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.
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 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.
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.
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.
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.
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.
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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.
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 (46-0-0) usually has the lowest cost per pound of nitrogen compared to other single-element nitrogen fertilizers. However, urea undergoes unique chemical transformations when field applied and severe losses in efficiency may result if special management practices are not followed. The purpose of this fact sheet is to briefly describe urea transformations and to suggest how urea-N may be conserved with proper management in the field.
In common with most commercial nitrogen fertilizers, urea is manufactured from anhydrous ammonia (NH3). The high analysis of urea—46% N—is the main reason for the low cost of this form of nitrogen fertilizer. Freight costs and storage and handling are all lower than with lower analysis fertilizers such as ammonium nitrate (34-0-0) or ammonium sulfate (21-0-0).
When field applied, urea changes to ammonium bicarbonate. This is a natural process resulting from the activity of the enzyme urease. Chemically the reaction is:
(NH2)CO + 2H2O ))) NH4HCO3 + OH-
This chemical reaction takes place after the urea is dissolved in water and will be complete within about 48 hours under field conditions. The water solution in which the reaction takes place has an alkaline pH, to as high as 8.5, and the ammonium (NH4+) tends to convert to ammonia gas (NH3). This gas will volatilize to the air if not protected. Urea placed on the soil surface or plant foliage may loose from 50% to 90% of its N as ammonia if not protected within a few hours of application.
If the urea-to-NH4 reaction takes place in the soil the nitrogen will be captured as exchangeable ammonium on the soil exchange complex and little if any loss of ammonia gas to the air will occur. Therefore, the key to conserving urea fertilizer nitrogen is to put the urea into the soil and not merely on the soil.
Soil incorporation of urea can be done several ways. Since urea is completely water soluble, when applied to the soil surface it can be moved down with irrigation water or rainfall, if one or the other occurs immediately after fertilization. Also, urea can be broadcast and plowed down immediately. And urea can be injected or banded into the soil.
Soil banding or injection is usually not feasible with an established crop such as pasture. Under these conditions the nitrogen fertilizer of choice would be either ammonium nitrate, ammonium sulfate, or one of the ammoniated phosphates (for example 11-52-0).
Urea is applied alone or in combination with other fertilizers. It is available in solid prills and in water solution. The latter includes a 50-50 mix of urea and ammonium nitrate, which is sold under various trade names and is guaranteed at 32% nitrogen (32-0-0). The urea-to-NH4 transformation will take place regardless of whether another nitrogen form or other fertilizer element is present in the fertilizer.
Urea is a low cost nitrogen fertilizer form. This is because of its high nitrogen composition and consequent low transport and storage costs. Urea may be the fertilizer of choice when only nitrogen is needed in a soil fertility program.
Urea converts to ammonium bicarbonate within about 48 hours after field application. Nitrogen in this form will tend to volatilize to the air as ammonia gas. This lost fertilizer investment risk can be minimized or eliminated by assuring that the urea gets into the soil and does not merely remain on the surface of the soil or crop foliage. This can be accomplished by irrigating in the urea; by plowdown soon after surface broadcast application; or by banding or injecting the urea directly into the soil.
D. W. James, Extension Soils Specialist
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