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AY-204
Fertilization
Purdue University
Cooperative Extension Service
West Lafayette, In 47907
Types and Uses of Nitrogen Fertilizers for Crop Production
David B. Mengel, Agronomy Department
Because of the tight supplies and soaring costs of
nitrogen materials, Indiana farmers are taking a critical second look at
their fertilizer programs. The object of such an evaluation is to insure the
wisest use and greatest returns from the application of nitrogen fertilizer.
Increasingly, farmers are asking such questions as: "How
do nitrogen fertilizers differ? What are the best kinds for the various
crops I raise? Which ones should or shouldn't be used on the types of soils
I have? Are there `best' times and ways to apply the different nitrogen
materials?"
The purpose of this publication is to answer these and
similar questions concerning types and uses of nitrogen fertilizers for crop
production. It is hoped that the information presented here will help
Hoosier farmers more accurately assess their current fertilizer programs and
make those adjustments that will maximize their fertilizer dollar.
FORMS OF NITROGEN IN FERTILIZER
Fertilizers common to crop production in Indiana usually
contain nitrogen in one or more of the following forms: nitrate, ammonia,
ammonium or urea. Each form has specific properties that determine when,
where and how various fertilizer materials can be used.
Here is a brief discussion of these four forms of
nitrogen, their characteristics, and under what conditions they should or
should not be applied.
Nitrate (NO3) Form
Nitrates "dissolve" in water and,therefore, move about in
the soil with the movement of soil water. Rainfall will wash nitrates
downward through the soil profile where they may enter tiles or drainage
channels and be lost for agricultural production. This is called leaching
and is the major cause of nitrogen loss from coarse-textured sandy soils.
During dry periods, on the other hand, when water is
evaporating from the soil, nitrates can move upward and may accumulate at
the soil surface. However, once leached below the root zone, upward movement
of large quantities of nitrates is unlikely, and thus they are considered
lost to the crop.
When soils become waterlogged, soil organisms take the
oxygen they need from nitrates, leaving the nitrogen in a gaseous form which
escapes into the air. This is known as denitrification and is the common
source of nitrogen loss in fine-textured clay soils.
Ammonia (NH3) and Ammonium (NH4) Forms
Ammonia is a gas at atmospheric pressure but can
be compressed into a liquid, as is the case with the nitrogen fertilizer
anhydrous ammonia. When anhydrous is applied, the ammonia reacts with water
in the soil and changes to the ammonium form. Ammonia in water, known as
aqua ammonia, is free to escape into the air and, therefore, when used as a
nitrogen fertilizer, must be injected under the soil surface.
Although water-soluble, ammonium attaches readily
to clay and organic matter particles (in much the same way iron is attracted
to and held on a magnet), thus preventing it from leaching away. Then during
the growing season, soil microorganisms convert the ammonium to nitrate,
which is the main form taken up by plants. The soil conditions most
favorable to this conversion process (called nitrification) include: a soil
pH of 7, moisture at 50% of the soil's water-holding capacity, and a soil
temperature of 80F. Conditions unfavorable would be: a pH below 5.5,a
waterlogged moisture condition, and temperature under 40F.
Urea (COCNH) Form
This form of fertilizer nitrogen usually undergoes a
three-step change before it is taken up by crops. First, enzymes in the soil
or plant residue convert the urea N to ammonia N. Next, the ammonia reacts
with soil water to form ammonium N. And finally, through the action of soil
microorganisms, the ammonium is converted to nitrate N.
Like nitrates, urea dissolves in and moves with soil
water and thus can be lost through leaching if not converted to ammonia and
then ammonium. The conversion to ammonia takes only 2 to 4 days when soil
moisture and temperature are favorable for plant growth. Lower temperatures
slow the process, but it will continue even down to freezing. Consequently,
leaching losses are seldom experienced under field conditions.
When ammonia is formed from urea applied on the soil
surface, some will be volatilized (escape into the air), the amount
depending on a combination of soil conditions. Greatest loss can be expected
when soil pH is above 7. soil temperature S high and soil moisture low.
