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Nitrogen Cycle

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🌱 Plant Nutrition By the Fed 'n Happy Team · Updated October 20, 2026

Definition

The nitrogen cycle is the continuous movement of nitrogen through the atmosphere, soil, and living organisms, cycling through fixation, ammonification, nitrification, assimilation, and denitrification. Each step is driven by different groups of bacteria, converting nitrogen between forms plants can and cannot use.

Corn field and soil cutaway showing nitrogen moving from the air through fixation, ammonium, nitrite and nitrate to plant uptake, decomposition and denitrification
Nitrogen cycles continuously between atmospheric gas, soil forms, and plant tissue, driven by distinct groups of bacteria at each stage. — Source: Fed 'n Happy (original illustration)
The Five Stages
Nitrogen moves through these forms continuously, driven by different bacteria at each step.
5 Steps
Fixation → Ammonification → Nitrification → Assimilation → Denitrification
~78%
Of Earth's atmosphere is nitrogen gas, the cycle's ultimate source and destination

Sources: University of Minnesota Extension, NOAA JetStream

Why It Matters

Nitrogen is the macronutrient plants need in the greatest quantity, but it exists in the atmosphere in a form plants can't use directly. The nitrogen cycle is what makes nitrogen usable in the first place, and understanding it explains both fertility and nitrogen loss in soil.

How It Works

Nitrogen fixation converts atmospheric N₂ into ammonia. Ammonification breaks down organic nitrogen from decomposing matter into ammonium. Nitrification converts ammonium into nitrate, the form plants absorb most readily. Assimilation is plants taking up that nitrate. Denitrification, occurring mainly in low-oxygen conditions, converts nitrate back into atmospheric nitrogen gas, completing the cycle.

Benefits of a Healthy Nitrogen Cycle

  • Sustained natural soil fertility
  • Reduced need for synthetic nitrogen inputs
  • Lower nitrogen loss to leaching or the atmosphere
  • Better long-term nutrient cycling balance

How to Support a Healthy Nitrogen Cycle

Working with the cycle beats bypassing it with a single synthetic form.

Feed the Whole Cycle With Organic Matter

Worm castings and compost supply organic nitrogen and the diverse bacteria that carry it through ammonification and nitrification naturally.

Avoid Waterlogged, Compacted Soil

Anaerobic conditions drive denitrification, converting valuable plant-available nitrogen back into gas that's lost to the atmosphere.

Plant Legume Cover Crops

Legumes contribute directly to nitrogen fixation, adding new nitrogen into the cycle beyond what's already cycling through the system.

Support Diverse Soil Biology Generally

Each stage of the cycle depends on different bacteria. A biologically diverse soil supports the full cycle rather than just one stage of it.

Sources: University of Minnesota Extension

The Five Stages of the Nitrogen Cycle

Each stage transforms nitrogen into a different chemical form, driven by a different group of bacteria.

Converts Driven By
1. Fixation Atmospheric N₂ → Ammonia (NH₃) Rhizobium (symbiotic) and free-living bacteria
2. Ammonification Organic nitrogen → Ammonium (NH₄⁺) Decomposer bacteria and fungi
3. Nitrification Ammonium → Nitrite → Nitrate (NO₃⁻) Nitrifying bacteria (requires oxygen)
4. Assimilation Nitrate/ammonium → Plant tissue Plant root uptake
5. Denitrification Nitrate → Atmospheric N₂ gas Denitrifying bacteria (low-oxygen conditions)

Sources: University of Minnesota Extension, NDSU Extension

Common Mistakes to Avoid

The nitrogen cycle is often reduced to just "fixation" or ignored entirely in favor of synthetic fertilizer.

Focusing only on nitrogen fixation
Fixation is just the first step. Ammonification and nitrification are equally necessary to get nitrogen into a form plants can actually absorb.
Overwatering or compacting soil
Both create the low-oxygen conditions that drive denitrification, converting valuable soil nitrogen back into gas that escapes to the atmosphere.
Assuming synthetic nitrogen bypasses the need for biology
Even synthetic nitrogen still moves through nitrification and denitrification once applied. The cycle's biology still governs how much of it stays plant-available.
Neglecting soil biology diversity
Different bacteria drive each stage of the cycle. A biologically thin soil can bottleneck at any single stage, regardless of how much nitrogen is present overall.

Sources: University of Minnesota Extension, Iowa State University Extension, Ohio State University Extension, University of Minnesota Extension (compaction)

Frequently Asked Questions

What is the nitrogen cycle? +
The nitrogen cycle is the continuous movement of nitrogen through the atmosphere, soil, and living organisms, cycling through fixation, ammonification, nitrification, assimilation, and denitrification. Each step is driven by different groups of bacteria, converting nitrogen between forms plants can and cannot use.
What are the main steps of the nitrogen cycle? +
The main steps are nitrogen fixation (converting atmospheric N2 to ammonia), ammonification (converting organic nitrogen to ammonium), nitrification (converting ammonium to nitrate), assimilation (plants taking up nitrate and ammonium), and denitrification (converting nitrate back to atmospheric nitrogen gas).
What is ammonification? +
Ammonification is the process by which bacteria and fungi decompose organic nitrogen, from dead organisms, waste, or organic matter, into ammonium, making it available for the next stage of the cycle.
What is nitrification? +
Nitrification is a two-step process where specialized bacteria first convert ammonium into nitrite, then convert nitrite into nitrate, the form of nitrogen plants most readily absorb through their roots. Both steps require oxygen.
What is denitrification and why does it matter? +
Denitrification is the process by which bacteria convert nitrate back into nitrogen gas, returning it to the atmosphere and completing the cycle. It happens mainly in low-oxygen conditions, like waterlogged or compacted soil, and can represent a real loss of plant-available nitrogen.
How can I minimize nitrogen loss from my soil? +
Avoiding waterlogged, compacted, or poorly aerated soil reduces the anaerobic conditions that drive denitrification. Supporting steady organic matter decomposition and healthy soil biology also keeps nitrogen cycling through the system rather than being lost.
How do worm castings interact with the nitrogen cycle? +
Worm castings introduce a diverse population of nitrogen-cycling bacteria along with organic nitrogen that feeds the ammonification and nitrification process, supporting the natural cycle rather than bypassing it with a single synthetic nitrogen form.
How does nitrogen fixation fit into the nitrogen cycle? +
Nitrogen fixation is the first step of the cycle, converting atmospheric nitrogen gas into a usable form. The rest of the cycle, ammonification, nitrification, assimilation, and denitrification, moves that fixed nitrogen through soil and eventually back to the atmosphere.
Fed 'n Happy Insight

Worm castings introduce a diverse population of nitrogen-cycling bacteria, supporting the natural cycle rather than bypassing it with a single synthetic nitrogen form.