3 Ways Nitrogen Can Be Fixed for Plants, With Numbers

Two ways nitrogen is fixed naturally for plant use are soil bacteria and lightning. Bacteria such as rhizobia, living in root nodules of legumes, turn nitrogen gas from the air into ammonia. Lightning splits the gas in the sky, and rain carries the products to the soil. Farms add a third route, the Haber-Bosch factory process, which made about 120 million tonnes of nitrogen in 2010 (Fowler et al., Royal Society, 2013).

This page compares all three with numbers from published studies and extension services. It also shows what a legume’s nitrogen is worth in dollars, with a calculator you can set to your own fertilizer price. If you want the chemistry first, read why plants cannot use nitrogen gas directly.

Two natural ways nitrogen is fixed for plant use

If a test or worksheet asks you to “name two ways nitrogen can be fixed naturally”, give these two answers and one line of detail for each.

1. Biological fixation. Soil bacteria use the enzyme nitrogenase to turn nitrogen gas (N₂) into ammonia (NH₃). Some live inside legume roots. Others live on their own in soil or water.
2. Lightning. The heat of a strike makes nitrogen and oxygen react. The nitrogen oxides dissolve in rain and reach the soil as nitrate.

The third way, industrial fixation, is made by people. It is not natural, but it now supplies a large share of the nitrogen on farms.

Why plants need fixed nitrogen

Nitrogen is part of every protein, enzyme and chlorophyll molecule. UF/IFAS Extension calls it the most limiting nutrient for plant growth. Most soil nitrogen sits in organic matter: about 98% of total soil nitrogen is organic, and it becomes available only after soil organisms convert it to ammonium and nitrate (UF/IFAS Extension, AG462). That is why a fixing route matters: without a steady new supply, the soil store runs down as crops remove nitrogen in grain, hay and straw.

Fixation is only the first step of a cycle. After the nitrogen is fixed, it moves through plants, animals, soil and water. Some returns to the air as gas, and some is lost to streams and groundwater as nitrate. Good management keeps more of it where the crop can use it.

Nitrogen fixation at a glance

Air is about 78% nitrogen gas, but its two atoms are held by a very strong triple bond. Plants cannot use it in that form. Fixation breaks the bond and makes nitrogen that roots can take up. Plants absorb it mainly as ammonium and nitrate, according to UF/IFAS Extension.

Tg means million tonnes. The figures come from the review by Fowler and colleagues (2013). They are global estimates with wide error ranges, not measurements for a single farm.

1. Biological fixation: bacteria and legumes

Biological fixation is the main natural source of usable nitrogen on land. Fowler and colleagues quote about 58 Tg N a year for land before the industrial era, with a range of 40 to 100. They put today’s input from crops and grazed savannah at about 60 Tg N a year, using a range of 50 to 70 from earlier work.

How the legume partnership works

Peas, beans, clovers, alfalfa and soybeans form nodules on their roots. Rhizobium or Bradyrhizobium bacteria live inside the nodules. The plant gives the bacteria sugars for energy. The bacteria give the plant ammonia. Wisconsin extension notes that each legume needs its own matching type of bacteria, and that nodules form when the right bacteria are in the soil (University of Wisconsin-Extension, A3517, 1997).

Not all fixing needs a plant partner. Azotobacter in soil and cyanobacteria in wet soils and water also fix nitrogen on their own. The global figures above count these natural land sources together with legume fixation.

How much do soybeans really fix?

Fixation drops when fertilizer nitrogen is added. In field trials at Michigan State University’s Kellogg Biological Station, fixed nitrogen made up 49% of the nitrogen in soybean grain without fertilizer. It fell to about 16% once fertilizer reached 85 kg N per hectare or more (Gelfand and Robertson, 2014). That is one site, so treat it as an example of the pattern, not a rule.

The same effect is why nodules need care. North Dakota State University says to consider inoculating soybean seed with Bradyrhizobium japonicum when a field has not grown soybeans in the past three years or more (NDSU Extension, soybean quick guide). Check the inoculant label and your local extension office for the right product.