Ammonia formed from urea applied under the soil surface, on the other hand,
s rapidly converted to ammonium. which will neither move with water nor be
lost to the air.
NITROGEN FERTILIZERS -
THEIR CHARACTERISTICS AND USES
Table 1 lists the various nitrogen fertilizers commonly
used for agronomic crops in Indiana. Shown for each fertilizer material is
the percent and form of nitrogen it contains and its recommended uses.
(Occasionally, there is need for technical facts about these nitrogen
fertilizers, such as weight, amount of N per gallon, pressure and
salting-out temperature. These data are given in Table 2.)
Following is additional information, first on the
adaptation and application of nitrogen fertilizers in general, and then on
each specific material. For more details, consult your fertilizer dealer,
county Extension agent or the related publications listed at the end of this
bulletin.
The Application Rate Suggestions
1. Rates for nitrogen fertilizers fall-applied ahead of
corn need to be 3 to 10 percent higher than spring pre-plant applications to
realize comparable yields.
2. If fertilizing corn at low nitrogen rates (i.e., up to
75 lb./acre), sidedressing permits greater N utilization and thus better
yield response than pre-plant application. At full rates (1-1 1/4 lb. N./bu.
yield), however, there is no difference in response between the two times of
application.
Ammonium Nitrate, Calcium Nitrate, Cal-Nitro
Ammonium nitrate is a 50-50 mixture of ammonium
and nitrate nitrogen. Although the modern-day "prilled" ammonium nitrate
material is much less hydroscopic (picks up moisture from the air) than that
of 20 years ago, it must still be protected by plastic when stored.
Calcium nitrate and cal-nitro are two different products
but both imported from Europe. Calcium nitrate (or nitrate of lime)
is produced by reacting nitric acid with crushed limestone and, therefore,
contains only the nitrate form of nitrogen. Cal-nitro is a mixture of
ammonium nitrate and crushed limestone and, thus, provides equal amounts of
ammonium N and nitrate N. Both products, being granulated, store and handle
well when dry: but they tend to pick up moisture more readily than our
domestic ammonium nitrate.
All three materials are excellent for topdressing wheat.
They are also equally satisfactory as spring plowdown applications for corn
on heavier-textured soils (silt loams, silty clay loams, clay loams and
clays). However, they are progressively less satisfactory for plowdown on
the courser-textured soils (loams, sandy loams, loamy sands and sands) but
can be used for sidedressing. The limitation to sidedressing corn with these
materials is often the lack of suitable equipment to do the job. Aerial
application should be considered only as a last resort, since granules
falling into the whorls of the leaves will cause salt burn.
For topdressing of grass pasture, if the goal is uniform
production for grazing, ammonium nitrate or cal-nitro is preferred, because
half the fertilizer N is in the slower-release ammonium form. If, on the
other hand, the goal is grass for hay or silage. then calcium nitrate might
be the first choice, since most of the N is in the immediately-available
nitrate form to give maximum early-season growth when soil moisture is most
plentiful.
Ammonium Sulfate
The special advantage of this dry form nitrogen
fertilizer is that it will not volatalize as a gas when surface-applied on
almost all Indiana soils, the exception being calcareous (high lime) soils
with pH 7.5 or higher. Therefore, ammonium sulfate makes an excellent
topdressing material for wheat and pastures. In addition, it will serve as a
fall plowdown fertilizer for corn if applied after soil temperature at the
4-inch depth is 50 or less. It is also a source of sulfur, an essential
plant nutrient.
One disadvantage of ammonium sulfate is that it is the
most acidifying of the nitrogen fertilizers. Thus, periodic soil tests are
necessary to monitor pH level of the soil.
Diammonium Phosphate
Dry diammonium phosphate (18-46-0) is used primarily in
bulk-blended fertilizers, but can be applied alone as a plowdown, sidedress
or topdress whenever nitrogen, phosphorus or both are needed. It ranks
second only to anhydrous ammonia as a source of nitrogen for crop
production.
