Urea fertilizer is a synthetic nitrogen source, manufactured as small white granules containing about 46% nitrogen by weight, making it the most concentrated solid nitrogen fertilizer available. Once spread on soil, it does not feed plants directly. Instead, a soil enzyme called urease breaks urea down into ammonium, which plants can absorb and which soil bacteria further convert into nitrate. That conversion process is both urea’s strength and its weakness, because a significant share of the nitrogen can escape into the air as ammonia gas before crops ever get a chance to use it.
How Urea Breaks Down in Soil
Urea itself is a simple organic molecule, two amine groups joined to a carbonyl group. It is highly soluble in water, which means granules dissolve quickly after rain or irrigation. Once dissolved, the enzyme urease, which is produced by soil microorganisms and is found throughout virtually all agricultural soils, catalyzes a reaction called hydrolysis. This splits urea into ammonia and carbon dioxide. The ammonia then picks up a hydrogen ion from soil water to become ammonium, the positively charged form of nitrogen that clings to negatively charged soil particles and is readily available to plant roots.
The speed of this hydrolysis matters. Under warm, moist conditions, most of the urea in a soil can be hydrolyzed within two to four days. That rapid breakdown is a double-edged sword. On one hand, nitrogen becomes available to crops quickly. On the other, the burst of ammonia produced during hydrolysis temporarily raises the pH around each granule, which pushes the chemical balance toward gaseous ammonia that can escape into the atmosphere rather than staying in the soil as ammonium.
Can Plants Take Up Urea Directly?
Most textbook descriptions treat urea purely as a precursor to ammonium, but plants can actually absorb intact urea molecules through their roots. Research on the model plant Arabidopsis showed that roots have a high-affinity urea transporter called DUR3, and that urea uptake was stimulated when urea was present in the surrounding medium. Plants that already had plenty of nitrogen from ammonium nitrate showed reduced urea uptake, suggesting roots adjust their transport machinery based on what nitrogen forms are available.1Plant Physiology. Physiological and Transcriptomic Aspects of Urea Uptake and Assimilation in Arabidopsis Plants Beyond DUR3, a separate family of membrane proteins called major intrinsic proteins can shuttle urea passively across cell membranes, acting as a lower-affinity backup route.2Plant Science. Molecular and physiological aspects of urea transport in higher plants
In practice, though, most of the nitrogen from a field application of urea reaches crops after conversion to ammonium and then nitrate, simply because urease works so fast that relatively little intact urea lingers in the root zone. The direct-uptake pathway is more relevant in situations like foliar sprays, where a dilute urea solution is applied to leaves and enters through stomata, bypassing soil chemistry entirely. Foliar urea feeding is common in fruit orchards and certain specialty crops where a quick nitrogen boost is needed without disturbing the soil.
Ammonia Volatilization and Why It Matters
The single biggest practical drawback of urea is ammonia loss. When urea hydrolyzes on or near the soil surface, the ammonia produced can escape as gas before it converts to the plant-available ammonium form. Globally, ammonia losses from surface-applied urea average around 16% of the nitrogen applied, and under hot, humid conditions those losses can climb to 40% or more.3PubMed Central. Agronomic efficiency of NBPT as a urease inhibitor: A review That is a lot of expensive fertilizer drifting away into the atmosphere.
The amount lost depends heavily on timing and weather. A three-year study on winter wheat found that the same urea applied in March or April lost about 4 to 5% of its nitrogen as ammonia, while urea applied in early June, when soils were warmer and drier, lost 17%.4Agriculture, Ecosystems & Environment. Ammonia volatilization after application of urea to winter wheat over 3 years affected by novel urease and nitrification inhibitors Soil moisture at the time of application had a strong effect: a dry surface gave the ammonia gas an easier escape route, while rainfall soon after application dissolved the urea and carried it into the soil where volatilization was much lower.
For anyone spreading urea on a lawn, garden, or farm field, the takeaway is straightforward. If you can incorporate it into the soil by light tillage or irrigation within a day or two of application, or if rain is expected shortly, losses shrink dramatically. Surface-broadcasting on a warm, dry day with no rain in the forecast is the worst-case scenario.
Nitrous Oxide Emissions
Ammonia is not the only gas to worry about. Urea application also increases emissions of nitrous oxide, a potent greenhouse gas with roughly 270 times the warming potential of carbon dioxide over a 100-year window. The mechanism is indirect: once urea hydrolyzes and ammonium accumulates, soil bacteria called ammonia oxidizers convert it to nitrite on their way to producing nitrate. In the first 36 hours or so after urea is applied, research has found that these ammonia oxidizers outpace the bacteria that should convert nitrite to nitrate, leading to a temporary nitrite buildup. That bottleneck drives nitrous oxide production, resulting in emissions roughly tenfold higher than those from an equivalent amount of ammonium applied directly.5PubMed Central. Urea Fertilization Significantly Promotes Nitrous Oxide Emissions from Agricultural Soils and Is Attributed to the Short-Term Suppression of Nitrite-Oxidizing Bacteria during Urea Hydrolysis
In absolute terms the fraction of applied nitrogen that escapes as nitrous oxide is small, typically less than 1%. A study in sugarcane fields measured cumulative nitrous oxide losses of about 0.7% of the nitrogen applied as urea over two cropping cycles.6PubMed. Enhanced-efficiency fertilizers in nitrous oxide emissions from urea applied to sugarcane But because the warming potential per molecule is so large, even that small fraction adds up across the hundreds of millions of tonnes of urea spread worldwide each year.
