Farm Scene
Volume 15 Issue 1
January 2015
Texas A&M AgriLife Extension Service—Lipscomb County
PO Box 119
Lipscomb, TX 79056-0119
Office: 806-862-4601
Fax: 806-862-3004
Email: [email protected]
Web site: http//lipscomb-co.tamu.edu
Soils - Characteristics of Fertilizer Materials
The various characteristics of fertilizer materials being sold on the market today will be discussed. You will learn to identify
some of the consequences of using each type of fertilizer material and how that material was developed and manufactured.
[This lesson, as well as the other nine lessons in the Soils series, is taken from the "Soils Home Study Course," published in 1999
by the University of Nebraska Cooperative Extension.]
Nitrogen Fertilizers
Anhydrous ammonia (82-0-0) is a gaseous material that is compressed and stored as a liquid. At 60oF, a gallon of
anhydrous ammonia weighs 5.15 pounds. It is manufactured by reacting nitrogen from the air with hydrogen
(usually from natural gas) in the presence of a catalyst at high temperature and high pressure.
Anhydrous ammonia is the source of nitrogen for other commercial fertilizers. It is also used for direct application.
Since it is a compressed gas, it must be injected in the soil to prevent loss from vaporization. When soil conditions
are favorable for injection and closure of the injection channel, ammonia is an effective source of nitrogen for
crops. Because anhydrous ammonia needs to go through the nitrification process (see Soils - Part 5), it is more resistant to losses from the soil by leaching or denitrification because it is converted by bacterial action to the nitrate
form more slowly than other nitrogen sources. N-Serve is an anti-bacterial agent which slows the conversion of
NH3 to the nitrate form.
When anhydrous ammonia is injected in the soil, the injection sites will have a high pH and a very high ammonium
concentration. This is lethal to most microorganisms in the band. It also provides a rich protein source for microbes
on the fringes of the band, which enhances their reproduction. Research studies show that recolonization is complete within a few months after application.
Aqua ammonia (20-0-0) is anhydrous ammonia dissolved in water. It is a low-pressure solution and contains free
ammonia. The amount of free ammonia increases as air temperatures increase. It is stored in closed low-pressure
tanks and is injected into the soil much like anhydrous ammonia. Since aqua ammonia is a low-pressure solution
and contains only a small amount of free ammonia, direct soil application does not need to be as deep as with anhydrous ammonia. Aqua ammonia is not commonly used because of the cost of handling the water in the product.
Ammonium nitrate (34-0-0) is a dry granular material manufactured by reacting nitric acid with anhydrous ammonia. The reacted material is concentrated, prilled and coated to prevent caking. In ammonium nitrate, one-half of
the nitrogen is in the nitrate form, and the other half is in the ammonium form.
Ammonium nitrate is the preferred form of nitrogen fertilizer when it is to be applied on the soil surface and not
incorporated. It does not volatilize as free ammonia, except on high pH soils. Ammonium nitrate use has declined
in recent years and is being replaced by urea, which is less expensive and easier to store and maintain. Under the
right conditions, ammonium nitrate can become explosive.
Educational programs of the Texas A&M AgriLife Extension Service are open to all people without regard to race, color, religion, sex, national origin, age, disability,
genetic information or veteran status.
The Texas A&M University System, U.S. Department of Agriculture, and the County Commissioners Courts of Texas Cooperating
Page 2
Farm Scene
Ammonium sulfate (21-0-0 + 24S) is a dry crystalline material produced by reacting anhydrous ammonia with sulfuric acid. It contains 21 percent nitrogen and 24 percent sulfur, which makes ammonium sulfate an excellent source
when both nutrients are needed. Ammonium sulfate has excellent storage properties. The low nitrogen content,
when compared to other sources, the crystalline form, and relative cost limit its use in Nebraska. While all nitrogen
fertilizers have an acidifying effect, ammonium sulfate is more acidifying than other nitrogen sources, which limits
its use in areas where soils require liming. Its most common use would be in sandy soil areas where sulfur is also
needed.