Urea
As discussed earlier, urea N passes through both ammonia
and ammonium forms before it is used by plants. As ammonia, it is in a
gaseous state and, thus, can escape into the air. For this reason, urea is
not recommended for topdressing pasture during the summer, but can be
applied in late winter or early spring as a topdressing for either pasture
or wheat.
If urea fertilizer is surface-applied at temperatures
above 50 degrees, it should be incorporated into the soil immediately with
chisel, disc or plow. If used as a fall plowdown ahead of corn, apply only
after soil temperatures at 4 inches deep drop to 50 degrees.
Anhydrous Ammonia
Anhydrous ammonia (a liquid under pressure) is an
excellent fall plowdown fertilizer for corn, if applied after soil
temperatures at the 4-inch depth are 50 degrees or less. Caution is advised,
however, if anhydrous is to be used in a spring pre-plant program, since
ammonia can injure germinating corn seed. Ordinarily, the ammonia will be
converted to the non-volatile ammonium form within 3 or 4 days. But this
conversion process will be slowed if either the soil is too dry or the
application rates are too high.
Do not apply anhydrous on heavy-textured soils (clay
loams, silty clays or clays) when they are wet. In the first place, it's
difficult to get a good "seal" behind the application knives, thus allowing
ammonia to escape; and secondly, running application equipment over such
fields when wet may destroy soil structure, making it more compact.
Aqua Ammonia
Sometimes water is added to anhydrous ammonia to reduce
the pressure needed to keep it in a liquid state and, in some respects, make
it easier to handle. The resulting material is called aqua ammonia. It
contains a certain amount of unattached or free ammonia and, therefore,
should be applied on the same basis as anhydrous. Aqua ammonia is not
suitable for surface application at any time.
Nitrogen Solutions
37-41% N Materials (Low-Pressure). Use of these
"low-pressure" nitrogen materials (consisting of various combinations of
ammonium nitrate-urea-ammonia-water) has been on the decline since the
mid-1960's. One reason is the limited conditions under which they can be
applied.
For instance, low-pressure N solutions are not
recommended for fall plowdown ahead of corn, because some of the nitrogen is
already in leachable nitrate form. Neither should they be surface-applied in
spring, but rather injected into the soil to prevent loss of the nitrogen
which is in the gaseous ammonia form. They are satisfactory as sidedress
fertilizers for corn, except on extremely sandy soils.
28-32% N Materials (Non-Pressure). Various
urea-ammonium nitrate-water (or UAN) mixtures are classified as
"non-pressure" nitrogen materials and commonly make up the "feed" portion of
a weed-and- feed program. They are also suitable for sidedressing of corn
and early spring topdressing of grasses and small grains, except on
calcareous soils of pH 7.5 and above.
Like the "low-pressure materials, UAN solutions contain
some nitrate N and, therefore, are not recommended for fall plowdown ahead
of corn or early pre-plant on low organic sands. Neither should they be
summer-applied to grass pasture because of excessive N volatilization when
the urea portion breaks down to ammonia at high temperatures.
Other Nitrogen Materials
Other nitrogen fertilizer materials include potassium
nitrate, slow-release urea-formaldehyde and organic nitrogen. These have
specific uses in greenhouses, lawn, turf or similar specialized programs and
are considered too expensive for agronomic crops.
During times of short nitrogen supplies, certain
industrial by-products containing nitrogen (usually ammonia) may appear on
the market. Information about such products can be obtained either from the
Office of the Indiana State Chemist and Seed Commissioner (ph. 317-494-1492)
or from the Purdue University Agronomy Department (ph. 317-494-4772).
RR 4/86
Cooperative Extension work in Agriculture and Home
Economics, State of Indiana, Purdue University and U.S. Department of
Agriculture cooperating: H.A. Wadsworth, Director, West Lafayette, IN.
Issued in furtherance of the acts of May 8 and June 30, 1914. The
Cooperative Extension Service of Purdue University is an equal
opportunity/equal access institution.
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