Assertion (A) : Nitrogen is an essential element for plant growth and is taken up by plants in the form of inorganic nitrates or nitrites. 
Reason (R) : The soil is the nearest and richest source of raw materials like Nitrogen, Phosphorus and other minerals for the plants.
(a) Both (A) and (R) are true and (R) is the correct explanation of (A). 
(b) Both (A) and (R) are true but (R) is not the correct explanation of (A). 
(c) (A) is true, but (R) is false. 
(d) (A) is false, but (R) is true

Solution:

Ans. Option (b) is correct:
Both (A) and (R) are true but (R) is not the correct explanation of (A). 

Plants take nitrogen from the soil by absorption through their roots in the form of either nitrate ions or nitrite ions. Plants cannot assimilate ammonium ions. Most nitrogen obtained by terrestrial animals can be traced back to the eating of plants at some stage of the food chain.  

Nitrogen is essential for plants because:

1. Plant uses nitrogen to produce protein (in the form of enzymes) and nucleic acids

2. Nitrogen can strengthen and support the roots, thus helping plants to take in more water and nutrients.

3. Nitrogen is also a part of chlorophyll, which helps in photosynthesis.

4. Nitrogen also helps in rapid growth and development.