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Ammonia Volatilization from
Urea: How large is the issue
and losses.
David E. Kissel
University of Georgia
Overview
• Urea Consumption
• Definitions
• UAN Solution Reactions
• Urea Dissolution and Diffusion
• Urea Hydrolysis
• Ammonia Volatilization
• Research Results
• Summary
Urea
Ammonium nitrate
Nitrogen solutions
Ammonium phosphate
Total compound N
0
1973 / 74
2004 / 05
10 20 30 40 50
www.fertilizer.org
51%
World Evolution of N Fertilizer
Consumption
Millions of tonnes
Urea
Ammonium nitrate
Nitrogen solutions
Total compound N
0
1973 / 74
2004 / 05
1 2 3 4 5
www.fertilizer.org
21%
Millions of tonnes
USA Evolution of N Fertilizer
Consumption
Anhydrous ammonia
Overview
• Urea Consumption
• Definitions
• UAN Solution Reactions
• Urea Dissolution and Diffusion
• Urea Hydrolysis
• Ammonia Volatilization
• Research Results
• Summary
Definitions• Dissolution: urea absorbs water
and converts from solid to liquid
• Hydrolysis: urea converts to ammonium (NH4
+)
• Diffusion: movement due to motion of molecules
– Urea has no charge, diffuses easily
– NH4+ has charge, diffuses slowly
Overview
• Urea Consumption
• Definitions
• UAN Solution Reactions
• Urea Dissolution and Diffusion
• Urea Hydrolysis
• Ammonia Volatilization
• Research Results
• Summary
Q. Does any ammonia volatilize
from UAN solutions before they are
applied or during the application?
A. None or very little, depending on
if a small amount of ammonia is
added to protect against corrosion of
mild steel.
Composition of UAN 28% N
• 14 % N from Ammonium Nitrate
– 7% ammonium N
– 7% nitrate N
• 14% N from Urea
• 30% water
• + small amounts of inhibitors to inhibit
corrosion of mild steel
Inhibitors to protect against
corrosion of mild steel
• 0.5 % ammonia (raises pH to about 7.5)
• Ammonium phosphates at 0.2 % P2O5
• Others
NH4 NH4 = NH3 + H+NH4 = NH3 + H+NH4 = NH3 + H+NH4
+ = NH3 + H+
(ion) (gas)
Urea Hydrolysis in UAN?
• DOES NOT OCCUR
– Therefore no ammonia is formed from the urea
portion of the UAN solution from this process.
– Therefore no ammonia loss from the urea
portion of UAN solution before soil
application.
Summary of ammonia loss from
UAN
• Urea in UAN does not hydrolyze in the fertilizer tank.
• The NH4+ from the ammonium nitrate
portion of the UAN cannot be lost as NH3.
• The amount of NH3 added to some UAN to inhibit corrosion is very small, around 10 lb per ton. A small portion of this NH3 may be lost during application.
Overview
• Urea Consumption
• Definitions
• UAN Solution Reactions
• Urea Dissolution and Diffusion
• Urea Hydrolysis
• Ammonia Volatilization
• Research Results
• Summary
Urea Dissolution
• Urea has to be dissolved before it can be hydrolyzed
Urea Dissolution
• Urea has to be dissolved before it can be hydrolyzed
• Urea can absorb water from the atmosphere and from the soil/crop residue
Urea Dissolution
• Urea has to be dissolved before it can be hydrolyzed
• Urea can absorb water from the atmosphere and from the soil/crop residue
• Absorption of water from the atmosphere depends on relative humidity (RH)
Urea Dissolution
• Urea has to be dissolved before it can be hydrolyzed
• Urea can absorb water from the atmosphere and from the soil/crop residue
• Absorption of water from the atmosphere depends on relative humidity (RH)
• Critical relative humidity (CRH) is the RH at which urea dissolves
Effect of Temperature on CRH
and Water Solubility of Urea
Air Temperature (oF)
20 40 60 80 100 120Critical R
ela
tive H
um
idity f
or
Ure
a (
%)
60
65
70
75
80
85
90
Ure
a S
olu
bili
ty (
lb/g
allo
n o
f w
ate
r)
4
6
8
10
12
14
16
CRH
Solubility
Urea Urea UreaUrea Urea Urea
RH > CRH (80%)
Crop residue
Urea dissolution
and diffusion
Dry Soil
www.agrium.com
