Nitrogen in the air is like a product packed in way too sturdy plastic (you probably know what I mean). It's technically there, but you can't use it without a tool. Some bacteria have scissors. They can open the packaging and make it useful - to them, and also to others afterwards.

Nitrogen usually forms three bonds. As gas, all three bonds are formed with another nitrogen atom. That's a very sturdy connection that is difficult to break apart. Plants don't have the tools for that. Some bacteria do, and they use the nitrogen to make other molecules where the nitrogen is bound to three different atoms. That's much more accessible, now you only need to break one bond at a time. Fertilizer has nitrogen in more accessible form, too.

Oxygen and CO2 are much easier to break apart.

Was it from "Veritasium"? If so he explains In the video I think how nitrogen in our atmosphere is diatomic. nitrogen and plants can't absorb this type of nitrogen easily it first needs to be split into single atoms of nitrogen. this process takes energy and with nitrogen it's an extreme amount of energy. Plants depend on microbes and mycelium in the soil to break this nitrogen down into a usable mono form look up nitrogen fixation on "Journey to the microcosmos" to learn more

N2 nitrogen in the air is not useful to plants, it needs to first be “fixed” (converted to usable organic forms). Bacteria in the soil do this and so plants have evolved to just obtain their nitrogen from the soil. They could have probably evolved to do it themselves but there was no need as bacteria already did it.

This only becomes a problem when you want to pack more plants into a smaller section of soil and bacteria cant provide enough nitrogen. For the crop yields we get today we need fertilizers to make up for what the soil bacteria cant do.

The process you are talking about is called nitrogen fixation. It is done by the nitrogenase enzyme. Plants do not have this enzyme but live in symbiose with bacteria living on the roots and in the soil which does the nitrogen fixation. So there is normally no need for plants to do this themselves.

The problem is that nitrogen fixation takes a lot of energy. Unlike carbon dioxide the dinitrogen in the air require much more energy to break the bonds. So nitrogen fixating bacteria consume a lot of energy, much more then the leaf cells on a plant generate from the sun. These bacteria get their energy mostly from sugars released by the plants through their roots. When the plants lack nitrogen compounds they release their sugar straight into the soil and the nitrogen fixating bacteria use this sugar to make amonia and other nitrogen fertilizers. Nematodes and amoeba will then eat the bacteria and release any excess nitrogen compounds which the plants then absorb through their roots.

However by making nitrogen fertilizers from energy rich natural gas or electricity we are essentially doing the bacterias jobs for them. By adding this nitrogen to the soil the plants are never low on nitrogen and therefore does not release sugar into the soil and the nitrogen fixating bacteria dies out. This of course means that the plants are able to use the sugars themselves to grow faster and larger.

The core of the issue is that it take a lot of energy to convert N2 (atmospheric nitrogen) into Nitrate (NO3^- ), and is difficult for a plant or animal to do in their cells.

To illustrate the energy intensity of the process, AFAIK, the main mechanism for N2 to get converted to NO3^- is lightning strikes.

The nitrogen-nitrogen triple bond is one of the Most stabile Bonds

Which translates to nitrogen beeing very unreactive, its Just difficult to turn it into anything but its diatonic Form, that it is in in air

Nitrogen atoms really, really like to be stuck to one other Nitrogen atom. So much so that it is very difficult to get atmospheric nitrogen (which is 2 nitrogen atoms stuck together) to do anything else, like form an amino acid.

Some plants pull it off by making friends with specific soil bacteria that can break apart Nitrogen-Nitrogen bonds.

I think this is the first time I’ve seen an ELI5 on the front page where all the answers are wrong!

Yes, atmospheric nitrogen takes a lot of energy to convert, but plants already provide all of that energy to their bacterial and fungal friends in the soil. So that’s not the reason.

The real issue is that photosynthesis and nitrogen fixation are incompatible because of oxygen. Photosynthesis makes oxygen molecules and oxygen molecules destroy the machinery that turns atmospheric nitrogen into usable forms.

There are organisms that do both photosynthesis and nitrogen fixation, but they don’t do it in the same place or at the same time. Some photosynthetic bacteria (like Anabaena) make special cells called heterocysts that fix nitrogen but don’t photosynthesize, while others (like Cyanothece) photosynthesize during the day, then fix nitrogen during the night.

When photosynthesis and nitrogen fixation were first evolving, this incompatibility wasn’t an issue because there wasn’t molecular oxygen in the atmosphere to any appreciable degree. Photosynthesis created it all. Once it had, created an environment that made nitrogen fixation much more challenging.

As u/writingtherongs pointed out, plants COULD have evolved a separate compartment to fix nitrogen in, but that just hasn’t happened in the last ~3 billion years. They’ve always relied on other organisms to provide their fixed nitrogen.

It takes a huge amount of energy to break the bonds between the atoms of nitrogen in the atmosphere. The bond is so strong that concentrated nitrate fertilizer is a high explosive. Fertilizer bombs are very real, and fertilizer factories sometimes explode The city of Lebanon was devastated when a warehouse full of fertilizer exploded. Basically, in order to make nitrogen based fertilizer, energy is pumped into nitrogen, and it is capable of releasing that energy with catastrophic speed. Some plants- mostly in the legume family (beans) can partner with bacteria in their roots, they pay the bacteria energy (sugar) in exchange for nitrate. But it is so expensive, most plants evolved to just use what is present in the environment.

bacteria evolved the ability to do this first, and are so good at it, that there is little evolutionary advantage to be gained by any plant that managed to evolve the ability. the plants don't need to fix nitrogen. They already have mutualistic relationships with microbes that do it for them.

