Why do plants absorb nitrogen from the ground and not the air?

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Why would the plant need nitrogen from the ground when the air is 78% nitrogen?

In: Biology
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Because the nitrogen in the air, N2, is very, very hard to actually make use of. It’s phenomenally stable.

There are some bacteria in the ground that are able to do it. They can turn atmospheric Nitrogen into ammonia (NH3) or similar compounds. Plants are much more readily able to make use of compounds like these to, say, construct proteins.

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).

The nitrogen is *there* but the plants can’t get it. The nitrogen needs to be taken from a gas and bound into organic compounds like ammonia, something plants rely on diazotrophs to do, bacteria and archaea that can “fix” nitrogen into such compounds.

Organisms evolved in ecosystems so while it might be convenient if they could fix nitrogen themselves it just didn’t turn out that way.

There actually are some plants that can absorb nitrogen from the air. Legumes for example do this, and it’s a process called nitrogen-fixing. However, plants didn’t evolve the ability to do it themselves. Instead, they evolved to form symbiotic relationships with nitrogen-fixing bacteria, that they incorporate into their own structure similar to the gut bacteria in humans. It’s much easier for plants to evolve this symbiotic relationship than to directly evolve nitrogen fixation mechanisms, so this is the thing that evolves.

The nitrogen in the air is useless, because it’s all in the same chemical form that plants can’t capture.

But, some bacteria can capture atmospheric nitrogen just fine, and they make it into forms that they (and plants) can use. And these bacteria live in the ground, meaning they can’t get sunlight so they can’t make their own energy.

So, when these bacteria and certain plants find each other, the plant makes extra sugar for the bacteria, and in return, the bacteria capture extra nitrogen for the plant. And since both organisms benefit from this relationship, the bacteria don’t infect and kill the rest of the plant, and the plant doesn’t fight and kill the bacteria.