survival. These elements are obtained through soil, air, water or from other organisms.
Some elements are naturally present in the plant in sufficient quantities and do not need to be refurbished frequently while there are approximately 20 different elements which must be supplied for the normal growth and development of the plant.
For example, C, H, O, N, S, P, K, Ca, Mg, Zn, Mo, Fe, Ni, Co, B, Na, Si, Cu, Cl, etc.
These elements are called essential elements if they fulfil the following three criteria given by Arnon and Stout,
1. A plant cannot complete its life cycle without the element. (Indispensable.)
2. No other element can replace it for performing the same function.(Irreplaceable.)
3. The element must be directly involved in plant metabolism.
The essential elements are further classified into macroelements and microelements, depending on the quantity in which they are required to be supplied.
Macronutrients are needed in comparatively larger quantities (approx. > 1 mg/l) than micronutrients.
For example, C, H, O, N, P, K, Ca, Mg and S are macronutrients whereas Fe, Mn, B, Mo, Cu, Zn, Cl, Ni, Co, S, Si are micronutrients.
There are certain non-essential elements which do not fit in all the above criteria of essential elements, but they still can be found in the plant ash, or in the soil or indirectly improve the plant health.
They cannot be considered as essential elements.