What is the nitrogen cycle in simple terms?

It's how nitrogen moves between the air, soil, plants and back again: bacteria pull nitrogen gas from the air into a usable form, plants absorb it, decomposers return it to the soil when living things die, and other bacteria eventually release some of it back into the air.

Why can't plants use nitrogen gas directly from the air?

The two atoms in N₂ are joined by an unusually strong triple bond that plants have no way to break. Only certain bacteria (and lightning, and industrial fertiliser plants) have enough energy to split it apart.

What plants fix nitrogen in soil?

Legumes — beans, peas, lentils, clover, vetch and alfalfa — host Rhizobium bacteria in nodules on their roots that fix nitrogen from the air. That's why they're used as cover crops and rotated with nitrogen-hungry vegetables.

What are the signs of nitrogen deficiency in plants?

Pale or yellowing older leaves, slow growth and thin stems. New growth often stays green longer because the plant redirects its limited nitrogen there first.

Does compost add nitrogen to soil?

Yes. Decomposing organic matter releases nitrogen as ammonium (ammonification), which soil bacteria then convert into nitrate that plant roots can absorb.

The Nitrogen Cycle in Your Garden

Nitrogen moves through five stages in a garden: bacteria fix nitrogen gas from the air into ammonium, other bacteria convert that to nitrate, plants absorb it, decomposers return it to the soil when plants and animals die, and denitrifying bacteria release some of it back into the air — completing the cycle.

Nitrogen loops from air to soil to plant and back again. Use the player to walk through each stage.

Show the math

1
Nitrogen fixation
N₂ (air) → NH₄⁺ (ammonium)
Rhizobium bacteria living in the root nodules of legumes — beans, peas, clover — pull nitrogen gas from the air and convert it into a form plants can use. Lightning and fertiliser factories fix nitrogen the same way.
= NH₄⁺ in soil

Read the steps as text

Nitrogen fixation. N₂ (air) → NH₄⁺ (ammonium) Rhizobium bacteria living in the root nodules of legumes — beans, peas, clover — pull nitrogen gas from the air and convert it into a form plants can use. Lightning and fertiliser factories fix nitrogen the same way.
Nitrification. NH₄⁺ → NO₂⁻ → NO₃⁻ Nitrosomonas bacteria oxidise ammonium into nitrite, then Nitrobacter oxidise that into nitrate — the form most plants prefer. This needs oxygen, so it slows in compacted or waterlogged soil.
Assimilation. NO₃⁻ / NH₄⁺ → proteins, chlorophyll, DNA Roots absorb nitrate and ammonium and build them into the amino acids and chlorophyll that drive leafy, green growth — the “N” in an N-P-K fertiliser.
Ammonification. Organic N (dead matter, waste) → NH₄⁺ When plants and animals die, or animals excrete waste, decomposer bacteria and fungi break that nitrogen back down into ammonium — exactly what happens inside a compost heap.
Denitrification. NO₃⁻ → N₂ (air) In waterlogged, low-oxygen soil, denitrifying bacteria use nitrate instead of oxygen to breathe, releasing nitrogen gas back to the atmosphere and closing the cycle — at the cost of nitrogen the garden could have used.

Why the nitrogen cycle matters for gardening

Nitrogen is the nutrient plants need in the largest quantity, and it drives leafy, green growth — it's the N in an N-P-K fertiliser label. But the air that surrounds every plant is nearly 78% nitrogen gas (N₂), and no plant can use it directly: the two nitrogen atoms are locked together by one of the strongest bonds in chemistry. The nitrogen cycle is the set of steps, mostly carried out by soil bacteria, that unlocks that nitrogen and eventually returns it to the air.

Every fertile soil is running this cycle constantly. Understanding it explains why legumes enrich soil, why compost works, why overwatered plants can still show nitrogen deficiency, and why more fertiliser isn't always better.

Feeding the cycle in your own garden

Grow nitrogen-fixing plants. Beans, peas, clover and vetch host Rhizobium bacteria in their root nodules, which pull nitrogen from the air for free. Many gardeners rotate these with heavy feeders like tomatoes, corn and brassicas, or grow them as a "green manure" cover crop and dig it into the soil.

Compost everything you can. Kitchen scraps, grass clippings and fallen leaves all contain organic nitrogen. Composting speeds up ammonification — the decomposition step — so that nitrogen becomes available to next season's plants instead of being lost.

Protect soil structure. Nitrifying bacteria need oxygen, so compacted or waterlogged soil slows the whole cycle and encourages denitrification instead, which releases nitrogen back into the air. Avoid working or walking on wet soil, and improve drainage in heavy clay beds.

Signs of a nitrogen imbalance

A shortage shows up first in older, lower leaves, which turn pale yellow (a symptom called chlorosis) while growth slows and stems stay thin, because the plant moves what little nitrogen it has to new growth. Too much nitrogen does the opposite: lush, dark leaves and weak, floppy stems, often with fewer flowers and less fruit, because the plant puts its energy into leaves instead of reproducing.

Because nitrate is water-soluble, it also washes out of soil easily after heavy rain or over-watering, which is why nitrogen often needs topping up more often than phosphorus or potassium.

How fertilisers fit into the cycle

Synthetic fertilisers supply nitrogen that has already been industrially "fixed" into ammonium or nitrate, so plants can use it almost immediately — fast, but it does nothing to build the soil's own cycle and can wash away quickly. Organic sources like compost, manure and cover crops feed the cycle more slowly: soil organisms have to break them down through ammonification and nitrification first, which releases nitrogen gradually and tends to stay in the soil longer.

Frequently asked questions

What is the nitrogen cycle in simple terms?: It's how nitrogen moves between the air, soil, plants and back again: bacteria pull nitrogen gas from the air into a usable form, plants absorb it, decomposers return it to the soil when living things die, and other bacteria eventually release some of it back into the air.
Why can't plants use nitrogen gas directly from the air?: The two atoms in N₂ are joined by an unusually strong triple bond that plants have no way to break. Only certain bacteria (and lightning, and industrial fertiliser plants) have enough energy to split it apart.
What plants fix nitrogen in soil?: Legumes — beans, peas, lentils, clover, vetch and alfalfa — host Rhizobium bacteria in nodules on their roots that fix nitrogen from the air. That's why they're used as cover crops and rotated with nitrogen-hungry vegetables.
What are the signs of nitrogen deficiency in plants?: Pale or yellowing older leaves, slow growth and thin stems. New growth often stays green longer because the plant redirects its limited nitrogen there first.
Does compost add nitrogen to soil?: Yes. Decomposing organic matter releases nitrogen as ammonium (ammonification), which soil bacteria then convert into nitrate that plant roots can absorb.