UREA
Introduction
Fertilizer Urea
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
Urea can be applied to soil as a solid or solution or to certain crops as a foliar spray.
Urea usage involves little or no fire or explosion hazard.
Urea's high analysis, 46% N, helps reduce handling, storage and transportation costs over other dry N forms.
Urea manufacture releases few pollutants to the environment.
Urea, when properly applied, results in crop yield increases equal to other forms of nitrogen.
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 (urea)
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 nitrified—that 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.
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.
