Why Can't Plants Use Nitrogen Gas Directly? 5 Key Facts

Plants can’t use nitrogen gas directly because the two atoms in N₂ are held by a triple bond that plant cells have no way to break. Breaking it takes an enzyme called nitrogenase, and only certain bacteria and archaea make it. So plants wait for microbes, lightning or fertilizer to turn the gas into forms their roots can take up, mainly ammonium (NH₄⁺) and nitrate (NO₃⁻).

That is the short answer. The five facts below explain the chemistry, show how much nitrogen each source supplies, and turn the science into a field decision: how much fertilizer a legume crop can replace. The same reasoning answers the wider question of why most organisms can’t use atmospheric nitrogen, including animals.

Fact 1: Nitrogen Gas (N₂) Is Abundant but Inaccessible

Nitrogen makes up 78.084% of dry air (NOAA). Every leaf and root sits in it, and gas moves into leaves through the small pores called stomata. Yet the N₂ does nothing for the plant once it arrives. It passes through unchanged, because no plant enzyme can react with it.

Plants still need a great deal of nitrogen. It is a building block of proteins and DNA, and Nature Education’s nitrogen cycle review explains that it must first be converted to another chemical form before it can be used for those compounds. New Mexico State University’s extension guide puts the problem plainly: N₂ is unusable by most living organisms, and plants, animals and microbes can all run short of nitrogen while surrounded by it.

Why plants and animals both miss out

Animals have the same gap. They breathe N₂ in and out without using it, and they get their nitrogen by eating plants or other animals. Most organisms lack nitrogenase, so only a select group of microbes opens the door for everyone else.

Fact 2: The Strong Triple Bond Is a Chemical Barrier

The two nitrogen atoms share three pairs of electrons. WebElements lists the bond energy of gaseous N₂ at 945 kJ per mole. That is about twice the energy of the double bond in oxygen gas and nearly four times the single bond in chlorine gas. This is what makes nitrogen so difficult to convert: the molecule barely reacts at the temperatures found in soil and leaves.

Chart

Bond energy of four common gases

kJ per mole, gaseous diatomic molecules

What it costs a cell to break the bond

Even with the right enzyme, the job is expensive. Reducing one N₂ molecule takes eight electrons and at least sixteen ATP molecules. ATP is the cell’s energy currency. Legumes pay this bill with sugar: a soybean plant may divert up to 50% of its photosynthate (the sugars made in its leaves) to its root nodules while they are actively fixing nitrogen.

Oxygen makes it harder still

Nitrogenase is very sensitive to oxygen and is deactivated by it. That is a real problem inside a green plant, whose leaves release oxygen. Microbes work around it: some cyanobacteria fix nitrogen in low-oxygen cells called heterocysts, and others fix it only at night. Legume nodules use a red protein, leghemoglobin, to control oxygen flow to the bacteria inside.

Fact 3: Microbial Nitrogen Fixation Makes Atmospheric Nitrogen Usable

Nitrogen fixation means converting N₂ into a reactive form. The enzyme reduces N₂ to ammonia (NH₃), which microbes and plants can then build on. So the common quiz statement “plants cannot use N₂ directly; bacteria must first convert it to ammonium” is essentially true. The fine print is that the first product is ammonia, and that lightning and factories can fix nitrogen as well.

Chart

Where fixed nitrogen comes from

million tonnes of nitrogen a year, global estimates

How soil bacteria solve the problem

• Partner bacteria: rhizobia enter the roots of legumes such as beans, clover and alfalfa, and fix nitrogen inside nodules. The plant supplies sugar and the bacteria supply nitrogen.
• Soil and water microbes: some bacteria and cyanobacteria fix nitrogen on their own, without a host plant.
• Bacteria do not make solid nitrogen or rain: if a quiz asks how soil bacteria help, the right answer is that they change nitrogen gas into a usable form in the soil that roots can absorb.

In a legume, almost all the fixed nitrogen goes directly into the plant. Some leaks into the soil, and most of the rest returns when roots, leaves and other tissue decompose. Harvest changes this: grain or cut forage takes most of the fixed nitrogen off the field.

How much nitrogen each source fixes

Fowler and colleagues put global fixation at 413 million tonnes of nitrogen a year, about half of it from human activity (Philosophical Transactions of the Royal Society B, 2013). Oceans lead among natural sources, while the Haber-Bosch process, the factory route to ammonia, supplied about 120 million tonnes in 2010. Lightning adds only about 5 million. Fowler’s review also notes that most nitrogen applied in agriculture is fixed industrially, with the remainder coming from nitrogen-fixing crops.

Are there any exceptions?

No crop has been shown to make its own nitrogenase, but two findings show how close partnerships can get. A maize landrace grown in Oaxaca, Mexico, drew 29% to 82% of its nitrogen from the air over five years of field trials, at least partly through bacteria living in mucilage on its aerial roots. And in 2024, researchers described a nitrogen-fixing structure called a nitroplast inside a marine alga, Braarudosphaera bigelowii. It began as a cyanobacterium. Neither case means a plant cell breaks the bond with its own enzyme.

Fact 4: Nitrogen Cycling Pathways and Plant Uptake

Once nitrogen is fixed, soil microbes keep moving it between forms. Roots take what is available at the time. The main steps are:

Infographic

From air to root in 4 steps

1. 1FixationMicrobes convert N₂ to ammonia.
2. 2AmmoniumAmmonia forms ammonium, which roots can absorb.
3. 3NitrateSoil microbes oxidize some ammonium to nitrate.
4. 4UptakeRoots build these forms into amino acids and proteins.

