Table 8. Effect of source and placement of urea and ammonium nitrate (AN) on corn yields. Lamberton, Minnesota Experiment Station, 1960-84. lb. N/A Treatment Source Av. Yield
bu/A 0 62 40 Plow-downfall AN 79 40 Plow-downfall Urea 86 40 Surfacefall AN 82 40 Surfacefall Urea 85 80 Plow-downfall AN 98 80 Plow-downfall Urea 97 160 Plow-downfall AN 104 160 Plow-downfall Urea 105 40 Topbressspring AN 89 40 Topdressspring Urea 88 80 Topdressspring AN 100 80 Topdressspring Urea 102 [SIZE=-1]Source: MacGregor, Malzer and Nelson, University of Minnesota[/SIZE]
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.
Fluid Urea
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 watersoluble. 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 Storage
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.
Table 9. Critical relative humidities (CRH) of urea, ammonium nitrate, and a mixture of the two. Material CRH% Urea 75.2 Ammonium Nitrate (A.N.) 59.4 Urea + ammonium nitrate 18.1
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
- Store separately from ammonium nitrate.
- Do not use small, fast-moving augers to move the urea.
- Do not exceed a spreading width of 50 feet when urea is applied.
- Do not place in direct contact with corn seed.
- Keep rates of nitrogen applied together with small grain in drill to 10 1b. on dry soils, 20 lb. when soil is moist.
- Apply urea on sod crops when atmospheric temperature is below 60 degrees F.
- When urea is broadcast on soils of high pH (above 7.5), the material should be incorporated into the soil as soon as possible.
Curtis J. Overdahl
Extension Soils Specialist
George W. Rehm
Extension Soils Specialist
Harvey L. Meredith
Department of Soil Science
COLLEGE OF AGRICULTURE, FOOD, AND ENVIRONMENTAL SCIENCES
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