Urea fertilizer is made by reacting ammonia with carbon dioxide under high pressure and temperature in an industrial process. It cannot be safely or practically produced at home. The reaction requires specialized equipment operating at extreme conditions, and the key raw material, anhydrous ammonia, is lethally toxic at relatively low concentrations. What follows is a clear breakdown of how urea fertilizer is actually manufactured, what makes it such a widely used nitrogen source, and how the finished product gets its final form.
Why You Can’t Make Urea at Home
The single biggest barrier is anhydrous ammonia. This compressed gas is stored at around -28°F and causes severe chemical burns on skin contact. Inhaling it at concentrations above 300 ppm is immediately dangerous to life, and concentrations of 2,500 to 4,500 ppm can be fatal within 30 minutes. The U.S. workplace exposure limit is just 25 ppm averaged over an eight-hour day. Beyond the ammonia hazard, the synthesis reaction itself demands pressures high enough to liquefy both ammonia and carbon dioxide, along with rapid heat removal in a specialized reactor tower. This is not chemistry you can replicate in a garage or workshop.
If you need urea fertilizer, it’s widely available and inexpensive at garden centers and farm supply stores. The rest of this article explains the industrial process for readers who want to understand what goes into making it.
The Two Raw Materials
Urea production starts with just two ingredients: ammonia (NH₃) and carbon dioxide (CO₂). Ammonia typically comes from a nearby ammonia plant, where natural gas provides the hydrogen that gets combined with atmospheric nitrogen. Carbon dioxide is a byproduct of that same ammonia production, so most urea plants sit right next to ammonia facilities to capture and pipe CO₂ directly into the process. Some newer designs capture CO₂ from industrial flue gases and pair it with ammonia produced using renewable energy, but the chemistry stays the same.
The Two-Step Reaction
The conversion happens through what’s known as the Bazarov reaction, and it unfolds in two distinct stages inside a high-pressure synthesis reactor.
In the first step, ammonia and carbon dioxide combine to form an intermediate compound called ammonium carbamate. This reaction releases a large amount of heat and happens quickly. In the second step, that ammonium carbamate loses a molecule of water and becomes urea. This dehydration step is slower and doesn’t go to completion in a single pass, which is why modern plants include recycling loops to push more of the intermediate toward the final product.
The overall result: two molecules of ammonia plus one molecule of carbon dioxide yield one molecule of urea and one molecule of water. Every ton of urea produced consumes a significant quantity of CO₂, though the process still generates a net 910 kg of CO₂ per ton of urea when you account for the energy used to make ammonia in the first place.
Inside a Modern Urea Plant
After the synthesis reactor, the urea solution still contains a lot of unconverted ammonium carbamate. The solution flows into a piece of equipment called a high-pressure stripper, a tall vertical heat exchanger where the leftover carbamate breaks back down into ammonia and CO₂. Fresh carbon dioxide flows upward through the stripper while the urea solution trickles down through heated tubes, and the released gases get recycled straight back into the reactor. This stripping step is what makes modern plants efficient: instead of discarding unreacted material, they continuously loop it back through.
After stripping, the urea solution passes through lower-pressure stages that concentrate it further by evaporating water. By the end, you have a molten urea melt that’s ready to be shaped into the solid granules or prills you’d recognize in a bag of fertilizer.
Prilling vs. Granulation
The molten urea needs to be turned into solid particles that are easy to store, transport, and spread on fields. There are two main ways to do this, and the choice affects the size, strength, and handling characteristics of the finished product.
Prilling involves spraying molten urea droplets from the top of a tall tower (often 50 meters or more) and letting them solidify as they fall through a rising current of cool air. The resulting prills are small, around 1.65 mm in diameter, with a fairly uniform size and strength. They’re weaker than granules, with an average crushing strength of about 3.8 newtons, which means they’re more prone to breaking during handling and storage.
Granulation builds up particles in a fluidized-bed drum, where layers of urea melt are sprayed onto seed particles that tumble and grow. Granules are larger, typically 2.8 to 3.1 mm, and significantly stronger at 10 to 17 newtons of crushing force. That extra toughness reduces dust and breakage during shipping and makes granular urea the preferred form for bulk blending with other fertilizers. Most modern plants use granulation for these reasons, though prilling towers are still common at older facilities.
What Makes Urea So Popular
Urea contains 46% nitrogen by weight, the highest concentration of any solid nitrogen fertilizer on the market. Its NPK rating is 46-0-0, meaning it supplies nitrogen only, with no phosphorus or potassium. That high nitrogen density means lower shipping and storage costs per unit of actual nutrient compared to alternatives like ammonium nitrate (34% nitrogen) or ammonium sulfate (21%).
It dissolves readily in water, which makes it versatile. You can apply it as dry granules, mix it into a liquid solution (a 50% urea solution by weight gives you a 23-0-0 liquid, though it will crystallize out below about 60°F), or blend it with other fertilizers to create custom nutrient mixes.
Storing and Handling the Finished Product
Urea is hygroscopic, meaning it pulls moisture from the air once humidity crosses a specific threshold. That critical relative humidity ranges from about 82% at 50°F down to 73% at 95°F. In practical terms, urea stored in a humid, warm environment will absorb water, clump together, and become difficult to spread evenly. Keeping it in sealed bags or covered bins, away from direct moisture, prevents most problems.
Once applied to soil, urea converts to ammonium through a natural enzyme reaction, and that ammonium is what plants actually take up. The conversion happens within a few days in warm, moist soil. Surface-applied urea is vulnerable to nitrogen loss as ammonia gas, especially on high-pH soils or when rain doesn’t incorporate it into the ground quickly. Lightly working it into the top few inches of soil or applying before expected rainfall reduces those losses significantly.
Quality Control: The Biuret Problem
When urea is heated during manufacturing, some of it can convert to a byproduct called biuret, which is toxic to plants at high concentrations. This is especially important for urea used in foliar sprays, where the solution contacts leaves directly. Fertilizer-grade urea intended for soil application can tolerate somewhat higher biuret levels, but urea marketed for foliar use is processed to keep biuret content well below 1%. If you’re buying urea for leaf feeding, check the label for a low-biuret designation.