1. Fixation: microbes convert N₂ to ammonia.
2. Ammonification: fungi and prokaryotes decompose dead tissue and release ammonia back to the soil, where plants and microbes can take it up.
3. Nitrification: soil bacteria and archaea oxidize ammonia to nitrite, then nitrate.
4. Plant uptake: roots absorb ammonium and nitrate and build them into amino acids and proteins.
5. Denitrification: in low-oxygen places such as waterlogged soil, microbes turn nitrate back into gas, and the nitrogen returns to the air.

Forms of nitrogen in soil: what plants can use

The leaching risk is the reason extra fertilizer is not a cure-all. EPA notes that nutrients plants do not use can wash from fields into waterways or leach into groundwater, and high levels can cause eutrophication, the excess growth that leads to low-oxygen dead zones. Most nitrogen fertilizer production starts with ammonia made from natural gas, so every wasted pound has a cost as well.

▶ Why soil organic matter matters in agriculture · 7:33

Fact 5: Sustainable Practices Maximize Natural Nitrogen Inputs

Since plants can’t take nitrogen from the air themselves, a grower’s practical choice is how much to get from biology and how much to buy. Legume fixation varies widely with species and stand. Alabama Extension’s table for forage legumes in the Southeast runs from 30 to 90 lb N per acre a year for cowpea up to 50 to 300 lb for alfalfa. These are regional estimates, and a thin or stressed stand will sit at the low end.

▶ Seven simple sustainable farming practices, including cover crops · 4:34

Chart

Nitrogen fixed by selected forage legumes

lb N per acre per year, low and high estimates, US Southeast

Practical steps for more biological nitrogen

• Add legumes to the rotation. Choose the species for your climate and for how the crop will be used.
• Inoculate when needed. Kansas State University notes that rhizobia are specific to their host plant, so the inoculant for soybeans does not suit every legume.
• Check the nodules. NMSU says small nodules should appear 2 to 3 weeks after germination. Pink or red inside shows that fixation has started.
• Do not over-apply nitrogen to legumes. High soil nitrogen reduces fixation, though it does not stop it altogether.
• Keep the ground covered. EPA lists cover crops and perennial species among the ways to protect soil and nutrients between cash crops.
• Feed the soil. Compost and manure supply organic nitrogen. See these organic farming and soil health tips for more.

How to count a legume nitrogen credit

A nitrogen credit is the amount you can subtract from the fertilizer planned for the next crop. University of Wisconsin guidance (A3517, June 1997) gives 190 lb N per acre after a good alfalfa stand and 40 lb after soybean, with lower credits for sandy soils and poor stands, and no soybean credit on sands. Kansas State (L-778, 2007) uses lower alfalfa figures, 120 lb for an excellent stand and 80 lb for a good one. Use your own state’s table, because credits are regional. Then price what remains; our guide to DAP cost and application amounts covers one common source.

Satellite Monitoring and Mineral Detection at Farmonaut

Satellite crop monitoring cannot see nitrogen molecules, but it can show where a crop is under stress, which tells you which part of a field to check first. Yellowing can have many causes, so confirm with a soil or tissue test.

This page sits in our mining library, where Farmonaut’s separate satellite-based mineral detection service is described. Mining readers can request a quote or contact the team.

▶ A full walkthrough of satellite crop monitoring and reading NDVI · 19:36

Frequently asked questions

Why can’t plants use nitrogen gas directly?

N₂ has a very strong triple bond, about 945 kJ per mole, and plants lack nitrogenase, the enzyme that breaks it. Roots rely on ammonium and nitrate made by microbes, decomposition or fertilizer.

Why can’t plants get nitrogen directly from the air without bacteria?

Air reaches leaves easily, but plant cells have no chemistry that reacts with N₂. Without bacteria, the other routes are lightning, which is small, and factory-made fertilizer.

Why can’t most organisms use atmospheric nitrogen directly?

Only select prokaryotes, meaning certain bacteria and archaea, make nitrogenase. Everything else gets fixed nitrogen from those microbes or through food.

Why can’t plants and animals use nitrogen from the atmosphere?

Both lack the enzyme. Plants take up fixed nitrogen through their roots, and animals get theirs by eating plants or other animals.

Can plants absorb nitrogen gas through their leaves?

N₂ can enter a leaf through its stomata, but it is not used. It stays unreacted because the leaf cannot break the triple bond.

Is it true that bacteria must convert N₂ to ammonium before plants can use it?

Broadly, yes. Nitrogenase makes ammonia, which forms ammonium in water. Other bacteria can then convert it to nitrate, which roots also absorb. Lightning and industry fix some nitrogen without bacteria.

What makes nitrogen difficult for organisms to break down?

The triple bond. Even with nitrogenase, a microbe spends at least 16 ATP for each N₂ molecule and must protect the enzyme from oxygen.

What forms of nitrogen can plants absorb?

Mainly ammonium (NH₄⁺) and nitrate (NO₃⁻). Both come from fixation, decomposition, nitrification or fertilizer.

Do legumes add nitrogen to soil for the next crop?

Yes, mostly as residue decomposes. How much depends on the crop and how much is harvested: Wisconsin credits 40 lb N per acre after soybean and 190 lb after a good alfalfa stand.

Could crops be engineered to fix their own nitrogen?

Researchers are working toward it, and the nitroplast and Oaxaca maize findings have added interest. It remains a research goal, not something a grower can buy today.