Urea Urea Urea
Urea dissolution
and diffusion
Urea Urea UreaCrop residue
Urea Urea Urea
www.agrium.com
1/2 inch
Wet Soil
Urea Diffusion into Soil (after 10 days)Urea Concentration in Soil (ppm)
0 100 200 300 400 500 600 700
Soil
Depth
(m
m)
0
10
20
30
40
50
Drier
Wetter
Urea Urea UreaUrea Urea UreaCrop residue
Urea Urea Urea2 inches
Overview
• Urea Consumption
• Definitions
• UAN Solution Reactions
• Urea Dissolution and Diffusion
• Urea Hydrolysis
• Ammonia Volatilization
• Research Results
• Summary
Urea Hydrolysis
Urea 2 Urease enzyme
+ H+2 NH4+
• Urease is derived from crop residues
and soil microorganisms
Urea Urea UreaUrea Urea Urea
Crop residue
Urease inhibitor
Urea Urea Urea
Overview
• Urea Consumption
• Definitions
• UAN Solution Reactions
• Urea Dissolution and Diffusion
• Urea Hydrolysis
• Ammonia Volatilization
• Research Results
• Summary
(aq)
Soil solution
NH4+
NH3 Volatilization Process
Urea
Atmosphere
Clay
OM
NH4+
NH4+
NH4+
Hydrolysis
NH4+
(aq)+
Equilibrium
NH3 H+
(gas)
Soil air
NH3
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
+ 2 H+ 2
Soil pH
5 6 7 8 9
NH
3 (
as %
of
To
tal A
mm
on
iaca
l N
)
0
5
10
15
20
25
30
68oF
Effect of soil pH on NH3 as % of Total N
(aq)
Soil solution
NH4+
NH3 Volatilization Process
Urea
Atmosphere
Clay
OM
NH4+
NH4+
NH4+
Hydrolysis
NH4+
(aq)+
Equilibrium
NH3 H+
(gas)
Soil air
NH3
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
+ H+
Ferguson et al. (1984)
Days Since Urea Application
0 2 4 6 8 10 12
So
il p
H
6
7
8
9
NH
3 L
oss (
kg
N h
a-1
)
0
4
8
12
High Buffering Capacity
Low Buffering Capacity
Low Buffering Capacity
High Buffering Capacity
•Increase in soil pH
depends on soil H+
buffering capacity
•Higher clay and OM
contents lead to higher
soil H+ buffering
capacity
Atmosphere
H+
H+H+
H+
H+
H+
H+
NH4+
Urea
Urea
NH4+
NH4+
NH4+
NH4+
NH4+
NH4+
NH4+
Urea
½ inch
pH 6 7 8 9
NH4+NH4
+
NH4+NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
CECH+
Urea
Urea hydrolyzes on the soil surface
H+
H+
H+H+
H+
H+
H+
H+
NH4+
Urea Urea
Urea
NH4+
NH4+NH4
+
NH4+
Urea
2 inches
pH 6 7
NH4+NH4
+
NH4+
NH4+NH4+
NH4+NH4+NH4
+NH4+
NH3
Urea Urea
Urea
Urea
CEC
Urea hydrolyzes below the soil surface
Soil
+
Effect of Temperature on NH3 Volatilization
NH4+Urea NH3 H+
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
½ inch
Temperature
Effect of Temperature on Urea Hydrolysis
Temperature (oF)
30 40 50 60 70 80 90 100 110
Rela
tive U
rease A
ctivity
0
1
2
3
4
5
6
Gould et al. (1973)
Soil
+
Effect of Temperature on NH3 Volatilization
NH4+Urea NH3 H+
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
½ inch
Temperature
Temperature (oF)
30 40 50 60 70 80 90 100 110
NH
3 (
as %
of
Tota
l A
mm
onia
cal N
)
0
5
10
15
20
25
30
35
Effect of Temperature on % of NH3
pH = 8.5
Soil
+
Effect of Temperature on NH3 Volatilization
NH4+Urea NH3 H+
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
½ inch
Temperature
Temperature (oF)
30 40 50 60 70 80 90 100 110Rela
tive D
rivin
g F
orc
e for
Convers
ion
of A
queous N
H3 to G
aseous N
H3
1
2
3
4
5
Effect of Temperature on Conversion of
Aqueous NH3 to Gaseous NH3
Soil
+
Effect of Temperature on NH3 Volatilization
NH4+Urea NH3 H+
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
½ inch
Temperature
Soil
Wind speed
+
Effect of Wind Speed on NH3 Volatilization
NH4+Urea
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
NH3 H+
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
Atmosphere
½ inch
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
Am
monia
Loss (
% o
f A
pplie
d N
)
0
5
10
15
20UAN UREA Nitamin
51mm
Replication II0
5
10
15
20 UAN UREA Nitamin
51mm
Replication I
Days After Application
0 20 40 60 80 100 1200
5
10
15
20UAN UREA Nitamin
51mm
Replication III Time
00:00:00 08:00:00 16:00:00 00:00:00
Win
d S
pe
