11th Class Biology Mineral Nutrition In Plants Essential Mineral Elements

An essential element is defined as 'one without which the plant cannot complete its life cycle, or one that has a clear physiological role'. Therefore, in 1939 Arnon and Stout proposed the following characters for judging the criteria of essentiality of an element in the plant :

(1) The element must be essential for normal growth and reproduction, which cannot proceed without it.

(2) The requirement of the element must be specific and cannot be replaced by another element.

(3) The requirement must be direct that is, not the result of any indirect effect e.g., for relieving toxicity caused by some other substance. 

Essential elements are divided into two broad categories, based on the quantity in which they are required by plants. Macro-elements and micro-elements. Their ionic forms are respectively called macronutrients and micronutrients. Mineral salts dissolved in soil solution are constantly passing downwards along with percolating (gravitational) water. The phenomenon is called leaching. Leaching is more in case of anions.

Macronutrients (Macroelements or major elements) : Which are required by plants in larger amounts (Generally present in the plant tissues in concentrations of 1 to 10 mg per gram of dry matter). Of the non-essential functional elements, silicon and sodium often occur in the range of macroelements. Macroelements are usually involved in the synthesis of organic molecules and development of osmotic potential.

Micronutrients (Microelements or minor elements or trace elements) : Which are required by plants in very small amounts, i.e., in traces (equal to or less than 0.1 mg per gram dry matter). Cobalt, vanadium, aluminium and nickel, may be essential for certain plants. Microelements are mostly involved in the functioning of enzymes, as cofactors or metal activators. The usual concentration of essential elements in higher plants according to D.W. Rains (1976) based on the data of Stout are as follows :