Rapid Determination of
the pH Buffering Capacity
of Soils
David E. Kissel
University of Georgia
Athens, GA
Why is pH buffering capacity
important?
• It can be used in making ag lime
recommendations for acid soils.
• It can potentially be used in making S
recommendations for decreasing the pH
for some soils (example, blueberry).
• It is a valuable soil property for estimating
pH changes from some nitrogen reactions
in soils such as urea hydrolysis,
nitrification, or soil retention of ammonia.
Soil pH Buffering Capacity can be expressed
in units of
meq H+ (kg soil)-1 (pH)-1
multiplied by 50 mg CaCO3 (meq)-1 it can be
expressed in units of
mg CaCO3 (kg soil)-1 (pH)-1
Example of Soil pH Buffering
6.5
6.0
5.5
pH
p
H
5.0
4.5
4.0
3
2
1
0
+ (kg+ soil)-1 -1pH-1-1
meq Hmeq
H (kg soil) pH
-1
-2
Acid/non-acid
cations vs. soil pH
pH – 5.3
H+
Al3+
Al3+
Soil
Soil
Al3+
H+
Mg2+
K+
Al3+
Ca2+
Al3+
Mg2+
BS = 0%
pH - 7.0
BS = 50%
Soil
pH - 3.5
Ca2+
Ca2+
Mg2+
Ca2+
Ca2+
Mg2+
Ca2+
K+
Ca2+
K+
BS = 100%
Soil pH Buffering Capacity can be
expressed in units of
meq H+ (kg soil)-1 (pH)-1
multiplied by 50 mg CaCO3 (meq)-1
it can be expressed in units of
mg CaCO3 (kg soil)-1 (pH)-1
The UGA titration method employs
two pH measurements
• The first measurement is soil pH prior to
adding calcium hydroxide (pH is measured
in 0.01M calcium chloride, rather than in
water).
• The second pH measurement is taken
after adding a measured amount of
calcium hydroxide and 30 minutes
equilibration.
How we do this operation?
LBC = mg CaCO3/kg/dpH
= (2.7 ml X 0.047 X 50)
/0.020 kg/(6.21-5.27)
= 338 mg CaCO3/kg/pH
Lab# pH1
pH2
LBC
127
5.27 6.21 338
128
5.43 5.81 835
Issues with Single-Addition Titration
• The equilibrium pH was not reached after
30 minutes of equilibration.
• By depending only on a stirrer to mix the
soil with the added base, the stirrer motors
and stirrers must be robust to keep all soil
in the cup moving to interact with the
added calcium hydroxide.
To address the lack of
equilibrium, we adjusted the
recommendation equation
lb/a ag lime = LBC X [target pH(measured pH-0.6)] X 2 X 1.5 X
(soil depth/6)
Objectives (John Thompson)
● Determine the time of Ca(OH)2 reaction
needed to reach an equilibrium LBC.
● Determine the relationship between the
LBC from 30 minutes equilibration and
the LBC at the time when a true
equilibrium pH is reached.
Materials and Methods
• Soils were identified and bulk samples
collected from three geographic provinces
in Georgia
– Ridge and Valley (northwest GA)
– Coastal Plain
– Atlantic Coast Flatwoods
• Soils with high organic matter content
were obtained from North Carolina
Materials and Methods (cont’d)
• Soils were dried overnight, then ground,
sieved (2 mm), and stored in Ziploc
freezer bags
• Soils were analyzed in groups of five due
to tray size
- Each tray can hold 40 samples: 5 wide
(columns) x 8 deep (rows)
Results
Soil 4
4.8
4.7
4.6
ph
4.5
4.4
4.3
4.2
4.1
0
20
40
60
80
100
120
Time (hours)
Soil 4
2200
2000
1800
LBC
1600
1400
1200
LBC = mg CaCO3/kg/dpH = (2.7 ml X
0.047 X 50) /0.020 kg/(pHtr-pHck)
1000
800
600
0
20
40
60
Time (hours)
80
100
120
Soil 15
5.6
5.4
5.2
5.0
pH
4.8
4.6
4.4
4.2
4.0
3.8
3.6
0
20
40
60
80
100
120
Time (hours)
Soil 15
360
340
320
300
LBC
280
260
240
220
LBC = mg CaCO3/kg/dpH = (2.7 ml X
0.047 X 50) /0.020 kg/(pHtr-pHck)
200
180
160
0
20
40
60
Time (hours)
80
100
120
Summary
• All soils were near equilibrium by 96 hr with
respect to their LBC.
• Ratio of LBC 96 hr/LBC 0.5 hr for Soil 15 and
Soil 4 were 1.8 and 2.7 respectively. Most of the
other soils were within this range.
• Short term incubations are being carried out to
test the validity of the new lime equation.
• Based on our incubation results we will be able
to improve our rapid method for soil pH buffering
capacity used to make lime recommendations.
Summary
• LBC equilibr. = LBC 30 min X approx. 2
• The LBC equilibr will be used to calculate
the lime requirement without using the
added factor of 0.6.
mg/kg= mg/kg/pH X (delta pH)
where (delta pH) is (target pHmeasured pH)
Present Equation
lb/a ag lime = LBC30min. X [target
pH-(measured pH-0.6)] X 2 X 1.5
X (soil depth/6)
New Equation
lb/a ag lime = LBCEquil. X [target
pH-measured pH] X 2 X 1.5 X
(soil depth/6)