Ammonium nitrate-sulfate (30-0-0 + 6.5S) is a dry nitrogen fertilizer relatively new to the United States. It is manufactured by reacting anhydrous ammonia with a mixture of nitric acid and sulfuric acid. The nitrogen content of the
product will vary depending on the proportion of nitric to sulfuric acid. A common grade is 30-0-0 that contains 6.5
percent sulfur. Ammonium nitrate-sulfate has good storage and handling properties. It is very satisfactory for direct
application, use in blended fertilizers, and is a good replacement for ammonium nitrate.
Urea (46-0-0) is a dry nitrogen material produced by reacting ammonia with carbon dioxide. Of the commonly used
dry fertilizers, urea contains the highest percentage of nitrogen and is rapidly replacing ammonium nitrate. When
surface applied, urea is the most readily volatilized of the dry nitrogen materials. After application to the soil, urea
will hydrolyze (combine with water) to form ammonium carbonate. This reaction is driven by the enzyme urease,
which is present in the soil and on crop residues. Ammonium carbonate is very unstable and decomposes into water, ammonia, and carbon dioxide. Thus, nitrogen is lost as ammonia.
This risk of loss can be minimized by incorporating urea into the soil with tillage, injection, or with one-half inch or
more of water from rainfall or pivot irrigation. A new urease inhibitor called Agrotain® retards the hydrolysis of urea
for about two weeks under most field conditions. Agrotain® also can be used with UAN solutions. Careful consideration should be given to the potential loss of urea applied to the soil surface without this protection.
Urea-ammonium nitrate (UAN) is a non-pressure solution of ammonium nitrate, urea, and water. Two grades are
most common: 28-0-0 (10.7 pounds/gal) and 32-0-0 (11.1 pounds/gal). The lower analysis material contains more
water and can be stored at lower temperatures. Salt crystals will form at about 0oF for 28 percent solution and at
about 32oF for the 32 percent solution.
Individual solutions can be made from urea or ammonium nitrate; however, higher analysis solutions are possible
when urea and ammonium nitrate are combined. Commonly available nitrogen solutions will contain about one-half
of the nitrogen from urea and the other half from ammonium nitrate.
UAN solutions have an advantage in terms of handling. They can be pumped, mixed with chemicals and sprayed.
They are corrosive and will quickly destroy brass, bronze and zinc fittings. Carbon steel and cast iron are also seriously corroded. UAN doesn’t corrode aluminum alloys, stainless steel, rubber, neoprene, polyethylene, vinyl resins, and
glass.
The performance characteristics of urea-ammonium nitrate solutions are the same as for the two main ingredients,
urea and ammonium nitrate. The ammonium nitrate portion will perform like ammonium nitrate, and the urea portion like urea. The urease enzyme is present on crop residues and on the soil surface, so the urea portion of this fertilizer is subject to loss unless incorporated by tillage or rain.
Plant & Soil Sciences eLibrary Pro
https://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447045
Volume 15 Issue 1
Page 3
Types and Uses of Nitrogen Fertilizers for Crop Production
Types and Uses of Nitrogen Fertilizers for Crop Production
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
Farm Scene
Page 4
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 . 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.
