The nitrogen in synthetic fertilizer comes from the air. Earth’s atmosphere is about 78% nitrogen gas, making it the most abundant source of this essential plant nutrient on the planet. But nitrogen gas is extremely stable and unreactive, so plants can’t use it directly. The industrial process that converts atmospheric nitrogen into a form plants can absorb is what makes modern fertilizer possible.
From Air to Ammonia: The Haber-Bosch Process
Nearly all synthetic nitrogen fertilizer starts with a single chemical reaction developed in the early 1900s called the Haber-Bosch process. It works by forcing nitrogen gas from the air to combine with hydrogen gas, producing ammonia. This reaction requires extreme conditions: temperatures around 500°C, pressures above 100 times normal atmospheric pressure, and an iron-based catalyst to help the reaction along. The ammonia that results is the building block for virtually every nitrogen fertilizer on the market.
That ammonia gets further processed into different fertilizer products depending on the application. Urea, the most common dry nitrogen fertilizer, is a white crystalline solid containing 46% nitrogen by weight. Its high nitrogen concentration keeps handling, storage, and transportation costs lower than other forms. Other products include ammonium nitrate and liquid nitrogen solutions, but they all trace back to ammonia made from atmospheric nitrogen.
Where the Hydrogen Comes From
Pulling nitrogen from the air is the easy part. The harder, more energy-intensive step is sourcing the hydrogen needed to combine with it. About 80% of the energy required for ammonia production goes toward generating hydrogen, and the dominant method is steam methane reforming. This process uses high-temperature steam (700°C to 1,000°C) to break apart methane molecules in natural gas, releasing hydrogen. In the United States, 95% of all hydrogen is produced this way.
This is why nitrogen fertilizer is so tightly linked to fossil fuels. The nitrogen itself is free, pulled from the air around us. But the hydrogen comes from natural gas, and the process of extracting it releases significant carbon dioxide. On average, producing one ton of ammonia generates 2.4 tons of CO2. Roughly 85% of the world’s ammonia goes toward manufacturing nitrogen-based fertilizers, making the fertilizer industry one of the largest greenhouse gas emitters in the chemical sector.
Nitrogen From Living Organisms
Long before industrial chemistry existed, nitrogen entered the soil through biological processes. Certain bacteria, collectively called rhizobia, form small nodules on the roots of legumes like soybeans, clover, and alfalfa. Inside those nodules, the bacteria convert atmospheric nitrogen into ammonia using an enzyme called nitrogenase, powered entirely by energy the plant captures from sunlight. This happens at normal temperatures and pressures, no industrial reactor required.
The amount of nitrogen these partnerships produce varies widely. Field studies of temperate legumes show fixation rates ranging from about 50 to over 300 kilograms of nitrogen per hectare per year, depending on the crop and growing conditions. That’s a meaningful amount. Farmers have used this for centuries by rotating legume crops with grain crops, letting the legumes replenish soil nitrogen naturally. It’s one reason soybeans and alfalfa remain staples in crop rotation systems.
Nitrogen in Manure and Compost
Animal manure is another significant nitrogen source, especially in organic farming where synthetic fertilizers aren’t permitted. The nitrogen content varies considerably by animal type. Poultry manure is the richest, containing roughly 19 to 21 pounds of nitrogen per ton in raw form and up to 52 pounds per ton when dried. Swine manure falls in the middle at about 10 to 13 pounds per ton, while beef manure contains roughly 7 to 11 pounds per ton. Dairy manure tends to be the lowest, at 5 to 10 pounds per ton.
These numbers are much lower than synthetic urea’s 46% nitrogen concentration, which means you need to apply far more material to deliver the same amount of nitrogen. Manure also releases its nitrogen more slowly as soil microbes break down the organic matter, so the timing of nutrient availability is less predictable. Poultry compost tested at Ohio State University ranged from about 1.75% to 4% total nitrogen content, illustrating how variable even a single manure type can be depending on bedding material, moisture, and composting time.
The Push Toward Greener Production
The core chemistry of nitrogen fertilizer hasn’t changed in over a century. Ammonia is the second most manufactured chemical in the world, and the Haber-Bosch process still dominates production. But the carbon footprint is driving efforts to replace natural gas with cleaner hydrogen sources. The concept is straightforward: instead of cracking methane for hydrogen, split water molecules using electricity from renewable sources like wind or solar. The nitrogen still comes from the air, the basic reaction stays the same, but the hydrogen arrives without fossil fuels.
Using renewable-powered electrolysis to generate hydrogen could cut greenhouse gas emissions from ammonia manufacturing by about 91%. The catch is scale. Despite the fact that renewable ammonia production has existed for over a century in small applications, it accounts for just 0.01% of global ammonia output today. Cost remains the primary barrier, since cheap natural gas has kept conventional ammonia affordable for decades. Several large-scale green ammonia projects are now under development worldwide, but displacing a meaningful share of conventional production will take years of investment and infrastructure buildout.
So whether your fertilizer bag contains urea, ammonium nitrate, or a blended formula, the nitrogen inside almost certainly started as a molecule of gas floating in the atmosphere. What changed it into plant food was an industrial process running on natural gas, a biological partnership between bacteria and plant roots, or the digestive system of a farm animal. The atmosphere remains the original and essentially limitless reservoir.