Nitrogenous Fertilizer

If urea is surface applied and not incorporated (either by rain or tillage), N losses to the air (as ammonia) can approach 40% of the applied N. In addition, a rapid pH increase after application caused by hydrolysis of urea can result in ammonia release that can damage seedlings if the urea is applied too close to the seed. Conversion of ammonium to nitrate results in the formation of hydrogen ions (H+), so, like most N fertilizers, repeated urea applications will cause a reduction in soil pH over time.

Ammonium sulfate is well-suited as a top dress application as it has a lower N volatilization risk than surface-applied urea. Also, where S is needed ,ammonium sulfate is a good source of S.  The drawbacks to using ammonium sulfate include a relatively high salt index and greater acidification potential per unit N applied than other ammonium-containing N sources, higher cost per lb of N, and relative low N content, requiring more frequent refilling of hoppers. 

 Anhydrous ammonia must be injected 6 to 8 inches deep into moist and friable soil to limit ammonia loss (liquid ammonia converts to gas when no longer under pressure). It must be stored under high pressure, which requires specially designed, well-maintained equipment and facilities should be well-protected for safety reasons. During  application, personal protective equipment  (gloves and goggles) should be used.

Urea Ammonium Nitrate

         Urea ammonium nitrate (UAN) is a soluble, readily available N source with 28-32% Nprepared by mixing of ammonium nitrate and urea. It is primarily used as a non-pressurized liquid fertilizer and is for many the preferred source of N for sidedressing of row crops. UAN can be broadcast or placed in the starter band. If broadcast, UAN should be incorporated into the soil as the urea portion is subject to volatilization. However, because of its lower % of N in urea and ammonium form, volatilization losses per pound of N from UAN will be lower than for urea. Banding with drop nozzles has been found to minimize volatilization losses. The benefits of this product are its uniformity, ease of storage, handling and application. Like urea, UAN will lower the pH because of conversion of ammonium to nitrate and subsequent release of H+. 

Potassium Nitrate

         Potassium nitrate, also  known as saltpeter or nitric acid, is considered a specialty fertilizer. It is a colorless transparent crystal or white powder with 14% N and 46% potassium (K). Potassium nitrate does not lower the soil pH.

Mono-Ammonium Phosphate

         Mono-ammonium phosphate (MAP) contains readily available sources of N (11%), P (52%) and S (1.5%). MAP is a dry granular material that is applied alone  or often blended with other materials such as potash. It can be broadcast, band-applied or placed in the seed furrow. MAP can lower the soil pH but is an excellent starter fertilizer. 

Di-Ammonium Phosphate

         Di-ammonium phosphate (DAP) is dry fertilizerthat contains readily available sources of N (18%) and P (46%). Formation of free ammonia produced after mixing of DAP with soil can cause seedling injury as described for urea. To prevent such injury using DAP, it is recommended to limit band-applications to (1) 65 lbs per acre of DAP, or (2) 30 pounds of urea N plus N from DAP. 

Calcium ammonium nitrate or CAN

         Calcium ammonium nitrate or CAN, also known as nitro-limestone, is a widely used inorganic fertilizer, accounting for 4% of all nitrogen fertilizer used worldwide in 2007.

         The term "calcium ammonium nitrate" is applied to multiple different, but closely related formulations. One variety of calcium ammonium nitrate is made by adding powdered limestone to ammonium nitrate; another, fully water-soluble version, is a mixture of calcium nitrate and ammonium nitrate, which crystallizes as a hydrated double salt: 5Ca(NO3)2•NH4NO3•10H2O. Calcium ammonium nitrate is hygroscopic. Its dissolution in water is endothermic, leading to its use in some instant cold packs.

         Most calcium ammonium nitrate is used as a chemical fertilizer. Fertilizer grade CAN contains roughly 8% calcium and 21-27% nitrogen.  CAN is preferred for use on acid soils,  as it acidifies soil less than many common nitrogen fertilizers. It is also used in place of ammonium nitrate where ammonium nitrate is banned.

Commercially available N fertilizers

Name                                                     Chemical Formula                                            % N

Anhydrous Ammonia                                    NH3                                                                                                         82

 Aqua Ammonia                                            NH4OH,                                                         20

Ammonium Nitrate                                       NH4NO3                                                                                              35

Ammonium chloride                                      NH4Cl                                                           25

Ammonium sulphate                                   (NH4)2SO4                                                                                          20.6

Ammonium sulphate nitrate                       (NH4)2SO4.NH4NO3                                                                      26

Diammonium phosphate                              (NH4)2HPO4                                                                                    18

Potassium nitrate                                           KNO3                                                                                                16

Sodium nitrate                                             NaNO3                                                                                                16

Calcium Nitrate                                           CaNO3                                                                                             15.5

Urea                                                               CO(NH2)2                                                                                       46

Ammonium Nitrate solution (sol)              NH4NO3 + H2O                                                 20

Calcium Ammonium Nitrate                     CaCO3.NH4 NO3                                                                                25

Nitrogen Transformations

Nitrogen Mineralization

         Once nitrogen is fixed, it is subject to several chemical reactions which can convert it to different organic or inorganic forms. Mineralization occurs in soil as microorganisms convert organic nitrogen to inorganic forms. The first step of mineralization is called aminization, in which microorganisms (primarily heterotrophs) break down complex proteins to simpler amino acids, amides, and amines. (Heterotrophic microorganisms require preformed organic compounds as sources of carbon and energy. Autotrophic microorganisms can derive energy from the oxidation of inorganic elements or compounds such as iron (Fe), sulfur (S), ammonium, nitrite, or from radiant energy.) They derive their carbon from carbon dioxide (CO2).

