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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.

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rapid determination of the ph buffering capacity of soils

Rapid Determination of the pH Buffering Capacity of Soils

David E. Kissel

University of Georgia

Athens, GA

why is ph buffering capacity important
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.
slide3

Soil pH Buffering Capacity can be expressed in units of meq H+ (kg soil)-1 (pH)-1multiplied by 50 mg CaCO3 (meq)-1 it can be expressed in units ofmg CaCO3 (kg soil)-1 (pH)-1

slide4

Example of Soil pH Buffering

pH

-2

3

2

1

0

-1

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

pH

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

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

pH - 3.5

H+

Soil

Al3+

pH – 5.3

pH - 7.0

Ca2+

H+

Ca2+

Al3+

Mg2+

Mg2+

Ca2+

K+

Soil

Ca2+

Soil

Al3+

Mg2+

Ca2+

Ca2+

K+

Al3+

Al3+

Ca2+

Mg2+

K+

BS = 0%

BS = 50%

BS = 100%

slide6

Soil pH Buffering Capacity can be expressed in units of meq H+ (kg soil)-1 (pH)-1multiplied by 50 mg CaCO3 (meq)-1it can be expressed in units ofmg CaCO3 (kg soil)-1 (pH)-1

the uga titration method employs two ph measurements
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
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

issues with single addition titration
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

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)

slide12

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
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
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)
summary
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.
summary20
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.
present equation lb a ag lime lbc 30min x target ph measured ph 0 6 x 2 x 1 5 x soil depth 6

Present Equationlb/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)

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