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.
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