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Explain with examples :
(1) Macronutrients,
(2) Micronutrients,
(3) Toxic elements ,
(4) Essential elements.
Answer
(a) Macronutrients :

(i) Macronutrients are generally present in plant tissues in large amounts in excess of 10m mole kg–1 of dry matter. For example - Carbon, hydrogen, nitogen, phosphorous, and oxygen 

(b) Micronutrients : Micronutrients or trace elements are needed in less than 10mmole kg–1  of dry matter eg. iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

(c) Toxic elements : Any mineral ion or element which reduces dry weight by 10% of plant is called toxic element. Toxicity of element differs from plant to plant. The elements when present in large amounts much more than what is needed becomes toxic. 

(d) Essential elements : These are the elements which are essential for the growth of the plants. The plant cannot complete their lifecycle in the absence of it.  They are C, H, O, N, P, S, K, Ca, Mg, Fe, Mn, Cu, Mo, Zn, B, CI and NL