Urease Inhibitors
The most widely used tool for taming ammonia loss is a chemical additive called NBPT (the abbreviation stands for a long chemical name you do not need to memorize). NBPT is mixed into or coated onto urea granules before sale. When the treated granule dissolves in soil, NBPT temporarily blocks the active sites of the urease enzyme, delaying the hydrolysis of urea by several days. That delay gives rain or irrigation time to move the urea deeper into the soil profile, where ammonia gas cannot escape.7Scientific Reports. Urease inhibitors technologies as strategy to mitigate agricultural ammonia emissions and enhance the use efficiency of urea-based fertilizers
Across a large body of field trials, NBPT-treated urea reduces ammonia losses by roughly half compared with untreated urea.3PubMed Central. Agronomic efficiency of NBPT as a urease inhibitor: A review That is a substantial improvement and the reason NBPT-treated products have become so popular: the market for urease inhibitors has been growing at about 16% a year. The slowed hydrolysis also helps reduce nitrous oxide emissions, because a gentler, more gradual release of ammonium avoids the nitrite bottleneck that spikes nitrous oxide production in the first day or two after application.5PubMed Central. Urea Fertilization Significantly Promotes Nitrous Oxide Emissions from Agricultural Soils and Is Attributed to the Short-Term Suppression of Nitrite-Oxidizing Bacteria during Urea Hydrolysis
Separate from urease inhibitors, there are also nitrification inhibitors, which target a different step. These slow the conversion of ammonium to nitrate in the soil, keeping nitrogen in the ammonium form longer. Two of the most studied, DCD and DMPP, have been shown in a meta-analysis to be equally effective at reducing nitrate leaching and nitrous oxide emissions.8PubMed Central. Efficiency of two nitrification inhibitors (dicyandiamide and 3, 4-dimethypyrazole phosphate) on soil nitrogen transformations and plant productivity: a meta-analysis Sometimes urease and nitrification inhibitors are combined in a single product, attacking both loss pathways at once. These are often marketed under the umbrella term “enhanced-efficiency fertilizers,” and their integration into nutrient management plans has become a growing focus of agronomic research.9Crops & Soils. Integrating Enhanced‐Efficiency Fertilizers in 4R Nutrient Management
Coated and Slow-Release Urea
A different approach to controlling nitrogen release is to physically coat each urea granule with a material that slows water from reaching the urea inside. Sulfur coatings have been used for decades, but newer formulations use biodegradable polymers. In one trial on spinach, polymer-coated urea granules increased dry foliage yield by about 47% and nitrogen uptake by roughly 60% compared with uncoated urea, simply by releasing nitrogen more gradually so the crop could use a larger share of it.10PubMed Central. Biodegradable Polymer Coated Granular Urea Slows Down N Release Kinetics and Improves Spinach Productivity
Newer experimental coatings combine polymers with biochar, a charcoal-like material. The biochar layer acts as an initial barrier that slows water penetration, and then a polymer membrane underneath gradually swells and releases nitrogen in a sustained pattern.11Industrial Crops and Products. Slow-release urea fertilizer with polymer and biochar-based organic coatings: Design, field trials and global economic implications In soil column tests, some coated formulations extended the overall nitrogen release period by up to 60 days compared with plain urea, following a multi-phase release curve rather than the sudden burst you get from uncoated granules.12Scientific Reports. Plasma-modified biodegradable coatings for controlled nitrogen release from urea
The downside of coated urea is cost. The coating materials and manufacturing processes add significantly to the price per tonne, which is why coated products tend to be used more in high-value crops, turfgrass, and ornamental horticulture than in commodity grain farming, where margins are tighter. As biodegradable coating technology improves, the price gap is expected to narrow.
Long-Term Soil Acidification
Beyond immediate nitrogen losses, repeated urea use over years gradually lowers soil pH. The acidification comes from the nitrification step: when soil bacteria convert ammonium to nitrate, hydrogen ions are released as a byproduct, making the soil more acidic over time. Long-term field comparisons have confirmed that soil acidity increases as nitrogen application rates go up. Interestingly, urea causes less acidification than anhydrous ammonia, a competing nitrogen source, partly because some of urea’s nitrogen volatilizes as ammonia before it gets the chance to acidify the soil, a silver lining to what is otherwise a wasteful process.13Soil Science Society of America Journal. Soil Acidification from Long‐Term Use of Anhydrous Ammonia and Urea
For farmers using urea year after year, periodic liming (applying calcium carbonate or a similar material) is usually necessary to maintain soil pH in the range most crops prefer. How often and how much lime you need depends on your soil’s natural buffering capacity, how much nitrogen you apply, and your local climate. Soil testing every two to three years is the standard way to stay ahead of the problem.
