The agricultural industry widely uses urea, a white crystalline solid containing 46 percent nitrogen, as an animal feed additive and fertilizer. Here, we’ll focus on its role as a nitrogen fertilizer.
In the past decade, urea has surpassed anhydrous ammonia as the main source of nitrogen fertilizer in Minnesota. This has brought up new questions about urea and how to use it.
Basics of fertilizer urea
You can purchase fertilizer urea as prills or as a granulated material.
In the past, it was usually produced by dropping liquid urea from a prilling tower while drying the product. The prills formed a smaller and softer substance than other materials commonly used in fertilizer blends.
Today, considerable urea is manufactured as granules. Granules are larger, harder and more resistant to moisture. As a result, granulated urea has become a more suitable material for fertilizer blends.
You can apply urea to soil as a solid, solution or, for certain crops, a foliar spray.
Usage involves little or no fire or explosion hazard.
Urea’s high analysis – 46 percent N – helps reduce handling, storage and transportation costs over other dry N forms.
Urea manufacturing releases few pollutants to the environment.
When properly applied, it results in crop yield increases equal to other forms of nitrogen.
Nitrogen from urea can be lost to the atmosphere if fertilizer urea remains on the soil surface for extended periods of time during warm weather.
The key to most efficiently using urea is to incorporate it into the soil during a tillage operation. You can also blend it into the soil with irrigation water. As little as 0.25 inches of rainfall is sufficient to blend urea deep enough into the soil so ammonia losses won’t occur.
Urea breakdown begins as soon as it’s applied to the soil. If the soil is totally dry, no reaction happens.
But with the enzyme urease, plus any small amount of soil moisture, urea normally hydrolyzes and converts to ammonium and carbon dioxide. This can occur in two to four days and happens more quickly on high pH soils.
Unless it rains, you must incorporate urea during this time to avoid ammonia loss. Losses might be quite low in the spring if the soil temperature is cold.
The chemical reaction is:
CO(NH2)2 + H2O + urease → 2NH3 +CO2 (urea)
The problem is the NH3, because it’s a gas. However, if it’s incorporated, it acts the same as incorporated anhydrous ammonia. Also, half of 28 percent liquid N is urea. The same thing happens with this half as with regular urea.
Urea’s volatility depends to a great extent on soil temperature and soil pH. Tables 1 and 2 show that, after a few days, warm temperatures or high pH would cause losses.
Table 1: Impact of temperature on urea losses
|Days||45 F||60 F||75 F||90 F|
|0 days||0% of added N volatilized||0% of added N volatilized||0% of added N volatilized||0% of added N volatilized|
|2 days||0% of added N volatilized||0% of added N volatilized||1% of added N volatilized||2% of added N volatilized|
|4 days||2% of added N volatilized||2% of added N volatilized||4% of added N volatilized||5% of added N volatilized|
|6 days||5% of added N volatilized||6% of added N volatilized||7% of added N volatilized||10% of added N volatilized|
|8 days||5% of added N volatilized||7% of added N volatilized||12% of added N volatilized||19% of added N volatilized|
|10 days||6% of added N volatilized||10% of added N volatilized||14% of added N volatilized||20% of added N volatilized|
Table shows percent of surface-added urea volatilized as ammonia at different temperatures and days on the surface. Urea was added on a silt loam soil at 100 pounds of N.
Table 2: Impact of soil pH on urea loss
|Days||Soil pH- 5.0||Soil pH-5.5||Soil pH- 6.0||Soil pH- 6.5||Soil pH- 7.0||Soil pH- 7.5|
|0||0 of added N volatilized||0 of added N volatilized||0 of added N volatilized||0 of added N volatilized||0 of added N volatilized||0 of added N volatilized|
|2||0 of added N volatilized||0 of added N volatilized||0 of added N volatilized||0 of added N volatilized||1 of added N volatilized||5 of added N volatilized|
|4||1 of added N volatilized||2 of added N volatilized||5 of added N volatilized||10 of added N volatilized||18 of added N volatilized||20 of added N volatilized|
|6||4 of added N volatilized||5 of added N volatilized||7 of added N volatilized||11 of added N volatilized||23 of added N volatilized||30 of added N volatilized|
|8||8 of added N volatilized||9 of added N volatilized||12 of added N volatilized||18 of added N volatilized||30 of added N volatilized||33 of added N volatilized|
|10||8 of added N volatilized||10 of added N volatilized||13 of added N volatilized||22 of added N volatilized||40 of added N volatilized||44 of added N volatilized|
Table shows the percent of surface-added urea volatilized as ammonia at various soil pH levels and days on the surface. Urea was added on silt loam soil at 100 pounds of N per acre.
