Topic 7



A. Introduction

- Nitrogen is usually the nutrient most limiting plant growth.

- It is important in increasing the protein content of plants.


N Applied (lb/acre)
Percent Protein
Coastal Bermuda
Corn Grain

- adequate N increases chlorophyll producing a dark green color.

(important in turfgrass quality ratings)

- Excess N may be detrimental causing excess vegetative growth and lodging. (lowers tobacco qualtiy).

- The ultimate source of nitrogen used by plants is the inert gas N2 which constitutes 78% of the earth's atmosphere. In this form it is unavailable to plants and must be converted to usable forms. Plants take up nitrogen in the nitrate or ammonium forms.

The Nitrogen cycle describes the processes by which Nitrogen circulates between the atmosphere, biosphere, soil and water.

For more information about the nitrogen cycle refer to Fundamentals of Physical Geography

B. Nitrogen Fixation

1. Biological

a. symbiotic N fixation- microorganisms growing in association with plants. both the host and the microorganism benefits. Ex. legumes, soybeans, peanuts, clover.

As much as 100 to 150 pounds per acre

b. non-symbiotic N fixation- free living bacteria and blue-green algae in soil.

20 lbs. per acre

2. Physical

a. natural oxidation - heat of lightning may combine N2 and O2. Brought to earth by rain or snow.

5-10 lbs per acre per year

b. Industrial - fertilizer manufacturing

N2 + 3H+ ==> 2NH3

Nitrogen from the air

Hydrogen from natural gas - cost of natural gas affects price of fertilizer

C. Soil N

1. inorganic - NO3-, NH4+

2. organic - unavailable to plants : 97-98% of total N in soils is in the organic form. 1-2% per year is mineralized.

- wheat straw is an example of a high C/N ratio material that can cause immobilization of N whenn incorporated into the soil.

mineralization occurs in three steps - aminization, ammonification and nitrification

In well aerated, warm soils with favorable pH, NH4+ is rapidly converted to nitrate by microorganisms (nitrification)

Important factors to remember about nitrification

1. Nitrification requires oxygen, so it occurs most readily in well areated soils.

2. The reaction releases H+ ions resulting in acidification.

3. Microorganisms are involved so the transformation is affected by the soil environment.

ammonification does not require oxygen and is accomplished by a diverse group of heterotrophic bacteria, fungi and actinomycetes.

Mineralization of Organic N (Summary)



D. Reactions of inorganic N in soil

1. Ammonium - NH4+ - is a cation that is adsorbed by the exchange complex in soil (O.M. and Clay). It is resistant to leaching as long as it remains in the NH4+ form. However under favorable conditions it is converted to nitrate.

2. Nitrate - NO3- - is a anion so it is not adsorbed and moves with the soil water; can be leached causing losses and pollution problems -- ex. Algal Blooms (Currently an issue of heated debates in NC)

3. Factors that affect the conversion of NH4+ to NO3

nitrification is a Biochemical Reaction

a. Soil temperature - 34 degrees F for bacteria to be active; 86 degrees F, 1 week conversion of NH4+ to NO3 - at 68 degrees F, 2 weeks for 80% of NH4+ to convert to nitrate.

b. soil pH - Under acid conditions nitrification is slow. 5.8 - 6.0 is optimum

c. soil aeration - O2 is needed for the reaction. Waterlogged soils contain ammonium and very little nitrate.

d. soil moisture - does not occur under saturated or very dry conditions.

e. concentration of NH4+ and presence of nitrifying organisms. Production of NH4+ can be inhibited. High C/N ratio.

f. nitrifying organisms may be inhibited by nitrapyrin. 0.5 - 1 lb / acre incorporated. Lasts 5-6 weeks.

4. Losses of nitrogen

a. Leaching of NO3- -- Important on sandy soils and high rainfall of the southeast.

b. Denitrification - Loss of N under waterlogged conditions ( no O2)

NO3- => NO2- => N2

caused by microorganisms under anaerobic conditions

c. Volatilization - Gaseous loss of N on soil surface or form sand soils.

1. Urea

a. incorporating the urea

b. applying when temperatures are low

c. irrigating immediately to carry urea into soil.

2. NH4+ fertilizers surface applied on alkaline soils. pH > 7 not a problem in N.C.

3. Anhydrous Ammonia (NH3)

applied as a gas

a. can be lost if soil is very dry

b. can be lost if CEC is low

c. deep placement can help reduce loss

d. Crop removal 100 bu corn grain 90# ; stalks 60#'s ; coastal Bermuda 8 tons 180#'s.

E. Factors to consider when applying N fertilizer

1. N requirements of the crop (form of N.) High for corn; low for tobacco, none for soybeans.

2. amount of N that can be supplied by the soil.

3. N supplied by biological fixation

4. Efficiency of fertilizer N uptake

Usually only 50-60% of applied N is taken up by the plants (leaching, volatilization, denitrification).

5. Time of application (when should it be applied)

6. Soil texture and depth to B horizon less leaching in fine textured soils and soils with B close to surface. CEC and water holding capacity.

7. Rainfall and Transpiration. If rainfall exceeds evaporation transpiration, leaching will occur; replacement N may be necessary. Split applications.

F. Nitrogen Fertilizers

Fertilizer , Fertilizer 101

Prices , Fertilizer use and Price , DAP Price

Turfgrass Fertilization

1. Natural organic materials

a. Guano - once the sole supply of N fertilizer in the U.S. bat and bird dung.

nutrient content is variable Bat Sea Bird

The Chincha Islands in Peru became the number one place for Guano mining.

Bat Caves in Texas.

Guano Island Act of 1856

b. animal byproduct, manure, plant residues, sewage sludge used in specialty fertilizersfor lawns and shrubs

c. most of N released is converted to N in three weeks.

