Set your location
Urea volatilization risk is highly location-specific: temperature, humidity, wind, and incoming rain all interact. Allow location access for accurate real-time conditions at your field.
Surface-applied urea loses the most nitrogen when the soil surface is warm, moist, and windy and no rain follows. Loss rises as the soil warms (Montana State lists soil above 50°F as a high-risk condition (MSU Extension Soil Scoop), and its volatilization guide uses above 70°F soil (C. Jones, EB0209, updated June 2020)), while near-freezing ground slows the urease enzyme sharply, but not to zero: Montana State trials on urea broadcast between October and April averaged 18% loss and up to 44%. At least a half inch of rain within 48 hours moves urea into the soil and stops the loss; less than a quarter inch can make it worse by dissolving granules without incorporating them. No-till residue and soils above pH 7 raise the risk further. That is when an NBPT stabilizer like Agrotain most often pays for itself. Cool mornings ahead of a reliable incorporation rain are the lowest-risk window to spread.
Air temperature vs. soil surface temperature
Air Temperature
Drives enzyme activity speed: how fast urease converts urea to ammonia. This tool uses air temperature as its primary input because it’s what your phone’s weather shows. Warmer air = faster reaction once the granule dissolves.
Soil surface temperature
Determines whether enzyme is active at all. In early spring, air may be 58°F but soil surface is still 38–42°F, so risk is genuinely lower than this tool shows. Conversely, a black soil surface in full sun can be 15–20°F warmer than air temperature, making risk higher. Use air temp as a starting point, but know your soils.
In spring: when daytime air hits 50–55°F but soils are still cold from winter (under 40°F), risk is lower than the score suggests. Once soils warm above 50°F (earlier in the southern US, late spring further north) risk rises quickly and the score becomes your most reliable guide.
Urea vs. UAN vs. ESN: picking the N source
All three are common nitrogen sources for corn and small grains. Volatilization risk, application flexibility, and cost differ significantly. Match the product to your application method and timing window.
| N Source | Analysis | Volatilization Risk | Best Applied | Key Tradeoffs |
|---|---|---|---|---|
| Urea (granular) | 46-0-0 | HIGH (surface) | Incorporated by ½”+ rain within 48h; or use NBPT stabilizer; or inject/till-in | Highest N analysis of the dry N products. Most versatile for spreading. Surface application without stabilizer carries real loss risk above 50°F. |
| UAN (liquid) | 28-0-0 or 32-0-0 | MODERATE | Banded or dribbled, reduces volatilization vs broadcast. Can be injected for lowest risk. | Contains ~50% urea, ~25% ammonium, ~25% nitrate. Ammonium and nitrate fractions do not volatilize. Can be blended with herbicides and applied in-season. Requires liquid applicator. |
| ESN (polymer-coated urea) | 44-0-0 | LOW | Fall or early spring; coating delays release by weeks. Soil temperature controls release rate. | Significantly more expensive per lb N. Release is temperature-driven: in a cold spring, may release slowly. Excellent for no-till or high-pH fields where surface urea loss is high. Pays best when N prices are high. |
| Anhydrous Ammonia | 82-0-0 | VERY LOW | Injected below soil surface, volatilization negligible when properly applied. | Highest N analysis available. Requires specialized injection equipment. Best applied fall or early spring when soils are not too wet or frozen. Cold injection preferred to minimize nitrification before spring uptake. |
Weighing urea against UAN or ESN on price? Co-op posted N-P-K prices are on the fertilizer price page.
Why urea volatilizes
The volatilization reaction
CO(NH₂)₂
+ H₂O
(urease) →
2NH₃
+
CO₂
NH₃
(dissolved)
⇌
NH₃↑
(escapes to atmosphere, N lost permanently)
Urease enzyme, present in soil and especially in crop residue, converts urea to ammonia. At the soil surface, that ammonia gas escapes into the air rather than converting to plant-available ammonium (NH₄⁺).
Temperature
Urease works faster as soil warms. Montana State lists soil above 50°F as a high-risk condition (Soil Scoop) and above 70°F in its volatilization guide (EB0209, updated 2020). The warmer it is, the less time you have to get rain or tillage on it. Cold weather helps, but below freezing urease only slows down. It does not stop. Urea broadcast between October and April averaged 18% and up to 44% loss in Montana State trials.
Higher as soil warms
Moisture
The worst case in Montana State's trials: urea spread on a wet or snowy surface that then dries slowly with little or no rain (30 to 44% of the N lost). Light rain under about 1/4 inch is also risky: enough to dissolve the granule and start the enzyme, not enough to move N below the surface.
¼” rain without ½” = worst case
Wind
Ammonia gas builds up in a thin layer above the soil surface. Wind continuously removes that layer, maintaining the concentration gradient that pulls more N from the soil into the air. Even light winds of 3–5 mph keep volatilization moving at full speed.
Amplifies all other risk factors
Soil pH
Alkaline soils (pH 7+) favor ammonia gas over the stable, plant-available ammonium ion. At pH 7.5, extension trials commonly report losses of 30–50% of surface-applied urea in warm, moist conditions.
pH 7.5+ = extreme background risk
Crop Residue
Urea granules sitting on stalk fragments or leaf litter are in direct contact with residue urease and never reach soil. Montana State puts the water needed to move urea off residue at about 1.5 in within two days, against 0.5 in on bare or tilled soil.