Applying and storing urea
Urea can readily be nitrified – that is, converted to nitrate (NO3) – even when applied in late fall, and can be quite susceptible to denitrification or leaching the following spring. Anhydrous ammonia (AA) applied in the fall doesn’t nitrify as quickly, due to the stunting of microorganisms in the AA application band.
A two-year study conducted in Waseca compared late-October applications of both AA and urea for continuous corn (Table 3).
Data show a 6-bushels-per-acre advantage for AA over urea when applied in the fall without a nitrification inhibitor. But when N-Serve was added, AA showed a 16-bushels-per-acre advantage. This indicates the inhibitor has a better degree of contact with the AA mix than what’s possible with urea.
Table 3: Yields after applying anhydrous ammonia (AA) and urea
|AA (82% N)||162 bushels per acre||168 bushels per acre|
|AA + N-serve||170 bushels per acre||172 bushels per acre|
|Urea (45% N)||156 bushels per acre||164 bushels per acre|
|Urea + N-serve||154 bushels per acre||162 bushels per acre|
Table shows corn yields as influenced by N source, time of application and nitrification inhibitor in Waseca. Yield figures are an average of 1981 to 1982, after applying 150 pounds of N per acre.
Studies that continuously use urea have been conducted in Lamberton since 1960. Corn yields over a 24-year period averaged 5 to 6 bushels per acre higher with spring application of urea compared to the fall plowed-down application (Table 4).
Table 4: Yield as influenced by fall and spring urea applications
|Time/method of urea application||24-year average yield|
|Fall, plowed down||97 bushels per acre|
|Spring, top-dressed||102 bushels per acre|
|Spring, side-dressed||103 bushels per acre|
Table shows corn yields after applying 80 pounds of N per acre in Lamberton.
Urea applied in the fall generally hasn’t been as effective as AA. This is especially true in south-central Minnesota and Iowa.
When fall soil-moisture conditions are dry, there’s little difference between AA and urea. But when soil-moisture content is high, fall applications of urea haven't performed as well as AA. Applying urea-ammonium nitrate (UAN) in the fall isn’t recommended due to rapid nitrification and a high potential for loss.
If properly applied, urea and fertilizers containing urea are excellent sources of nitrogen for crop production.
After application to the soil, urea undergoes chemical changes and ammonium (NH4 +) ions form. Soil moisture determines how rapidly this conversion takes place.
When an urea particle dissolves, the area around it becomes a zone of high pH and ammonia concentration. This zone can be quite toxic for a few hours. The free ammonia that has formed can kill the seed and seedling roots within this zone.
Fortunately, this toxic zone becomes neutralized in most soils as the ammonia converts to ammonium. Usually it's just a few days before plants can effectively use the nitrogen.
Although urea imparts an alkaline reaction when first applied to the soil, the net effect is to produce an acid reaction.
How and how much to apply
Urea or materials containing urea should, in general, be broadcast and immediately incorporated into the soil.
If applying urea-based fertilizer in a band, separate it from the seed by at least 2 inches of soil. Under no circumstances should urea or urea-based fertilizer be seed-placed with corn.
With small grains, you can generally apply 10 pounds of nitrogen as urea with the grain drill at seeding time, even under dry conditions. Under good moisture conditions, you can apply 20 pounds of nitrogen as urea with the grain drill.
Research from North Dakota State University indicates that, under dry conditions, urea can reduce wheat stands more than 50 percent (Table 5). This was for urea applied with a grain drill in a 6-inch spacing, at the rate of more than 20 pounds of nitrogen per acre.
University of Wisconsin research indicates that seed-placed urea with corn, even at low rates of nitrogen, is very toxic to the seed and greatly reduces yields (Table 6). However, when urea was side-placed as a 2-by-2-inch starter, researchers noted little, if any, damage (Table 7).
