From Havlin, Beaton, Tisdale and Nelson (1999)

Urea CO(NH₂)₂
Favorable economics of manufacturing, handling, storage, and transportation have made urea a very competitive source of fertilizer N. Worldwide urea use is almost five times that of NH4NO3- Urea is the principal form of dry fertilizer N in the United States, approaching 16% of total N use.

Granular urea has noteworthy characteristics, including (1) less tendency to stick and cake than NH4NO3, (2) lack of sensitivity to fire and explosion, and (3) less corrosiveness to handling and application equipment. Substantial savings in handling, storage, transportation, and application costs are possible because of urea's high N content.

Biuret LEVELS
The concentration of biuret (NH2-CO-NH-CO-NH2) in urea is of special concern because of its phytotoxicity. Biuret levels of 2% can be tolerated in most fertilizer programs. Because citrus, pineapple, and other crops are sensitive to biuret in urea applied as a foliar spray, less than 0.25% biuret is recommended. Solutions made from urea containing 1.5% biuret are acceptable for foliar application on corn and soybeans. Urea high in biuret should not be placed near or in the seed row.

BEHAVIOR OF UREA IN SOILS
When applied to soil, urea is hydrolyzed by the enzyme urease to NH4+. Depending on soil pH, the NH4+ may form NH3, which can be volatilized at the soil surface, as represented in the following reactions:

CO(NH2)2 + H+ + 2H20 --> 2NH4+ + HC03-
NH4+ --->  NH3 + H+
Urea hydrolysis proceeds rapidly in warm, moist soils, with most of the urea transformed to NH4+ in several days. Urease, an enzyme that catalyzes the hydrolysis of urea, is abundant in soils. Large numbers of bacteria, fungi, and actinomycetes in soils possess urease. Urease activity increases with the size of the soil microbial population and with OM content. The presence of fresh plant residues often results in abundant supplies of urease. Urease activity is greatest in the rhizosphere, where microbial activity is high and where it can accumulate from plant roots. Rhizosphere urease activity varies depending on the plant species and the season of the year. Although temperatures up to 37'C favor urease activity, hydrolysis of urea occurs at temperatures down to 2'C and lower. This evidence of urease functioning at low temperatures, combined with urea's ability to melt ice at temperatures down to I IF (-12C), suggests that a portion of fall or early-winter-applied urea may be converted to NH3 or NH4+ before the spring. The effects of soil moisture on urease activity are generally small in comparison to the influence of temperature and pH. Hydrolysis rates are highest at soil moisture contents optimum for plants. Free NH3 inhibits the enzymatic action of urease. Since significant concentrations of free NH3 can occur at pH values above 7, some temporary inhibition of urease by free NH3 occurs after the addition of urea because soil pH in the immediate vicinity of the urea source may reach values of up to 9.0. High rates of urea fertilization in localized placement could create conditions restrictive to the action of urease.

MANAGEMENT OF UREA FERTIIIZER
Careful management of urea and urea- based fertilizers will reduce the potential for NH3 volatilization losses and in- crease the effectiveness of urea fertilizers.

Surface applications of urea are most efficient when they are washed into the soil or applied to soils with low potential for volatilization. Conditions for best performance of surface-applied urea are cold, dry soils at the time of application and/or the occurrence of significant precipitation, probably more than 0.25 cm (0. I in.), within the first 3 to 6 days following application. Movement of soil moisture containing dissolved NH3 and diffusion of moisture vapor to the soil surface during the drying process probably contribute to NH3 volatilization at or near the soil surface. Incorporation of broadcast urea into soil minimizes NH3 losses by  increasing the volume of soil to retain NH3. Also, NH3 not converted in the soil must diffuse over much greater distances before reaching the atmosphere. If soil and other environmental conditions appear favorable for NH3 volatilization, deep incorporation is preferred over shallow surface tillage. Band placement of urea results in soil changes comparable to those produced by applications of anhydrous NH3. Diffusion of urea from banded applications can be 2.5 cm (I in.) within 2 days of its addition, while appreciable amounts of NH4+ can be observed at distances of 3.8 cm (1.5 in.) from the band. After dilution or dispersion of the band by moisture movement, hydrolysis begins within 3 to 4 days or less under favorable temperature conditions. Placement of urea with the seed at planting should be carefully controlled be- cause of the toxic effects of free NH3 on germinating seedlings. The harmful effects of urea placed in the seed row can be eliminated or greatly reduced by banding at least 2.5 cm (I in.) directly below and/or to the side of the seed row of most crops. Seed placed urea should not exceed 5 to 10 lbs N/a. The effect on germination of urea placed near seeds is influenced by available soil moisture. With adequate soil moisture in medium-textured loam soils at seeding time, urea at 30 lb N/a can be used without reducing germination and crop emergence. However, in low-moisture, coarse-textured (sandy loam) soils, urea at 10 to 20 lb N/a often reduces both germination and crop yields. Seedbed moisture is less critical in fine-textured (clay and clay loam) soils, and urea can usually be drilled in at rates of up to 30 lb N/a. To summarize, the effectiveness of urea depends on the interaction of many factors, which cause some variability in the crop response to urea. However, if managed properly, urea will be about as effective as the other N sources.