UREA AS A SOURCE OF FERTILIZER NITROGEN FOR CROPS IN KENTUCKY
K. L. Wells, L. W. Murdock, and H. F. Miller
Kentucky farmers are finding proportionately
more nitrogen being marketed each year in the chemical form of urea. This
trend has resulted from a greatly expanded production capacity for urea
by basic nitrogen producers. Judging from present projections, this increase
in the production of urea is likely to continue. Although urea nitrogen
has been marketed in Kentucky for several years, there are still many questions
asked by farmers about its agronomic characteristics.
Urea nitrogen is manufactured by reacting
ammonia with carbon dioxide. It is commonly prilled or granulated into
a solid, free-flowing material containing 45-46 percent total nitrogen.
As such, it is a highly concentrated nitrogen material suitable for direct
application to crops or for blending mixed fertilizers. From the standpoint
of manufacturing, transportation, and direct application, urea has the
advantage over other solid nitrogen materials of being cheaper to manufacture
and having a higher content of total nitrogen.
Use of Urea for Making Other Fertilizers
Solid Mixed Fertilizers
Because of its good physical characteristics,
granulated rather than prilled urea is commonly used as a major supplemental
nitrogen source in blending mixed fertilizers. Urea is now widely used
by solid fertilizer blenders in Kentucky in blending fertilizers for use
on all nitrogen-requiring crops.
Urea is used in the manufacture of
nitrogen solutions. When mixed with ammonium nitrate, non-pressure, aqueous
solutions of nitrogen can be prepared which are of higher nitrogen content
than aqueous solutions of either urea or ammonium nitrate alone. Nitrogen
content of nitrogen solutions is usually 28-32 percent total N. Half the
nitrogen is from urea and half is from ammonium nitrate. Weight per gallon
generally ranges from 10-11 pounds, depending on the nitrogen content.
These solutions are widely used in Kentucky for direct application of supplemental
nitrogen, particularly on corn. Blenders of fluid mixed fertilizers also
use nitrogen solutions as a material in formulating various grades.
Agronomic Evaluation of Urea
To predict what may happen from use
of urea, an understanding of how it reacts when applied to soils is needed.
The following sections summarize research findings on what happens when
urea is used as a fertilizer under the soil and climatic conditions of
Reaction with Soil
In its chemical form, urea is unreactive
with soil and is readily soluble in water. When it is added to soil, naturally
occurring reactions in the soil cause urea to break down to the ammonium
form of nitrogen. Soil pH immediately surrounding the particle of decomposing
urea is rapidly increased to the range of pH 7 - pH 10 as this reaction
takes place. This is the soil reaction which is critical to the efficiency
of urea nitrogen applied to soil, because in the high-pH zone surrounding
the decomposing particle of urea, a high concentration of ammonia (NH3)
develops. Under some soil conditions, part of this ammonia nitrogen may
be volatilized from the soil. Much research has been conducted during the
past several years to determine how much nitrogen loss can take place and
to identify what conditions are related to such losses. The amount of loss
been found to range from none to severe, depending largely on soil properties,
temperature, moisture level and application methods. Some soil conditions
which have been found to increase nitrogen losses from urea reaction in
soil are: high soil pH (above 6.5); low soil cation exchange capacity;
surface application; high level of soil organic matter; warm soil temperatures;
application to moist soil with fast-drying conditions at soil surface.
When using urea under conditions which are associated with nitrogen losses
from urea, the most effective method of lowering such risk is to incorporate
urea into the soil while applying or immediately after applying it.
Urea Is Acid-Forming
Although the immediate reaction of
urea in soil is to raise the pH surrounding the urea particles, the long-term
effect from use of urea on soil is to lower soil pH. This increased acidification
results from soil microbes oxidizing the ammonium nitrogen produced from
the breakdown of urea to nitrate nitrogen. This effect can be important,
particularly on soils which are already acid to moderately acid (pH 5-6),
when a large amount of nitrogen is applied. In such instances, soil pH
during the plant growing season may drop low enough to increase the concentration
of manganese (Mn) or aluminum (Al) around plant roots to toxic levels,
or to result in a lower solubility of soil phosphorus (P) and molybdenum
(Mo). This can lower yields of some crops. It should be emphasized that
other acid forming nitrogen fertilizers such as ammonium nitrate, nitrogen
solutions, and anhydrous ammonia can also bring about these same reactions.
Crop Response to Urea
Urea has been evaluated as a nitrogen
source in Kentucky for burley tobacco, corn, wheat and barley, and grass
pastures. Results will be discussed by crop.
Research in Kentucky has shown that
urea is as effective for producing burley tobacco as the non-acid forming
sodium nitrate when soil pH is maintained at the recommended level. Results
from a field test on Maury silt loam soil are shown in Table 1.
