Mineral elements that are essential

The inorganic nutrients are classified as essential elements and non essential elements.

17 elements have been placed under essential elements.

These are the elements without which the reproduction and life cycle of a plant cannot be completed.

The essential elements are : C, H, O, N, P, K, S, Mg, Ca, Fe, Mo, Mn, Ni, Zn, B, Cl, Cu.

Essentiality of Minerals

A variety of mineral elements are present in the soil but all of them are not essential for plant growth.

Besides, a particular element may be needed for the growth of one plant and may not be required at all by other plants.

For example, sodium is required in very small amount by the desert shrub Atriplex, but is not required by most of the other plants.

Following criteria given by D.I. Arnon and P.R.Stout (1939) are used to determine essentiality of minerals:

1.The element must be absolutely necessary for normal growth and reproduction. The plant do not complete its life cycle or set the seed in the absence of that particular element

2.The element must not be replaceable by another element.

3.The element must play a direct role in the metabolism of plant.

4.Absence of a specific element causes deficiency in the plant which is corrected only by adding the specific mineral in the soil.

Types of Essential Elements

On the basis of concentration in plant, Hoagland divided essential elements into two groups.

(i) Macronutrients : These are present in more concentration like 1-10 mg per gram of dry weight. These are easily detectable. e.g., C, H, O, N, P, S, Ca, K, Mg.

(ii) Micronutrients : These elements occur in plant body in concentration of equal or less than 0.1 mg per gram of dry weight. Infact these are required in traces, so called trace elements. e.g., Mo, Mn, Zn, B, Cu, Cl, Fe, Ni.

In addition to the 17 essential elements, there are some beneficial elements such as sodium silicon, cobalt and selenium. They are required by higher plants.

General Functions of Mineral Elements

(a) Frame work elements – Form carbohydrates which form cell wall, e.g., C, H, O.

(b) Protoplasmic elements – Form protoplasm, e.g., C, H, O, N, P, S.

(c) Catalytic elementse.g., Fe, Cu, Zn, Mo, Mg, Mn, K (activator of over 40 enzymes)

(d) Balancing element – Ca, Mg and K counteract the toxic effect of other minerals.

(e) Storage elements – C, N, S, P.

(f) Critical elements – N, P, K.

(g) Minerals influence OP and TP.

(h) Monovalent cations (Na+, K+) Increases permeability of membrane, while divalent and trivalent ions decrease it.

(i) Toxic elments e.g., Al, As, Hg, Pb, Ag.

(j) Non mineral elements e.g., C, H, O, N. N is both mineral and non mineral.

(k) Functional elements: They are non essential in most plants but have a definite activity in some species e.g., silicon in grasses.