Aminization: In this step protein (an organic form of nitrogen) break down to yield amines, amino acids, carbon dioxides and other reaction products.

         Proteins ——> R----- NH2 +CO2  + Energy + other reaction products

Ammonification is the second step of mineralization in which amino (NH2) groups are converted to ammonium. Again, microorganisms (primarily autotrophic) accomplish this action.

R-NH2 + H2O —> NH3 + R-OH + energy

         This conversion is caused by another group of heterotrophic soil micro organisms such as bacteria, fungi and actinomycetes.  Ammonia, NH+3, so released converted into ammonium ions NH4+.

Reaction 1

NH3 + H2O ------- NH4 OH

NH+OH ---------- NH4+ + OH-

         In this reaction, ammonia, NH3, reacts with water H2O, to form ammonium hydroxide. Being unstable ammonium hydroxide NH4OH, breaks apart to yield ammonium ions NH4+ and hydroxyl ions OH-.

Reaction 2                                                                    

2NH3 + H2CO3 = (NH4)2CO3           2NH4+ + CO3-

                              NH3 + HNO3 = NH4NO3    NH4+ + NO3-

         In this reaction ammonia, NH3, react with carbonic acid, H2CO3 or nitric acid HNO3, formed in soil to yield ammonium carbonate NH4CO3 or Ammonium nitrate NH4NO3 which in turn break apart to yield ammonium NH4+ and carbonate ions  CO3- or ammonium NH4+ and nitrate NO3- ions.

Nitrification

         Microbial activity is also responsible for the two steps of nitrification. Nitrosomonas (obligate autotrophic bacteria) convert ammonium to nitrite. Nitrification inhibitors, such as nitrapyrin (N-ServeR) or dicyandiamide (DCD), interfere with the function of these bacteria, blocking ammonium conversion to leachable nitrate. The second step of nitrification occurs through Nitrobacter species, which convert nitrite to nitrate. This step rapidly follows ammonium conversion to nitrite, and consequently nitrite concentrations are normally low in soils.

                          Nitrosomonas              Nitrobacter
Organic Nitrogen ——————> Nitrite ——————> Nitrate

2NH4+ + 3O2 —> 2NO2– + 2H2O + 4H+ + 2NO2- + O2 —>2NO3

         Mineralization and nitrification are influenced by environmental factors that affect biological activity such as temperature, moisture, aeration, pH, and so forth. Nitrification, for example, occurs very slowly at cold temperatures and ceases once the temperature declines below freezing. The rate increases with increasing temperature until bacterial viability is reduced (around 95oF to 100oF), and then nitrification begins to decline as the temperature increases. Moisture and oxygen are necessary for microbial function in both the mineralization and nitrification processes. Excessive moisture limits oxygen availability, reducing mineralization and nitrification rates, which, perhaps, leads to anaerobic soil conditions. Rates of mineralization and nitrification proceed most rapidly at pH levels near 7, and decline as soils become excessively acid or alkaline.

Immobilization

         Immobilization, or the temporary tying up of inorganic nitrogen by soil microorganisms decomposing plant residues, is not strictly a loss process. Immobilized nitrogen will be unavailable to plants for a time, but will eventually become available as residue decomposition proceeds and populations of microorganisms decline. Fertilizer nitrogen immobilization can be reduced by placing fertilizers below crop residues instead of incorporating fertilizer into the soil with residue. The producer can accomplish this most directly by knifing in anhydrous ammonia or solutions.

            The duration of the nitrate depression period during immobilization depends on environmental factors such as moisture and temperature and the carbon-to-nitrogen (C:N) ratio of the residue. Soil organic matter contains an average of about 50 percent carbon and 5 percent nitrogen. This ratio (10:1) is relatively constant for organic matter. The C:N ratio of plant residue ranges from 10:1 for young leguminous plant tissue to as high as 200:1 for straw of some grains. Plant tissues low in nitrogen generally are more resistant to decomposition and require a longer time before the nitrogen is available to plants.

         When a plant residue with a wide C:N ratio is incorporated into the soil, microbial decomposition starts. Microorganism populations increase greatly, evidenced by increased release of CO2 leaving the soil through respiration. The microorganisms take nitrogen from the soil for proteins. Consequently, for a time the concentration of inorganic nitrogen in the soil declines, and may be deficient for plant growth. As residue decomposes, the C:N ratio narrows. At a ratio of approximately 17:1, nitrogen becomes available for plant use. Decomposition continues until the ratio is approximately 11:1 or 10:1.