In Minnesota, good crop production usually requires an application of more than 20 pounds of nitrogen per acre. Farmers can avoid damage from urea by broadcasting most of the urea nitrogen fertilizer ahead of seeding. Data in Table 8 indicate that urea broadcast prior to seeding is equal to or more effective than similar ammonium nitrate treatments.
Table 5: Seed-placed ammonium nitrate (AN) and urea comparisons
|N (lbs per acre)||N source||Absaraka, N.D.||Williston, N.D.||Casselton, N.D.|
|0 lbs||-||600 seedlings per 40 ft. of row||270 seedlings per 40 ft. of row||760 seedlings per 40 ft. of row|
|20 lbs||AN||570 seedlings per 40 ft. of row||220 seedlings per 40 ft. of row||600 seedlings per 40 ft. of row|
|30 lbs||AN||590 seedlings per 40 ft. of row||240 seedlings per 40 ft. of row||690 seedlings per 40 ft. of row|
|40 lbs||AN||590 seedlings per 40 ft. of row||260 seedlings per 40 ft. of row||660 seedlings per 40 ft. of row|
|20 lbs||Urea||400 seedlings per 40 ft. of row||200 seedlings per 40 ft. of row||550 seedlings per 40 ft. of row|
|30 lbs||Urea||280 seedlings per 40 ft. of row||110 seedlings per 40 ft. of row||430 seedlings per 40 ft. of row|
|40 lbs||Urea||220 seedlings per 40 ft. of row||70 seedlings per 40 ft. of row||220 seedlings per 40 ft. of row|
Table 6: How urea and ammonium nitrate placed with seed affect corn grain yield
|Pounds of N per acre||Urea yield||Ammonium nitrate yield|
|0 lbs||137 bushels per acre||137 bushels per acre|
|5 lbs||60 bushels per acre||142 bushels per acre|
|10 lbs||36 bushels per acre||143 bushels per acre|
|20 lbs||33 bushels per acre||92 bushels per acre|
Table 7: How side-placed urea and ammonium nitrate affect corn grain yield
|Pounds of N per acre||Urea yield||Ammonium nitrate yield|
|0 lbs||142 bushels per acre||142 bushels per acre|
|25 lbs||145 bushels per acre||145 bushels per acre|
|50 lbs||146 bushels per acre||146 bushels per acre|
|100 lbs||150 bushels per acre||141 bushels per acre|
Table 8: How urea and ammonium nitrate source and placement affect corn yields
|Pounds of N per acre||Treatment||Source||Average yield|
|0 lbs||-||-||62 bushels per acre|
|40 lbs||Plow-down: Fall||Ammonium nitrate||79 bushels per acre|
|40 lbs||Plow-down: Fall||Urea||86 bushels per acre|
|40 lbs||Surface: Fall||Ammonium nitrate||82 bushels per acre|
|40 lbs||Surface: Fall||Urea||85 bushels per acre|
|80 lbs||Plow-down: Fall||Ammonium nitrate||98 bushels per acre|
|80 lbs||Plow-down: Fall||Urea||97 bushels per acre|
|160 lbs||Plow-down: Fall||Ammonium nitrate||104 bushels per acre|
|160 lbs||Plow-down: Fall||Urea||105 bushels per acre|
|40 lbs||Topdress: Spring||Ammonium nitrate||89 bushels per acre|
|40 lbs||Topdress: Spring||Urea||88 bushels per acre|
|80 lbs||Topdress: Spring||Ammonium nitrate||100 bushels per acre|
|80 lbs||Topdress: Spring||Urea||102 bushels per acre|
Urea can be bulk-spread, either alone or blended with most other fertilizers. It’s recommended that the spreading width not exceed 50 feet when combined with other fertilizer materials.
Urea often has a lower density than other fertilizers it’s blended with. This lack of weight produces a shorter distance-of-throw when applying the fertilizer with spinner-type equipment. In extreme cases, this will result in uneven crop growth and wavy or streaky fields.
Blending urea with other fertilizers
Urea and fertilizers containing urea can be blended quite readily with monoammonium phosphate (11-52-0) or diammonium phosphate (18-46-0).
