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.


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.


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.

Table 1. Characteristics and Adaptation of Nitrogen Fertilizers Commonly Used for Indiana Crop Production

                                                                 Adaptation for
                                           Form of        Fall                     Side-      Top-dressing
                                 Percent   nitrogen in   plow-down   Spring        dressing   small grains
Fertilizer material              nitrogen  fertilizer    for corn    pre-plant      corn      and grasses
Dry Solid Forms

Ammonium nitrate                 33.5%    1/2 ammonium   Unadapted   Good*         Excellent  Excellent*
                                          1/2 nitrate

Ammonium sulfate                 20.5%    Ammonium       Excellent   Excellent*    Excellent  Good*

Calcium nitrate                  15.5%    Nitrate        Unadapted   Good*         Excellent  Excellent*

Cal-nitro (ammonium nitrate +     26%     1/2 ammonium   Unadapted   Good*         Excellent  Excellent*
  limestone)                              1/2 nitrate

Diammonium phosphate              18%     Ammonium       Excellent   Excellent     Excellent  Excellent

Urea                              45%     Ammonium-      Excellent   Excellent*    Excellent  Good-winter
                                           forming                                            Poor-summer
Liquid Forms

Anhydrous ammonia1 (liquid        82%     Ammonium-      Excellent   Good*         Excellent  Unadapted
  under pressure)                            forming

Aqua ammonia1 (anhydrous       2O-24.6%   Ammonium-      Excellent   Good*         Excellent  Unadapted
  ammonia + water                           forming

Low-pressure N solutions1       37-41%    2/3 ammonia2   Poor        Good*         Excellent  Unadapted
  (ammonium nitrate-urea-                 1/4 - 1/3 
  ammonia-water)			    nitrate

Non-pressure N solutions (urea- 28-32%    1/4 nitrate2   Poor        Excellent     Excellent  Exc-spring
  ammonium nitrate-water or               3/4 ammonium                                        Poor-summer
* 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

1 Must be injected into the ground when applied to avoid N loss to the
air as gas

2 Approximate proportions.

Table 2. Physical Properties of Liquid Nitrogen Fertilizers.

                                                                                Pounds of
                                                         Weight    Pounds of   pressure per   Approximate
                                             Percent   per gallon   nitrogen   square inch     salting-out
Material                                     nitrogen   at 6OF     per gallon   at104F        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%     10.70 lb.  3.00 lb.                    -1F
                                             30.0%     10.85 lb.  3.27 lb.                   15F
                                             32.0%.    11.05 lb.  3.55 lb.                   32F

Ammonia, ammonium nitrate, urea combinations 37.0%      9.87 lb.  3.66 lb.       2 lb.       36F
                                             41.0%      9.5O lb.  3.90 lb.      10 lb.       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.

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.


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.