Essential elements may be classified into three groups

 1. MAJOR ELEMENTS OR PRIMARY NUTRIENTS

The essential elements, which are required by the plants in comparatively larger amounts are called

Major Elements or Primary Nutrients. The list includes C, H, O, N, P, and K.

2. SECONDARY ELEMENTS or NUTRIENTS

The secondary elements are also required by the plant in larger quantities next to primary nutrients. Examples

are: Ca, Mg and S.

3. MINOR ELEMENTS or MICRONUTRIENTS or TRACE ELEMENTS

The essential elements required in smaller amounts or traces by the plants are called Minor or

Trace Elements. They are Fe, Mn, Cu, Zn, Mo, B, and Cl. Apart from these elements, recently some more

elements have also been shown to the essential for the normal growth of some plants such as Na for Atriplex,

Si for rice and Cl for coconut and Al, Va and Co for ferms.

Micronutrients are usually present in the plants in different chemical forms as :

1. Inorganic ions

2. Undissociated molecules or

3. Organic complexes as chelates

Crieria of Essentiality of Elements (Arnon and Stout, 1939)

In order to show that element is truly essential, it is necessary to show not only that :

i. A deficiency of the element makes it impossible for a plant to complete

its vegetative and reproductive cycle but also that

ii. It cannot be replaced by another element and that

iii. The element should also have some part to play in metabolism

The effect is not simply the result of interaction with other non-essential elements,

organism etc. outside the plant.

The effect is not simply the result of interaction with other non-essential elements,

organism etc. outside the plant.

These three requirements form the criteria of essentiality of mineral elements. However, recent

studies indicated that the functions of some of the elements could be partly replaced by others. (e.g. potassium by

rubidium; magnesium by manganese).