Ammonia Volatilization from Urea: How large is the issue and losses. David E. Kissel University of Georgia
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 Time00:00:00 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?