Imagine nitrogen is like tape, and it's easy to stick it onto something.

Normally, when you apply tape to something, you can do a little bit of work to remove the tape. But imagine two pieces of tape stuck to each other-- Nearly impossible to separate, at least without a lot of very careful work.

The particular form of nitrogen in the air, N2, is two nitrogen atoms stuck together, and like the tape, and they're very hard to separate.

Many/most plants rely on nitrogen fixation, which is a collection of processes for separating the two nitrogen atoms and sticking them to something else that's easier to for the plant to work with/consume.

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Nitrogen in the air is actually formed as two atoms of Nitrogen "triple bonded" together. This is very strong and hard to break into each individual atom of nitrogen.

Soil bateria use atmospheric Nitrogen to eat and as a result they can break the "triple bond" leaving Nitrogen in other forms which the plant can use.

Because they simply do not have the required biological machinery. Animals and fungi also don't have it, luckily some bacterial cultures do so they basically do the nitrogen binding for the entire rest of the ecosystem(ignoring industrially produced fertilizers).

Similarly, all the vitamins, they are classified as vitamins because your body needs them, but can't produce them on it's own. You have to consume them from your food.

Nitrogen in the air is in the form of N2 and it rather stable because it has a triple bond between the N atoms. It takes more energy to break the N2 triple bond and bacteria have been selected to do that energy requiring step. It gets broken down in the soil by bacteria in to amino-compounds which are more easily reactive and accessible for the plant. It's all governed by thermodynamic. For the plant, it is easier to take up N from the soil because it is usually broken in to molecules that are only single bonded to it and N has an extra pair of unpaired electrons that plants access much more easily and with a favorable thermodynamic.

For the same reason we can't eat oxygen. The leaves are not designed to absorb nitrogen in its airborne form

Along with what others have said, here's an analogy. We need to breathe oxygen. H2o and co2 both contain oxygen but we definitely can't breathe them as our supply of oxygen. Technically having the atom doesn't mean it can be used the same.

Was the video about Fritz Haber? Did Sabaton lead you there?

Atmospheric nitrogen is more or less inert; two atoms form a N2 molecule that is very stable; the bond between them is well balanced. Nitrogen in the form of nitrates in soil will easily break free in water solutions and form N^2- ions; plants can incorporate those easily into more complex molecules since it is unstable and needs to form compounds with something.

You know how cling wrap sticks to itself really well? And if you pull out a lot of cling wrap, it is almost impossible to pull all of it apart at the same time? The Nitrogen in air (N2) is similar to that. It sticks together very strongly. Nitrogen sticks to itself about 3 times stronger than it sticks to a single Hydrogen. It takes a lot of energy to break apart N2, so plants instead get the Nitrogen from sources where it is bonded more weakly with other atoms, like nitrates (bonded to Oxygen) and ammonia (bonded to Hydrogen).

Alfalfa fixes nitrogen through bacterial action - nitrogen fixing bacteria. That's why crop rotation is done. It adds nitrogen to the soil and feeds the cattle.

Nitrogen in the air is found in a molecule that is like a coconut. Without a tool like a hammer (or the chemical equivalent, which plants don't have) it's really hard to break open and get to the good stuff. Nitrogen in the ground is found in a molecule that is like a banana. It's easy to peel and get to the good stuff.

Nitrogen gas really likes being a gas. You have to put a lot of energy into breaking it apart. Like lightning strike levels of energy. Indeed lightning strikes are responsible for turning a significant amount of nitrogen into ammonia. But, lightning is not the main source of usable nitrogen. Aside from human production of ammonia for fertilizer, bacteria are main fixers of nitrogen.

There are a family of enzymes (Nitrogenase) that can be used to reduce the amount of energy required to turn nitrogen gas into something useable by plants. However, this enzyme breaks down in the presence of oxygen. Unlike nitrogen, oxygen gas isn't particularly happy being a gas all itself and will react with nitrogen compounds quite readily (fertilizer is known for being explosive, and the N in TNT stands for nitrogen). Plants are heavily involved with oxygen, either producing it during the day or consuming it at night. If plants tried to produce this enzyme for themselves it would rapidly break down in the presence of the oxygen being produced by photosynthesis.

As such, nitrogen fixation is left to the bacterial specialists.

Nitrogen in the air is very strong, and cannot be broken by plants.

So this needs to be fixed, or broken down. In nature, lightning has enough energy to do this, breaking nitrogen down. But a lot of man made processes have been invented to do this as well.

Man the answers are all over the place on this one. Maybe because what your asking sounds more like a philosophy question . Plants can absorb nitrogen from the air btw. I'll assume you are asking why they can't turn it into protein. The short answer is they absolutely could but lack the genes for it. Why can't you turn sunlight into sugar? Why can't we breathe under water? The answers to questions like this is that we don't know for sure. Evolution took the paths it took and here we are. Maybe it's not worth the bother to plants when they get enough nitrogen from bacteria or lightning strikes. Another part of your question involved farming. Farmed plants are not natural. We use fertilizer to increase the yield, but those plants will still grow without fertilizer.