Why would the plant need nitrogen from the ground when the air is 78% nitrogen?
In: Biology
The nitrogen in the air is bound up in bonds between two nitrogen atoms. Those bonds are *extremely* strong – in fact, they’re just about the strongest bond in any molecule. Breaking that bond is very difficult as a result, and most living things can’t do it, which means they can’t put that nitrogen into the molecules necessary for life.
The technical term for “converting N2 from the air into useful atoms of nitrogen” is [nitrogen fixation](https://en.wikipedia.org/wiki/Nitrogen_fixation). In the natural world, it’s done mostly by specific species of bacteria. Figuring out how to do it artificially took a very long time, and led to the development of modern fertilizer (without which we couldn’t produce anywhere *close* to the same level of farm yields that we do today; nitrogen is often the bottleneck for plant growth).
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As a side note, the reverse process – nitrogen in other compounds turning into nitrogen gas – releases a ton of energy, which is why many high explosives contain nitrogen. In particular, it’s why fertilizers are often explosive in large amounts (like the Beirut explosion last year).
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