What are essential elements for plants? Give the criteria of essentiality? How are minerals classifieds depending upon the amount in which they are needed by the plants?

An element which fulfills the criteria of essentiality are called an essential element for plants. Following are the various criteria for essentiality. (a) The element must be absolutely essential for supporting normal growth and reproduction in plant. (b) The requirement of element must be specific and not replaceable by any other element (c) The element must be directly involved in metabolism in the plant. On the basis of the amount in which minerals are needed by plants, they are divided into two categories, viz. macronutrients and micronutrients. Macronutrients: Elements which are present in large amounts, i.e. more than $10\mathrm{m}$ mole per kg of dry matter are called macronutrients. Examples: carbon. hydrogen, oxygen, nitrogen. etc. micronutrients: Elements which are present in small amounts, i.e. less than 10 $\mathrm{m}$ mole per kg of dry matter are called micronutrients. Examples: iron, calcium, magnesium. etc.