Rapid Determination of Rapid Determination of the pH Buffering Capacity the pH Buffering Capacity

of Soilsof Soils

David E. KisselDavid E. KisselUniversity of Georgia

Athens, GA

Why is pH buffering capacity Why is pH buffering capacity important?important?

• It can be used in making ag lime It can be used in making ag lime recommendations for acid soils.recommendations for acid soils.

• It can potentially be used in making S It can potentially be used in making S recommendations for decreasing the pH recommendations for decreasing the pH for some soils (example, blueberry).for some soils (example, blueberry).

• It is a valuable soil property for estimating It is a valuable soil property for estimating pH changes from some nitrogen reactions pH changes from some nitrogen reactions in soils such as urea hydrolysis, in soils such as urea hydrolysis, nitrification, or soil retention of ammonia.nitrification, or soil retention of ammonia.

Soil pH Buffering Capacity can be expressed Soil pH Buffering Capacity can be expressed in units of in units of meq Hmeq H++ (kg soil) (kg soil)-1-1 (pH) (pH)-1-1

multiplied by 50 mg CaCOmultiplied by 50 mg CaCO33 (meq) (meq)-1 -1 it can be it can be

expressed in units ofexpressed in units of mg CaCOmg CaCO33 (kg soil) (kg soil)-1-1 (pH) (pH)-1-1

pH

3 2 1 0 -1 -2

meq H+ (kg soil)-1 pH-1

Example of Soil pH Example of Soil pH BufferingBuffering

4.0

4.5

5.0

5.5

6.0

6.5pH

meq H+ (kg soil)-1 pH-1

Acid/non-acid Acid/non-acid cations vs. soil pHcations vs. soil pH

pH – 5.3 pH - 7.0

Al3+

Al3+

Al3+

H+

K+

Ca2+

Mg2+

Mg2+

K+

Ca2+

Mg2+

Mg2+

Ca2+

Ca2+

So

il

So

il

Ca2+

Al3+

K+

Ca2+

BS = 0% BS = 50% BS = 100%

pH - 3.5

Al3+

H+

So

il

Ca2+

Soil pH Buffering Capacity can be Soil pH Buffering Capacity can be expressed in units of expressed in units of

meq Hmeq H++ (kg soil) (kg soil)-1-1 (pH) (pH)-1-1

multiplied by 50 mg CaCOmultiplied by 50 mg CaCO33 (meq) (meq)-1-1

it can be expressed in units ofit can be expressed in units of mg CaCOmg CaCO33 (kg soil) (kg soil)-1-1 (pH) (pH)-1-1

The UGA titration method employs The UGA titration method employs two pH measurementstwo pH measurements

• The first measurement is soil pH prior to The first measurement is soil pH prior to adding calcium hydroxide (pH is measured adding calcium hydroxide (pH is measured in 0.01M calcium chloride, rather than in in 0.01M calcium chloride, rather than in water).water).

• The second pH measurement is taken The second pH measurement is taken after adding a measured amount of after adding a measured amount of calcium hydroxide and 30 minutes calcium hydroxide and 30 minutes equilibration.equilibration.

How we do this operation?How we do this operation?

LBC = mg CaCOLBC = mg CaCO33/kg/dpH /kg/dpH

= (2.7 ml X 0.047 X = (2.7 ml X 0.047 X 50) /0.020 kg/(6.21-50) /0.020 kg/(6.21-5.27)5.27)

= 338 mg CaCO= 338 mg CaCO33/kg/pH/kg/pH

Lab# pH1 pH2 LBC

127 5.27 6.21 338

128 5.43 5.81 835

Issues with Single-Addition TitrationIssues with Single-Addition Titration

• The equilibrium pH was not reached after The equilibrium pH was not reached after 30 minutes of equilibration.30 minutes of equilibration.

• By depending only on a stirrer to mix the By depending only on a stirrer to mix the soil with the added base, the stirrer motors soil with the added base, the stirrer motors and stirrers must be robust to keep all soil and stirrers must be robust to keep all soil in the cup moving to interact with the in the cup moving to interact with the added calcium hydroxide.added calcium hydroxide.