What a legume leaves for the next crop

Extension services turn this into “nitrogen credits”: pounds of nitrogen you can subtract from the fertilizer plan for the next crop. The University of Nebraska-Lincoln gives 45 lb N per acre after soybeans on both soil groups. After a good alfalfa stand it gives 150 lb on medium and fine soils and 100 lb on sandy soils. North Dakota State uses 40 lb after soybeans, peas and lentils (NDSU, SF-882). Credits vary by state, so use the table your own extension service publishes.

Chart

First-year nitrogen credit after a legume

lb N per acre for the next crop, sandy vs medium and fine soils

Signs a crop is short of nitrogen

Nitrogen shortage shows on older leaves first, as a light green to yellow colour, because the plant moves nitrogen to new growth (UF/IFAS Extension). A sulfur shortage does the opposite and yellows young leaves first. Many other problems look similar, so test the soil or leaf tissue before you act. The video below shows how yellow leaf colour links to nitrogen shortage.

▶ Yellow leaf colour and nitrogen shortage in plants, with signs to look for and organic ways to respond. · 12:32

2. Industrial fixation: the Haber-Bosch process

The Haber-Bosch process combines nitrogen from air with hydrogen at high temperature and pressure, over an iron-based catalyst, to make ammonia. Factories turn the ammonia into urea, ammonium nitrate and other fertilizers. In 2010 the process fixed about 120 Tg N, twice the estimate for natural land sources (Fowler et al., 2013).

Erisman and colleagues estimated that synthetic nitrogen fertilizer fed 44% of the world’s people in 2000 and 48% in 2008, as summarised by Our World in Data. That page adds that such figures are hard to pin down, and that estimates cluster around half.

Fertilizer use has grown, then levelled

World use of nitrogen fertilizer rose from 11.5 million tonnes of nitrogen in 1961 to about 113 million tonnes in 2021 (FAO data via Our World in Data). The chart below shows how growth slowed after 2011.

Chart

World nitrogen fertilizer use

Million tonnes of nitrogen applied, selected years

The cost: energy, emissions and price

Making ammonia uses a lot of energy. The International Energy Agency reports that ammonia production took about 2% of final energy use in 2020, over 95% of it from fossil fuels, and emitted about 450 Mt of CO₂, or 1.3% of energy-sector emissions (IEA, Ammonia Technology Roadmap, 2021). Just over 70% of output came from natural gas. In the IEA’s Net Zero Emissions by 2050 scenario, near-zero-emission methods reach nearly 95% of production by 2050 (electrolysis-based “green” ammonia more than 40%, gas with carbon capture about 20%), up from less than 1% today. That is a modelled pathway, not a forecast.

In the United States, 38 ammonia plants operated in 19 states in 2025 (one sat idle all year). About 57% of capacity sits in Louisiana, Oklahoma and Texas near natural gas supplies, and about 88% of domestic ammonia went to fertilizer (USGS Mineral Commodity Summaries, 2026). The USGS estimated the 2025 average Gulf Coast ammonia price at US$450 per short ton. The weekly price was US$357 in late June 2025 and US$492 in late September 2025.

Retail prices differ by product. DTN reported US retail prices for the last week of September 2026. They are list averages from retailers, not what every farmer paid.

Chart

US retail price per pound of nitrogen

US$ per lb N, DTN retail averages, last week of September 2026

3. Lightning and other abiotic fixation

Lightning heats air to extreme temperatures. Nitrogen and oxygen then combine to form nitric oxide, which turns into nitrogen dioxide and nitric acid and dissolves in rain. Global estimates of the source range from 2 to 10 Tg N a year, with newer values nearer 5 (Fowler et al., 2013).

That is small next to the other two ways, so lightning will not replace a fertilizer plan. It is a steady background supply that falls on all land and sea, and it varies with storms. Do not count it as a nitrogen credit for a field.

Compare the three ways

The chart below plots each source from the same review. It also includes the ocean, which the plant-focused table above leaves out.

Chart

Where fixed nitrogen comes from

Tg N per year (million tonnes of nitrogen); land, ocean, factories and lightning

What is a legume’s nitrogen credit worth?

A legume only pays off if the nitrogen it leaves saves more than it costs to grow and manage. The calculator below converts a nitrogen credit into dollars at your fertilizer price. It does not count the legume’s own crop income or other benefits.