Do not blend urea with superphosphates unless applied shortly after mixing. Urea will react with superphosphates, releasing water molecules and resulting in a damp material that’s difficult to store and apply.
Particle size uniformity is important with dry solid urea, whether applied directly or in blended formulations.
Some imported urea appears to be below U.S. quality standards on granule uniformity. Dissolving urea and marketing the liquid solution is an attempt to overcome this lack of uniformity while taking advantage of the favorable urea price.
The liquid mix of urea and ammonium nitrate (UAN 28 percent N) has been on the market for a long time. However, the solution’s characteristics aren’t the same as when urea alone is dissolved in water.
A solution of 50 percent urea by weight results in 23-0-0 and has a salting-out temperature of 60 degrees Fahrenheit. To store and handle liquid urea during cooler temperatures, the nitrogen concentration must be lowered to reduce salting problems. Several possible formulations can be used for this, such as adding small amounts of ammonium nitrate, ammonium sulfate or anhydrous ammonia.
Research, particularly on liquid urea, is very limited. Generally, where dry urea successfully functions, the fluid urea should perform equally well and may have the advantage of better uniformity over some dry urea sources.
Biuret in urea can cause agronomic problems if placed near the seed, or even if it’s added preplant in bands where seeds will later be planted.
Biuret in the manufacturing process
Most U.S. urea manufacturers keep biuret content low by keeping high temperatures to a minimum. Biuret content is typically around 0.3 percent, although urea of foreign origin appears to be higher.
High heat is normal during urea manufacturing. If heat exceeds 200 degrees Fahrenheit, there’s a slight conversion of urea to biuret, but this takes place only during the manufacturing process. No such conversion happens in storage or in the soil.
Biuret converts to ammonia, but conversion is much slower than for urea. Because biuret remains in the soil for several weeks, the potential for seed damage continues beyond the brief period of urea’s conversion to ammonia.
Biuret’s major damage is to germinating seeds. There’s little damage through plant absorption, although some citrus crops have been affected.
Urea can be applied to sod crops, winter wheat or other small grains. However, make this application in cool seasons. During warm periods (60 degrees Fahrenheit or above), urea in contact with vegetative material tends to give off ammonia.
If urea must be applied on grass pastures in the summer, apply when there’s a high probability of rainfall.
You can also apply urea as a foliar spray on some crops, such as potatoes, wheat, vegetables and soybeans.
Urea is highly water soluble. At normal atmospheric temperatures, approximately 1 pound of urea can be dissolved in 1 pound of water.
Research indicates that urea should contain no more than 0.25 percent biuret for use in foliar sprays. For many crops, the quantity of nitrogen applied at one time shouldn’t exceed 20 pounds of nitrogen per acre.
Urea isn’t combustible or explosive. Under normal circumstances, it can be safely stored with no loss of quality.
Don’t use small or fast-moving augers to move granular urea. Urea particles are generally soft and abrasion can break the granules. Use belt conveyors whenever possible.
Also, don’t store urea with ammonium nitrate. When in contact, these materials rapidly absorb water when the relative humidity is above 18 percent. Table 9 indicates the relative humidity at which urea and ammonium nitrate absorb moisture from the air.
Table 9: Critical relative humidities of urea, ammonium nitrate and a mixture of the two
|Material||Critical relative humidities (CRH)|
|Ammonium nitrate (AN)||59.4%|
|Urea + ammonium nitrate||18.1%|
Urea fertilizer can be coated with certain materials, such as sulfur, to reduce the rate at which the nitrogen becomes available to plants. Under certain conditions, these slow-release materials result in more efficient use by growing plants.
Urea in a slow-release form is popular for golf courses, parks and other special lawn situations.
Urea do’s and don’ts
- Store separately from ammonium nitrate.
- Don’t use small, fast-moving augers to move the urea.
- Don’t exceed a spreading width of 50 feet when urea is applied.
- Don’t place urea in direct contact with corn seed.
- Keep rates of nitrogen applied together with small grain in drill to 10 pounds on dry soils and 20 pounds when soil is moist.
- Apply urea on sod crops when atmosphere temperature is below 60 degrees Fahrenheit.
- When broadcasting urea on soils of high pH (above 7.5), incorporate the material into the soil as soon as possible.
Reviewed in 2021