Eli5 why plants which need nitrogen from the ground do not take it from the air which is 70% Nitrogen

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Eli5 why plants which need nitrogen from the ground do not take it from the air which is 70% Nitrogen

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Nitrogen, as used by plants, is usually in the form of the nitrate ion. This ion is relatively difficult and expensive to form, so most plants cannot create it from atmospheric nitrogen.

Most of the nitrogen in the air is diatomic, pairs of nitrogen atoms bound together. That’s a very strong bond, which is difficult to break apart with the kind of energy plants have available to them, so the nitrogen they consume mostly comes from compounds which contain nitrogen and are held together by much weaker bonds.

The nitrogen in the air is N2, two nitrogen atoms attached with an extremely strong bond. Plants need single nitrogen atoms to build stuff.

It’s like if you were surrounded by pairs of LEGO bricks superglued together, and to survive you needed to build structures requiring single bricks. Even if they’re pairs of bricks of the type you need, you’re gonna have to get them elsewhere.

Because evolution is not a design. Why can’t animals photosynthesise? Why can’t mammals extract oxygen from water? Why do complex organisms not have the ability to asexually reproduce? Species evolve alongside each other and develop niches. Some evolve in high organic nitrogen niches. Others rely on symbiosis with nitrogen fixing bacteria.

A useful analogy here is burning wood.

Why don’t dry forests burst into flame when there’s all those dry needles or leaves and wood around? The answer is because it takes a certain amount of energy to activate the process of chemically converting wood into carbon dioxide and water vapor gases, so you need to invest that energy to start the process.

Nitrogen has the same initial problem. In order to use it, plants need to bring it out of the air and convert it into a more useful form. But plants don’t have the high amounts of energy required to perform that process.

Some plants have developed a relationship with certain bacteria that can do this special conversion for them. Clover is a good example – it has nodules on its roots that these bacteria live in. They specialize in grabbing nitrogen and converting it into a more useful form that the plant can take advantage of. And that’s why farmers like to plant clover to let a field “rest” for a season – it’s effectively recharging the soil.