Macronutrients: 

Such essential elements which are found in a plant in the range of 1.0 to 10.0 mg per gram of dry weight of plant are called macronutrients or mega nutrients. 

These are further divided into two categories. 

• Primary macronutrients – Nitrogen, Phosphorus and Potassium are also called as critical elements and soils normally show a deficiency of these elements.
• Secondary macronutrients – Calcium, Magnesium, and Sulphur are secondary macronutrients. Essential elements: 
• Green plants synthesize carbohydrate by photosynthesis using absorbed from atmosphere and water absorbed from the soil. 
• Plants require several inorganic mineral elements and these are derived from soil or atmosphere. 
• Out of 105 elements found on earth, about 60 elements have been found present in plants, but all these are not essential for plants. 
• Various studies have revealed that for normal growth of a plant, seventeen (17) elements are necessary. These are called essential elements. 
• Many non-essential elements present beyond a certain limit may be toxic to plants. 
• A criterion for determining the essentiality of 17 essential elements: 
•  These elements are necessarily required for normal growth, development and reproduction of plants. 
• The function of the essential elements should be specific and its deficiency can be fulfilled only by this element and not by any other element. 
• The element must be directly involved in the metabolism of the plant. 
•  A specific symptom must appear in the plant due to deficiency of a specific element and the symptom must disappear by a supply of that element. 

Macronutrients: 

• Elements which are found in the plant in an amount from 1.0 to 10.0 mg per gm of the dry weight of the plant are called macronutrients. 
• These are nine in number – C, H, O, N, P, K, S, Mg, and Ca. 
• Among this N, P and K are considered as primary macronutrients and also called as critical elements whereas Ca, Mg and S are considered as secondary macronutrients.