The only real difference in yield occurred
when no lime was used. In that case, soil pH dropped to 4.7 during the
growing season where urea was used, but hardly changed where the all-nitrate
N source was used. When the soil was limed to offset the acid-forming potential
of urea, there was no significant difference between N sources. These data
also show that leaf content of manganese (Mn) was greatly increased when
urea was used without liming the initially acid (PH 5.4) soil. Leaf content
of molybdenum (already deficient) correspondingly dropped to even lower
levels. But with use of lime on this initially acid soil, urea performed
agronomically as well as the nitrate source of N.
Table 1. Effect of rate of lime application and N source on yield,
pH, and Mn and Mo concentration of burley tobacco leaves, average of two
1Both N sources applied at 225 lbs N/A preplant.
|Rate of lime Tons/A
2Soil pH (water) measured at midseason of each year.
Soil pH before liming was 5.4.
Wheat and Barley
Urea has also been compared with ammonium
nitrate as a source for spring topdressed N on wheat and barley in field
tests conducted by U.K. Results (Table 2) of a 3-year study on "Arthur"
wheat and a 2-year study on "Barsoy" barley show no differences between
the two sources from an early to mid-March topdressing.
Table 2. Effect of spring topdressed ammonium nitrate and urea on
yield of wheat and barley.
1 Topdressed in early to mid-March
2 Av. 3 years data
3 Av. 2 years data
UK has conducted several field experiments
comparing urea with ammonium nitrate (AN) for both conventionally grown
corn and no-till corn, either all applied at planting or as a delayed application
5-7 weeks after planting. Results have been variable, but in general the
following trends are evident:
(1) Conventionally grown corn
a.When N is broadcast just before planting
and incorporated into the soil, there is little difference between sources
b.When N is broadcast at planting and
not incorporated into soil, there is a trend for AN to be slightly more
effective on poorly drained soils, but little difference on well-drained
c.When N is topdressed over corn 5-7
weeks after planting and not incorporated, there is little difference between
(2) No-till corn (sod, stalkland, or small grain stubble)
a.Variable results have been obtained
when all N was topdressed at planting, but collectively the data would
indicate that there is a risk for urea to be slightly less effective than
AN, particularly at low to moderate rates of N.
b.When all N is topdressed over corn
5-7 weeks after planting, the risk increases for urea to be less effective
than AN, although results have been variable.
Due to the large number of factors
which determine the amount of nitrogen which will be lost, the results
between experiments is quite variable. Apparently, under most conditions
in Kentucky, the loss of nitrogen from surface applied urea will be less
than 5% as compared with AN. Although uncommon, these losses can be much
larger. The highest losses found were about 25%.
(4) Rate of N
Results of field experiments show the
risk that urea will result in lower corn yields than AN is greatest when
there is nitrogen stress (low to moderate rates of applied N) on the crop.
In experiments there was little difference between sources when 150 lbs
N/A was used.
Results from several experiments comparing
urea with ammonium nitrate (AN) show that use of urea topdressed onto grass
fields is not likely to be as effective as AN. These studies show that
yields of forage and total content of N are usually lower from urea than
AN. The degree of agronomic loss measured in Kentucky is in the range of
10-30 percent. In other words, each pound of urea N topdressed will likely
yield 70-90 percent of that from a pound of topdressed ammonium nitrate
N. Studies conducted in central Kentucky have shown such losses for March,
June and August topdressings. A study conducted at Princeton, however,
has shown little difference between urea and AN topdressed onto fescue
at 50 lbs N/A in early spring (March-early April). But topdressings from
mid-April to early October resulted in urea being only 80-90 percent as
effective as AN.
Is Urea a Slow-Release Form of N?
Because of the fact that urea must
be converted to ammonium and nitrate nitrogen for its use by plants, some
people describe urea as being a "slow-release" form of N. Field performance
data show this is not the case. The conversion of urea-N to ammonium-N
and on to nitrate-N is rapid, and under Kentucky's soil, climatic, and
cropping conditions is not considered to represent a "slow release" of
N either for application to fallow land ahead of the intended crop, for
application at or after time of planting, or for application onto grass
Field studies conducted in Kentucky
indicate that urea can be a good source of N for most N-requiring crops.
The greatest risk of inefficiency of urea is from topdressing it onto grass
fields and no-tillage corn. By keeping in mind the principles of urea reaction
when applied to soil, it can satisfactorily be used in many crop production
systems. The following statements point up the thoughts to keep in mind
when using urea:
•It is acid forming and, when used
in large amounts, results in an appreciable drop in soil pH during the
growing season. By keeping soil adequately limed, urea can be as effective
as non-acid forming N sources.
•Its initial reaction when applied
as fertilizer results in high concentrations of ammonia N immediately around
each particle. Therefore, care should be taken not to place it close to
seeds or young seedlings to reduce likelihood of ammonia toxicity.
•Incorporation of urea into soil lowers
risk of N loss from urea under conditions where N loss might be expected.
•Urea is a highly concentrated nitrogen
material (45-46% N) and generally more economical per unit of N than other
solid forms. When purchasing nitrogen, make your decision on cost per pound
of N, not cost per pound of material.