By: Overdahl, Rehm and Meredith
In the past decade urea has surpassed and nearly replaced ammonium nitrate as a fertilizer. This has brought about new questions on urea and its use.
Urea, a white crystalline solid containing 46% nitrogen, is widely used in the agricultural industry as an animal feed additive and fertilizer Here we discuss it only as a nitrogen fertilizer.
Physical Forms of Urea
Commercially, fertilizer urea can be purchased 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, though, 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.
Advantages of Fertilizer Urea
Incorporate Urea for Best Use
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 the most efficient use of urea is to incorporate it into the soil during a tillage operation. It may also be blended into the soil with irrigation water. A rainfall of as little as 0.25 inches is sufficient to blend urea into the soil to a depth at which ammonia losses will not occur.
Urea Losses to the Air
Urea breakdown begins as soon as it is 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 hydrolizes and converts to ammonium and carbon dioxide. This can occur in 2 to 4 days and happens quicker on high pH soils. Unless it rains, urea must be incorporated 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 as follows:CO(NH2)2 + H2O + urease 2NH3 +CO2
The problem is the NH3, because it's a gas, but if incorporated the NH3, acts the same as incorporated anhydrous ammonia. Also, half of 28% liquid N is urea and the same thing happens with this half as with regular urea.
Urea Losses Related to Soil Temperature and pH
The volatility of urea 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.
Fall Application Comparisons
Urea can be readily nitrifiedthat is, converted to nitrate (NO3) even when applied late in the fall, and can be quite susceptible to denitrification or leaching the following spring. Anhydrous ammonia (AA) applied in the fall does not nitrify as quickly, due to the stunting of microorganisms in the AA application band.
A two-year study conducted at Waseca compared late-October applications of both AA and urea for continuous corn (Table 3). These data show a 6 bu/A advantage for AA over urea when applied in the fall without a nitrification inhibitor. But when N-Serve was added, a 16 Bu/A advantage was shown with AA. This indicates that the inhibitor has a better degree of contact with the AA mix than is possible with urea.
Studies with continuous use of urea have been conducted at 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).
Urea applied in the fall has generally not been as effective as AA. This is especially true in south-central Minnesota and Iowa. When fall soil-moisture conditions are dry, there is little difference between AA and urea. But when soil-moisture content is high, fall applications of urea haven't performed as well as AA. Applications of urea-ammonium nitrate (UAN) in the fall are not recommended due to rapid nitrification and a high potential for loss.
Soil Application and Placement of 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 a 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. Seed and seedling roots within this zone can be killed by the free ammonia that has formed. 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. Urea-based fertilizer applied in a band should be separated from the seed by at least two inches of soil. Under no circumstances should urea or urea-based fertilizer be seed-placed with corn.
With small grains, 10 lb. of nitrogen as urea can generally be applied with the grain drill at seeding time even under dry conditions. Under good moisture conditions, 20 lb. of nitrogen as urea can be applied with the grain drill. Research results at North Dakota State University indicate that under dry conditions, urea at the rate of more than 20 lb. nitrogen per acre, applied with a grain drill in a 6-inch spacing, can reduce wheat stands more than 50% (Table 5) Research at the University of Wisconsin 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). When urea was side-placed as a 2" x 2" starter, however, little if any damage was noted (Table 7).
In Minnesota, good crop production usually requires an application of more than 20 lb. 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.
Spreading of Urea
Urea can be bulk-spread, either alone or blended with most other fertilizers. It is recommended that the spreading width not exceed 50 feet when combined with other fertilizer materials.
Urea often has a lower density than other fertilizers with which it is blended. This lack of "weight" produces a shorter "distance-of-throw" when the fertilizer is applied 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).
Urea should not be blended with superphosphates unless applied shortly after mixing. Urea will react with superphosphates, releasing water molecules and resulting in a damp material which is difficult to store and apply.
Uniformity of particle size 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 and still take advantage of the favorable urea price.
The liquid mix of urea and ammonium nitrate (UAN 28% N) has been on the market for a long time. The characteristics of this solution, however, are not the same as when urea alone is dissolved in water. A solution of 50% urea by weight results in 23-0-0 and has a salting-out temperature of 60 degrees F. In order to store and handle liquid urea during cooler temperatures, the nitrogen concentration must be lowered to reduce salting problems. There are several possible formulations that 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 functions successfully, the fluid urea should perform equally well and may have the advantage of better uniformity over some dry urea sources.
Biuret in Urea
Biuret in urea can cause agronomic problems if placed near the seed. or even if added preplant in bands where seeds will later be planted.
Most U.S. manufacturers of urea keep biuret content low by keeping high temperatures to a minimum. Biuret content is typically around 0.3%, although urea of foreign origin appears to be higher.
High heat is normal during the manufacture of urea. If heat exceeds 200 degrees F there is 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. Since biuret remains in the soil for several weeks, the potential for seed damage continues beyond the brief period of conversion of urea to ammonia. The major damage of biuret is to germinating seeds. There is little damage through plant absorption, although some citrus crops have been affected.
Application of Urea to Growing Crops
Urea can be applied to sod crops, winter wheat. Or other small grains. This application, however, should be made during cool seasons. During warm periods (60 degrees F or above), urea in contact with vegetative material will tend to give off ammonia.
If urea must be applied on grass pastures in the summer, apply when there is a high probability of rainfall.
Foliar Application of Urea
Urea can also be applied 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 lb. Of urea can be dissolved in 1 lb. of water.
Research data indicate that urea should contain no more than 0.25% biuret for use in foliar sprays. For many crops the quantity of nitrogen applied at one time should not exceed 20 lb. of nitrogen per acre.
Urea is neither combustible nor explosive. It can be stored safely with no loss of quality under normal circumstances. Small or fast-moving augers should not be used to move granular urea. Urea particles are generally soft and abrasion can break the granules. Belt conveyers should be used whenever possible.
Urea should not be stored with ammonium nitrate. These materials, when in contact, rapidly absorb water when the relative humidity is above 18%. Table 9 indicates the relative humidity at which urea and ammonium nitrate absorb moisture from the air.
Slow Release Of Urea
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 use on golf courses, parks, and other special lawn situations.
Urea Do's and Don'ts