No-till: assume higher risk
The two to three week window
Most volatilization from urea runs over the two to three weeks after application (Montana State EB0209), faster when it is warm and the surface is moist. Manitoba Agriculture reports losses measured over 5 and 7 days. A dry spell after spreading is not a safe spell: the first rain or heavy dew that wets the granules without moving them in starts the loss.
Two to three weeks of exposure
NBPT stabilizer: how it works and when it pays
What NBPT Does
NBPT (N-butyl thiophosphoric triamide), the active ingredient in Agrotain and most generic stabilizers, temporarily blocks urease enzyme activity. It slows loss, it does not stop it, for roughly two to 10 weeks (Montana State, EB0209), less on warm, moist or acid soil: time for incorporation rain or irrigation.
The Economics
At $8–12/acre, NBPT pays for itself when the N it saves is worth that much: $8 to $12 divided by what your N costs a pound. At Moderate or higher (or on no-till ground, high-pH soils, or when forecasts show no incorporation rain) the economics strongly favor treatment.
Stabilizer is insurance. You won’t need it on every load; use the score to decide which ones.
Common questions
Risk rises as the soil warms. Montana State lists soil above 50°F as a high-risk condition (MSU Extension Soil Scoop), and its volatilization guide uses above 70°F soil (C. Jones, EB0209, updated June 2020). The warmer the surface, the faster loss starts. Below 40°F, urease enzyme activity is slow. Below 32°F urease works very slowly, so loss starts slowly, but it does not stop. Montana State University field trials found an average 18% and up to 44% of the N lost from urea broadcast between October and April, highest on a moist or snowy surface with no rain or only sprinkles for the next 2 to 3 weeks. A urease inhibitor (NBPT) still helps. Runoff on slopes is a second concern on frozen ground. Do not assume cold months are safe: Montana State found losses from fall and winter broadcast can run higher than from spring broadcast. The soil vs. air temperature section above has more.
At least ½ inch within 48 hours will incorporate urea into the soil and stop the volatilization reaction. This is the incorporation threshold: once N is in the soil, it converts to ammonium (NH₄⁺) and is protected. Less than ¼ inch can actually make things worse: just enough moisture to dissolve the granule and activate the urease enzyme, but not enough to move N below the surface. If the forecast shows scattered showers under ¼ inch, that is not your incorporation event.
At $8–12 per acre, NBPT pays for itself when the N it saves is worth that much: divide $8 to $12 by what your N costs a pound. The urea price line on this page shows today's posted price per lb of N. The math favors treatment when the risk score is Moderate or higher. No-till residue, soil above pH 7.0 and a forecast with little rain are all high-risk conditions in Montana State's guide (C. Jones, EB0209, updated June 2020), which says to add NBPT to surface-applied urea; that is where NBPT is most likely to pay. On cold soils with a solid rain forecast and tilled ground, you can safely skip it. Use this risk score to make the call on every load.
Usually more, though how much varies by field and weather. Montana State says it takes about 1.5 inches of water within two days to move urea off residue and into the soil, against half an inch on bare or tilled soil. The reason: crop residue contains concentrated urease enzyme. A urea granule that lands on a corn stalk fragment or soybean stubble is in direct contact with enzyme and never reaches the protective soil environment. The granule can be fully hydrolyzed to ammonia while sitting on the residue surface, and that ammonia escapes into the air rather than converting to plant-available ammonium. On no-till or strip-till ground, always assume higher baseline risk and use NBPT stabilizer or plan for reliable incorporation rain.
Urea (46-0-0) is the most cost-effective but carries the highest volatilization risk when surface-applied. UAN (28 or 32-0-0) is liquid, contains roughly half urea nitrogen plus ammonium and nitrate fractions that don’t volatilize. Can be banded to reduce surface contact. ESN (polymer-coated urea, 44-0-0) releases N slowly over weeks with very low volatilization risk: excellent for no-till and high-pH fields, but significantly more expensive per pound of N. For most corn acres, the economical answer is: use urea with NBPT stabilizer when risk is Moderate or higher, and save ESN for the highest-risk fields where protection has the biggest payoff. See the comparison table above.
Below 32°F urease works very slowly, so loss starts slowly, but it does not stop. Montana State University field trials found an average 18% and up to 44% of the N lost from urea broadcast between October and April, highest on a moist or snowy surface with no rain or only sprinkles for the next 2 to 3 weeks. A urease inhibitor (NBPT) still helps. A second loss mechanism adds to it: runoff. If frozen ground thaws quickly or if rain falls on saturated, frozen soil, urea granules sitting on the surface can wash off the field entirely before any incorporation occurs. On sloped or tile-drained fields, fall and early spring applications to frozen ground carry real runoff risk. This tool caps the score low below 32°F because loss starts slowly; that cap is a design choice, not a promise of zero loss, and it does not account for runoff, which can be a much larger loss on the wrong slope.
More AGSIST tools
Made by Sigurd Lindquist, CCA, Chetek, Wisconsin.