Less than a half percent of fertilizer sold in the U.S. today is organic.

Dairy 9.7 5.2 8.1
Beef 12.3 7.2 8.7
Swine 11.5 8.0 8.0
Laying Hens 27.1 22.0 11.6
Horse 12.2 6.4 10.8
Sheep 20.7 9.3 19.6

2. Natural inorganic N

a. Nitrate of soda (NaNO3)

1. mined and refined in Chile

ore is called caliche

Chilean Nitrate of Soda, is a highly soluble, quick acting granular fertilizer that is 16 percent nitrogen. Chilean nitrate is mined from a desert in northern Chile, most likely the only known deposit of this mineral salt. It was the preferred source of N for sidedressing tobacco for many years, and was known as Bull Dog Soda.(Bag)

SQM mines sodium nitrate and other minerals in Chile.


2. nitrate of soda contains 16% NO3-N

3. was once the sole supply of inorganic chemical fertilizer N in U.S.

4. used mainly for topdressing row crops in Southeastern U.S.

b. NH3 from coal (Europe)

3. Synthetic chemical fertilizers.

Nitrogen Fertilizer Production

The Haber Bosch process

Natural Gas Prices

natural gas vs ammonia


A Minnesota plant will use wind power to produce ammonia

Anhydrous NH3 is a starting point for making other N fertilizers. Natural gas is the H+ source. N comes from the atmosphere.

a. Anhydrous ammonia

Used as an applied fertilizer or as a building block for other fertilizer products.

  • 82% N; stored as a liquid under pressure or refrigerated; becomes a gas when released and exposed to air.
  • Applied directly to soil by injection


  • Some is lost to atmosphere during application.
  • To minimize loss apply to a moist soil 6 to 7" deep.
  • Need special equipment
  • safety concerns
  • Illegal drugs , propane tanks

b. Urea [CO(NH2)2]

A solid nitrogen product typically applied in granular or prilled form. It can be combined with ammonium nitrate and dissolved in water to make liquid nitrogen fertilizer known as urea ammonium nitrate or UAN solution.

Prill- to form pellet-sized crystals or agglomerates of material by the action of upward-blowing air on falling hot solution; used in the manufacture of ammonium nitrate and urea fertilizers.

Granules are particles produced by deposition of sucessive layers on smaller particle from a liquid or slurry.

When applied to soil, urea-N hydrolyzes to ammonium N. Within 1 day after application, about 66% of urea-N is hydrolyzed, and within 1 week all is hydrolyzed.. When not incorporated, significant N loss by volatilization can occur.

  • 46% N
  • rapid hydrolysis to NH4+
  • incorporate to pervent volatilization
  • biuret is an impurity that can reduce growth if it is present in quantities more than 2%. Usually a problem only in foliar feeding.

c. Ammonium Nitrate [NH4NO3]

A solid nitrogen product typically applied in granular form is valued for its use on pasture lands and specialty crops such as citrus.

  • 33.5 -34.5 % N
  • absorbs moisture (hygroscopic)
  • granulated to reduce caking
  • Safety considerations Explosive properties

d. Nitrogen Solutions (non-pressure)

UAN solutions

Typically, a combination of urea and ammonium nitrate dissolved in water to form a highly soluble liquid fertilizer, typically containing 28 or 32 percent nitrogen.

  • NH4NO3 and or urea
  • urea and NH4NO3 dissolved in water called UAN/solutions

e. Aqua ammonia - low pressure liquid must be injected 2-3" deep. Ammonia gas dissolved in water (21% N)

All N solutions are classed as pressure or non-pressure solutions.

f. ammonium sulfate 20.5% N

Made by reacting ammonia with sulfuric acid

It is also a byproduct of coke ovens, where sulfuric acid is used to remove ammonia evolved from the coal.

NH3 + H2SO4 =====> (NH4)2SO4

good handling and storage

g. ammonium phosphates - used primarily as a source of P

NH3 + H3PO4 =====> NH4H2PO4 .............11-48-0

1. Monoammonium phosphate

a.k. as MAP ..... 11-48-0

NH3 + H3PO4 =====> NH4H2PO4

2. Diammonium phosphate a.k as DAP .... 18-46-0


h. potassium nitrate .... 13-0-44

6KCl + 12 HNO3 ==> 6KNO3 + 3Cl2 + 6NO2 + 6H20

used mostly as a sidedressing

i. calcium nitrate

16% N

j. Slow release fertilizer materials (2)[more expensive] used for turf and shrubs, potted plants.

1. sulfur coated urea 37% N 17% S, poly plus coated urea

2. Urea formaldehyde 38% N

3. IBDU - isobutylidene diurea 31%

4- OSMOCOTE- semipermeable membrane NH4NO3 usual source of N. History

5. Mag AMP 7-40-6 -- Particle size determines solubility.

6. Natural organic 1.5 - 8.5% N

high cost -- no problem with burning -- long period of release.

G. Equivalent Acidity and Basicity of N Fertilizers

2NH4 + 4O2 ====> 2NO2- + 4H+ +2H20

Material Ca/CO3 Needed to Neutralize Acidity From 100 lbs. N
Anhydrous NH3

 Tillage Radish ?

H. Characteristics of N Source

1. Slow Release N Sources

a. insoluble or slowly soluble in water

b. gradual supply of N - Plants show slow initial response

c. little danger of foliar burn

d. can be applied at higher rates allowing less frequent applications

f. produces more uniform growth response

2. Quick Release

a. soluble in water

b. can be used immediately by the plant. Rapid initial response

c. high potential for foliar burn

d. applications at low rates at frequent intervals

e. leach more rapidly


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