Table 1. Characteristics and Adaptation of Nitrogen Fertilizers Commonly Used for Crop Production
Adaptation for
Percent
nitrogen
Form of
nitrogen in
fertilizer
Ammonium nitrate
33.5%
Ammonium sulfate
Fall
plow-down
for corn
Spring
pre-plant
Side-dressing Top-dressing small
corn
grains and grasses
½ ammonium
½ nitrate
Unadapted
Good*
Excellent
Excellent*
20.5%
Ammonium
Excellent
Excellent*
Excellent
Good*
Calcium nitrate
15.5%
Nitrate
Unadapted
Good*
Excellent
Excellent*
Cal-nitro (ammonium
nitrate + limestone
26%
½ ammonium
½ nitrate
Unadapted
Good*
Excellent
Excellent*
Diammonium phosphate
18%
Ammonium
Excellent
Excellent
Excellent
Urea
45%
Ammonium—
forming
Excellent
Excellent*
Excellent
Good—winter
Poor—summer
82%
Ammonium forming
Excellent
Good*
Excellent
Unadapted
20-24.6%
Ammoniumforming
Excellent
Good*
Excellent
Unadapted
37-41%
⅔ ammonia2
¼- ⅓ nitrate
Poor
Good*
Excellent
Unadapted
Fertilizer Material
Dry Solid Forms
Excellent
Liquid Forms
Anhydrous ammonia1
(liquid under pressure)
1
Aqua ammonia
(anhydrous ammonia +
water
Low-pressure N solutions1
(ammonium nitrate-ureaammonia-water
Chart continued on next page
Volume 15 Issue 1
Page 5
Adaptation for
Fertilizer Material
Percent
nitrogen
Form of
nitrogen in
fertilizer
Fall
plow-down
for corn
28—32%
¼ nitrate2
¾ ammonium Poor
Spring
pre-plant
Side-dressing Top-dressing small
corn
grains and grasses
Excellent
Excellent
cont. - Liquid Forms
Non-pressure N solution
(urea-ammonium nitratewater or UAN)

This asterisk means that, if the fertilizer is used for the purpose
indicated at the top of the column, certain limitations or caution are
involved. These are spelled out in the section discussing that
fertilizer.
1
Must be injected into the ground when applied to avoid N loss to the
air as gas.
2
Approximate proportions.
Excellent-spring
Poor-summer
Table 2. Physical Properties of Liquid Nitrogen Fertilizers.
Material
Percent
nitrogen
Weight
per gallon
at 60F
Pounds of
nitrogen
per gallon
Pounds of
pressure per
square inch
at 104F
Approximate
salting-out
temperature
Anhydrous ammonia
82.2%
5.15 lb.
4.23 lb.
211 lb.
No salt-out
Aqua ammonia
20.6%
7.60 lb.
1.56 lb.
2 lb.
No salt-out
Ammonium nitrate, urea combinations
28.0%
30.0%
32.0%
10.70 lb.
10.85 lb.
11.05 lb.
3.00 lb.
3.27 lb.
3.55 lb.
Ammonia, ammonium nitrate, urea combinations
37.0%
41.0%
9.87 lb.
9.50 lb.
3.66 lb.
3.90 lb.
- 1F
15F
32F
2 lb.
10 lb.
36F
44F
Application Suggestions
1. Three of the four liquid nitrogen fertilizers - anhydrous ammonia, aqua ammonia and low-pressure 37-41% N
solutions - must be injected into the ground to avoid loss of ammonia (gaseous) nitrogen to the air The dry or
solid fertilizers plus liquid non-pressure 28-32% N, on the other hand, can be surface-applied. On sloping
cropland, however, they too should be incorporated into the soil to prevent loss from surface runoff.
2. Ammonium sulfate, diammonium phosphate, anhydrous ammonia, aqua ammonia and urea are all suitable
for fall application ahead of corn, except on poorly-drained or excessively-drained soils. Application should not
be made until soil temperatures at the 4-inch depth have dropped to at least 50F.
3. Ammonium and ammonium-forming fertilizer materials, over time, will cause soil to become more acid (lower
the pH). Where these fertilizers are regularly used, soil samples should be taken periodically to determine when
limestone is needed
Page 6
Farm Scene
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.
Volume 15 Issue 1
Page 7
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.
Farm Scene
Page 8
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.
Adapted: https://www.extension.purdue.edu/extmedia/AY/AY-204.html, David B. Mengel, Agronomy Department.
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