How is nitrogen useful for plants? Remember that you learnt this in the chapter 'The Invisible Living World: Beyond Our Naked Eyes', in your Grade 8 Science textbook.

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Accepted Answer

Nitrogen is the raw material plants need to build proteins — and therefore to build leaves, stems, roots and seeds. It is also part of chlorophyll, the green pigment that lets a plant make its own food. Without nitrogen a plant stays stunted and yellow.

But there is a problem, and it is the point of the question. Nitrogen makes up 78% of the air — the single most abundant gas around us — and yet plants cannot use it straight from the air. The nitrogen in the atmosphere is locked in a very tightly bound form that roots cannot absorb.

The solution comes from the invisible living world. Certain nitrogen-fixing bacteria — Rhizobium living in the root nodules of leguminous plants such as gram, peas, beans and groundnut, and free-living bacteria in the soil — convert atmospheric nitrogen into nitrogen compounds the plant roots can take up. Lightning does a small share of the same work. This is why the chapter you read in Grade 8 Science put nitrogen under the invisible living world: the bacteria are the link between the air and the plant.

Why this connects back to Chapter 3: it shows that the composition of the atmosphere is not just a list of percentages to memorise. The 78% nitrogen figure is the reservoir that every protein in every living body is ultimately drawn from — through bacteria, into plants, and then into the animals and people who eat them.

Try this: this is also why farmers in India rotate a cereal such as wheat or rice with a pulse such as gram or moong. The pulse crop, with its Rhizobium nodules, puts nitrogen back into the soil for the cereal that follows — a very old Indian practice with a very modern explanation.

NCERT Solutions for Class 9th Social Science Chapter 3 Composition of the Atmosphere — LET'S RECALL