ed
(m
s-1
)
0
1
2
3
4
5
High
Low
Time
00:00:00 08:00:00 16:00:00 00:00:00
Win
d S
pe
ed
(m
s-1
)
0
1
2
3
4
5
High
Low
Optimum Conditions for
Volatilization
• Small diffusion depth into soil
• RH > CRH
• High temperature
• High wind speed
• Low pH buffering capacity
Overview
• Urea Consumption
• Definitions
• UAN Solution Reactions
• Urea Dissolution and Diffusion
• Urea Hydrolysis
• Ammonia Volatilization
• Research Results
• Summary
Urea Applied to Pasture - Fall 2004
Days after application
NH
3lo
ss (
% o
f applie
d N
)
0
10
20
30
40
50
0.1
0.2
0.3
0.4
10
20
30
40
50
Soil
wa
ter
conte
nt
(g g
-1)
20 40 60 80 100 120
Rain
fall
(mm
)
Nitamin®
UAN
Urea
Rainfall
Soil water content
Temp = 47oF
Days after application
NH
3lo
ss (
% o
f applie
d N
)
0
10
20
30
40
50
10
20
30
40
50 Nitamin®
UAN
Urea
0.1
0.2
0.3
0.4
Soil
wa
ter
conte
nt
(g g
-1)
20 40 60
Rain
fall
(mm
)
Rainfall
Soil water content
Temp = 63oF
Urea Applied to Pasture – Spring 2005
UreaNH4+
Atmosphere
Clay
OM
NH3
Urea Urea Urea
Urea UreaNH4+
NH4+
(aq)+
Equilibrium
NH4+
(aq)NH3 H+
Days after application
NH
3lo
ss (
% o
f applie
d N
)
0
10
20
30
40
50
10
20
30
40
50Nitamin®
UAN
Urea
Rainfall
Soil water content
0.1
0.2
0.3
0.4
Soil
wa
ter
conte
nt
(g g
-1)
20 40 60
Rain
fall
(mm
)Temp = 62oF
Urea Applied to Pasture - Fall 2005
Time of RH above CRH in the first 30 d
Tim
e o
f R
H >
CR
H (
h)
0
5
10
15
20
25
0 5 10 15 20 25 30
Total: RH > CRH =19 days63 %
Days after application
Urea Urea Urea
Atmosphere
Urea Urea Urea
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
Soil water content (0.07 g g-1)
RH > CRH (80%)
Permanent wilting point (0.04 g g-1)
Fescue thatch
NH4+
NH4+
NH4+
NH4+NH4
+NH4+NH4
+
Days after application
NH
3lo
ss (
% o
f applie
d N
)
0
10
20
30
40
50
10
20
30
40
50Nitamin®
UAN
Urea
0.1
0.2
0.3
0.4
Soil
wa
ter
conte
nt
(g g
-1)
20 40 80
Rain
fall
(mm
)
60
Rainfall
Soil water content
Temp = 69oF
Urea Applied to Pasture – Spring 2006
Urea Urea Urea
Urea Urea UreaNH4+
Atmosphere
NH4+
NH4+
(aq)+
Equilibrium
NH4+
(aq)NH3 H+
NH3
NH3 NH3
NH3
NH3
NH3NH3
NH3
Soil water content (0.15 g g-1)
Field capacity (0.18 g g-1)CEC
Ammonia volatilization losses under field conditions
Fertilizer
--------------- Ammonia loss (% of applied N) -----------------
Fall 2004
Urea
UAN
Nitamin®
19 a*
6 b
6 b
Spring 2005
12 a
13 a
14 a
•Within a column, values followed by the same letter are not significantly different according to
Fisher’s LSD at p=0.05
Summary of Pasture Results
Fall 2005 Spring 2006
46 a
33 b
34 b
24 a
18 a
18 a
Summary for Pastures/No-Till
Dry Soil, Dry Residue, RH < CRH: OK
Dry Soil, Dry Residue, RH > CRH: not OK
Wet Soil, Wet Residue: not OK
• Apply on dry soil, dry residue, expected low RH, and
hope for 1 inch of rain whenever it rains.
• Apply on wet soil if 1 inch rain/irrigation is expected.
Summary for Clean-Till Crops
Dry Soil: OK
Wet Soil: not OK
• Apply on dry soil and hope for 1 inch of rain whenever
it rains.
• Apply on wet soil if 1 inch of rain/irrigation is expected.
Summary for Pastures/CropsKnifed or disked in
Dribbled
Broadcast
• Knifed or disked in better than Dribbled
• Dribbled better than Broadcast
Laboratory Results (77oF, 90% RH)
NH3 volatilization in the lab
Days after application0 10 20 30
NH
3Lo
ss (
% o
f A
pplie
d N
)
0
10
20
30 Urea
UAN
Nitamin®
a
bb
Some Results from Central Great
Plains, Manhattan, Kansas
Topdress N on Wheat
Kansas State Univ., NE KS
N, pounds/acre N Source Yield, Bu/acre
0 45.3
30 Urea 60.2
60 Urea 68.1
90 Urea 71.0
30 Amm. Nitrate 59.1
60 Amm. Nitrate 64.0
90 Amm. Nitrate 70.1
Ammonia loss from N uptake by
Coastal Bermudagrass- Temple, TX
Bill Hargrove – 1975, 1976
Treatment N rate, pound/A NH3 Loss (%)
Urea 100 0
Urea 200 0
Urea 400 3
UAN 100 8
UAN 200 2
UAN 400 0
Questions?