The Biuret Contaminant
Urea fertilizer is manufactured industrially by reacting ammonia with carbon dioxide under high pressure and temperature. A byproduct of that process is biuret, a compound formed when two urea molecules join together and lose an ammonia molecule. Biuret is toxic to plants, although the precise physiological mechanisms behind that toxicity are still not well understood.14PubMed Central. Overexpression of exogenous biuret hydrolase in rice plants confers tolerance to biuret toxicity
Most agricultural-grade urea contains less than about 1.5% biuret, which is generally low enough to be harmless when the fertilizer is applied to soil, because soil microbes can degrade small amounts over time. The concern grows when urea is used as a foliar spray, because leaves are more sensitive than roots. Foliar-grade urea is produced to a tighter specification, often below 0.25% biuret. If you are buying urea for foliar feeding on citrus, ornamentals, or other sensitive crops, checking the biuret content on the product label is worth the few seconds it takes.
How Urea Compares With Other Nitrogen Fertilizers
Urea’s 46% nitrogen content gives it a logistics advantage: you need to ship and spread fewer tonnes per unit of nitrogen compared with ammonium nitrate (about 34% N) or ammonium sulfate (about 21% N). That saves on freight and storage, which is a big deal for large-scale operations. Field comparisons of urea and ammonium nitrate on bermudagrass forage showed that urea-ammonium nitrate (UAN) solutions, which blend the two, generally produced results intermediate between pure urea and pure ammonium nitrate.15Crop Science. Comparison of Enhanced‐Efficiency Nitrogen Fertilizers for Reducing Ammonia Loss and Improving Bermudagrass Forage Production
Ammonium nitrate is less prone to volatilization because only half its nitrogen starts as ammonium (the other half is already nitrate), and there is no hydrolysis step that spikes pH around the granule. But ammonium nitrate is regulated more tightly in many countries because of its potential misuse as an explosive precursor, which has made urea the default choice in many markets. Ammonium sulfate supplies sulfur along with nitrogen, which can be an advantage on sulfur-deficient soils, but its lower nitrogen concentration means higher application and transport costs.
Choosing among these fertilizers usually comes down to local price per unit of nitrogen, soil conditions, application equipment, and whether volatilization can be managed through incorporation, irrigation, or inhibitor use. There is no universally “best” nitrogen source; the best one is the one whose risks you can manage given your situation.
Urea as a Protein Substitute in Animal Feed
Urea’s role in agriculture extends beyond plant nutrition. Ruminant animals, cattle and sheep in particular, harbor microorganisms in their rumen that can convert non-protein nitrogen into microbial protein. Feed-grade urea exploits this biology: when added to a ruminant diet, the rumen microbes break down the urea and use the resulting nitrogen to build their own proteins, which the animal then digests further along in the gut. This allows urea to partially replace more expensive protein ingredients like soybean meal.16PubMed Central. Scientific Opinion on the safety and efficacy of Urea for ruminants
From a sustainability perspective, this is a genuinely interesting trick. Neither humans nor monogastric animals like pigs and chickens can derive meaningful protein from urea, but ruminants can, which improves the human-edible protein efficiency of livestock systems. In other words, ruminants convert a cheap industrial chemical into meat and milk protein that humans can eat, freeing up plant protein sources for direct human consumption or other uses.17Animal. Unlocking the limitations of urea supply in ruminant diets by considering the natural mechanism of endogenous urea secretion
The catch is that urea must be introduced carefully and at controlled levels. If an animal consumes too much urea too quickly, the rumen bacteria release ammonia faster than the liver can detoxify it, leading to ammonia toxicity, which can be fatal. Feed-grade urea is typically mixed into total mixed rations at concentrations that keep intake gradual and safe, and livestock nutritionists calculate inclusion rates based on the overall energy and nitrogen balance of the diet.
Storage and Handling Quirks
Urea is hygroscopic, meaning it absorbs moisture from the air. If stored improperly in humid conditions, granules clump into hard lumps that are difficult to spread evenly. The critical relative humidity above which urea starts absorbing significant moisture is around 75% at typical room temperatures, which is lower than many people expect. Storing bags in a dry, well-ventilated shed and keeping them sealed until use prevents most caking problems.
Temperature swings compound the issue. If stored urea cycles between warm daytime and cool nighttime temperatures, moisture condenses on granule surfaces and then evaporates, leaving behind salt bridges that glue granules together. For bulk storage, keeping the pile under cover and minimizing temperature fluctuations is more important than any particular additive. Some commercial urea is treated with a coating agent like formaldehyde or a waxy anti-caking compound, which helps but does not eliminate the need for reasonable storage conditions.
Urea also has a relatively low melting point compared with most fertilizers, around 133°C (271°F). In extreme heat during transport or storage in tropical regions, granules can soften and fuse. This is less of a concern in temperate climates but worth knowing if you are ordering urea for use in very hot environments and it will sit in storage for weeks.