To address the lack of To address the lack of equilibrium, we adjusted the equilibrium, we adjusted the recommendation equationrecommendation equation

lb/a ag lime = LBC X [target pH-lb/a ag lime = LBC X [target pH-(measured pH-0.6)] X 2 X 1.5 X (measured pH-0.6)] X 2 X 1.5 X

(soil depth/6)(soil depth/6)

Objectives (John Thompson)Objectives (John Thompson)

● ● Determine the time of Ca(OH)Determine the time of Ca(OH)22 reaction reaction

needed to reach an equilibrium LBC.needed to reach an equilibrium LBC.

●● Determine the relationship between theDetermine the relationship between the LBC from 30 minutes equilibration andLBC from 30 minutes equilibration and the LBC at the time when a truethe LBC at the time when a true equilibrium pH is reached.equilibrium pH is reached.

Materials and MethodsMaterials and Methods• Soils were identified and bulk samples Soils were identified and bulk samples

collected from three geographic provinces in collected from three geographic provinces in GeorgiaGeorgia– Ridge and Valley (northwest GA)Ridge and Valley (northwest GA)– Coastal PlainCoastal Plain– Atlantic Coast FlatwoodsAtlantic Coast Flatwoods

• Soils with high organic matter content were Soils with high organic matter content were obtained from North Carolinaobtained from North Carolina

Materials and Methods (cont’d)Materials and Methods (cont’d)

• Soils were dried overnight, then ground, Soils were dried overnight, then ground, sieved (2 mm), and stored in Ziploc sieved (2 mm), and stored in Ziploc freezer bagsfreezer bags

• Soils were analyzed in groups of five due Soils were analyzed in groups of five due to tray sizeto tray size

- Each tray can hold 40 samples: 5 wide Each tray can hold 40 samples: 5 wide (columns) x 8 deep (rows)(columns) x 8 deep (rows)

ResultsResults

Soil 4

Time (hours)

0 20 40 60 80 100 120

LB

C

600

800

1000

1200

1400

1600

1800

2000

2200

Soil 4

Time (hours)

0 20 40 60 80 100 120

ph

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

LBC = mg CaCO3/kg/dpH = (2.7 ml X 0.047 X 50) /0.020 kg/(pHtr-pHck)

Soil 15

Time (hours)

0 20 40 60 80 100 120

pH

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

Soil 15

Time (hours)

0 20 40 60 80 100 120

LB

C

160

180

200

220

240

260

280

300

320

340

360

LBC = mg CaCO3/kg/dpH = (2.7 ml X 0.047 X 50) /0.020 kg/(pHtr-pHck)

SummarySummary

• All soils were near equilibrium by 96 hr with All soils were near equilibrium by 96 hr with respect to their LBC.respect to their LBC.

• Ratio of LBC 96 hr/LBC 0.5 hr for Soil 15 and 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 Soil 4 were 1.8 and 2.7 respectively. Most of the other soils were within this range.other soils were within this range.

• Short term incubations are being carried out to Short term incubations are being carried out to test the validity of the new lime equation.test the validity of the new lime equation.

• Based on our incubation results we will be able Based on our incubation results we will be able to improve our rapid method for soil pH buffering to improve our rapid method for soil pH buffering capacity used to make lime recommendations. 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 pH-measured pH)

Present Equationlb/a ag lime = LBClb/a ag lime = LBC30min30min.. X [target X [target

pH-(measured pH-0.6)] X 2 X 1.5 pH-(measured pH-0.6)] X 2 X 1.5 X (soil depth/6)X (soil depth/6)

New Equation

lb/a ag lime = LBClb/a ag lime = LBCEquil.Equil. X [target X [target

pH-measured pH] X 2 X 1.5 X pH-measured pH] X 2 X 1.5 X (soil depth/6)(soil depth/6)