The default values are examples chosen to sit near break-even. They are not benchmarks or forecasts. Use your own credit, price and extra cost. For reference, DTN’s last-week-of-September 2026 retail average for urea was US$683 per ton.

Try the numbers for your own farm below.

How to put biological nitrogen to work

1. Check the field history. If soybeans have not grown there for three years or more, consider inoculating (NDSU advice above); for other legumes, ask your extension office.
2. Test the soil. A soil nitrate test shows what is already there. Extension services in many states publish nitrate-test and credit guidance.
3. Credit the legume. Subtract the credit your extension service lists from the fertilizer plan. Alfalfa and clover credits depend on stand density, so count plants first.
4. Limit extra fertilizer on the legume. The Michigan trials above show fixation falls as fertilizer rises.
5. Keep records. Note yields, credits and any fertilizer left over, so next year’s plan starts from real numbers.
6. Check the label and local rules. Fertilizer, inoculant and manure rules differ by state and province. Follow the product label and ask a certified crop adviser for advice on your fields.

▶ Seven plain sustainable-farming practices, including cover crops, explained by a cartoon earthworm. · 4:34

How satellites help with nitrogen decisions

Satellite images show how green and dense a crop is. They cannot tell you the cause. A pale patch can be a nitrogen shortage, but water stress, disease, compaction or sulfur shortage can look alike. Use the map to decide where to walk and sample, then confirm with a soil or tissue test. The Farmonaut Satellite + AI Farm Monitoring System offers vegetation maps for fields drawn on a map; see the web and mobile app or the satellite API.

▶ A real demo field where satellite passes show a patch of crop stress that is hard to see on foot. · 1:42

Common mistakes

• Counting the legume credit and then applying the full fertilizer rate as well.
• Assuming every legume field has working nodules, when the field is new to the crop.
• Treating lightning as a source that can replace fertilizer.
• Applying nitrogen ahead of heavy rain, which can wash nitrate toward streams. Wisconsin extension notes that nitrate is a common groundwater pollutant in that state.
• Comparing fertilizer by price per ton instead of price per pound of nitrogen.

Related pages on our site

More reading: adding nitrogen to soil naturally, crimson clover as a cover crop, alternating nitrogen-fixing crops, carbon footprinting, product traceability, crop loans and insurance, large scale farm management and weather API developer docs.

FAQ

What are two ways nitrogen can be fixed naturally for plant use?

Soil bacteria and lightning. Bacteria, including rhizobia in legume nodules, convert nitrogen gas to ammonia. Lightning forms nitrogen oxides that rain carries to the soil as nitrate. Estimates are about 58 Tg N a year for natural land before the industrial era, 140 in the ocean, and about 5 for lightning (Fowler et al., 2013).

What is needed to fix nitrogen so plants can access it?

Energy, and a way to break the triple bond in N₂. Bacteria use the enzyme nitrogenase and energy from plant sugars. Lightning uses extreme heat. Factories use high temperature, high pressure and a catalyst.

How is fixed nitrogen made into usable forms?

Fixation makes ammonia. Soil organisms then change organic nitrogen into ammonium and nitrate, the inorganic forms roots take up (UF/IFAS Extension). Fertilizer urea is converted to ammonium quickly in moist, aerated soil.

Do soybeans need nitrogen fertilizer?

In the Michigan State trials, fixation supplied 49% of grain nitrogen with no fertilizer and about 16% with 85 kg N per hectare or more. Ask your extension service whether nitrogen fertilizer pays in your area.

How much nitrogen does a legume leave for the next crop?

It depends on the crop, stand and soil. Nebraska lists 45 lb N per acre after soybeans and up to 150 lb after a good alfalfa stand on medium and fine soils. North Dakota uses 40 lb after soybeans.

Is industrial nitrogen fixation bad for the environment?

It uses a lot of fossil energy. The IEA puts ammonia at about 2% of final energy use and 1.3% of energy-sector CO₂ in 2020. Used at the right rate and time, the fertilizer it makes feeds a large share of people, so the aim is efficient use and lower-carbon production, not removal.