Single Addition of Ca(OH)2 for Determining Lime Requirement
David E. Kissel and Leticia S. Sonon
Application and Principle
The Single Addition of Ca(OH)2 method was designed to estimate a soil’s pH
buffering capacity, from which its lime requirement (LR) can be calculated. This method
is based on the difference in the soil’s initial pH and the pH measured 30-min after the
addition of a single aliquot of Ca(OH)2. This difference in pH is used to calculate the
soil’s pH buffering capacity based on a 30-min equilibration time. The method requires
that titration curves of acid soils be linear in the working range from pH 4 to 6.5, which is
the case for surface soils. The single addition method was developed to work best with an
automated pH analyzer, although it could also be adapted to a manual pH meter.
The Single Addition of Ca(OH)2 method was developed by Kissel et al. (2007) to
determine the lime requirement for soils in Georgia. One distinct advantage of the
method is the minimal amount of chemicals required and the resulting products being
soil, water, and a small amount of CaCl2.
Equipment and Apparatus
1. Soil scoop and leveling rod
2. pH cups
3. Holding rack for pH cups
4. Dispenser for 0.01 M CaCl2 or other electrolyte solution added to soil
5. Manual pH meter or automated pH analyzer
6. Glass pH electrode with an internal reference element or a separate
reference electrode
7. Analytical balance and glassware for making electrolyte solutions if they are
added to soil
8. Burette pump to add Ca(OH)2 to the soil
9. Reciprocating shaker capable of 180 opm
Reagents
1. 0.05 M KHC8H4O4 (potassium hydrogen phthalate, KHP) solution: Crush 15 to
20 g primary standard (formula weight = 204.23 g/mole) to about 100 mesh and
dry at 120oC for 2 hours. Cool in a desiccator. Weigh 10.000 ± 0.001 g, transfer to
1-L volumetric flask, and dilute to 1000 mL with deionized water.
2. Saturated Ca(OH)2 solution: Weigh about 42 g Ca(OH)2 powder and place it in
20-L carboy container. Add approximately 10-L of deionized water and shake or
stir vigorously to maximize dissolution of Ca(OH)2. Fill the carboy to the 20-L
mark. Cap tightly to prevent any CO2 from reacting with the Ca(OH)2 solution.
Shake or stir the solution again. Some Ca(OH)2 particles remain undissolved and
settle at the bottom of the container.
3. 1% Phenolphthalein Indicator: Available from any chemical vendor.
Standardization of saturated Ca(OH)2 solution
1. Place 5 mL of 0.05 M KHP solution into 100 mL beaker.
2. Add 5 drops of 1% phenolphthalein indicator.
3. Fill the burette with saturated Ca(OH)2 solution earlier prepared.
4. Slowly titrate saturated (Ca(OH)2) into the KHP solution. When OH- from
Ca(OH)2 has neutralized hydrogen from KHP, the pink color of phenolphthalein
appears. Stop the addition of calcium hydroxide when only the faintest pink
appears and remains for two minutes. Record the volume of Ca(OH)2 used.
5. Calculate the molarity of Ca(OH)2 as shown below.
Molarity of Ca(OH)2 = (5 mL x 0.05) / (mL of Ca(OH)2 used x 2)
Where 5 mL = volume of KHP
0.05 = molarity of KHP
2 = moles of KHP to react with one mole of Ca(OH)2
Procedure
1. Follow the procedure for making a 1:1 soil:0.01 M CaCl2 pH measurement (see
Chap 4).
2. Add a volume of saturated Ca(OH)2 to the slurry to raise soil pH. The volume of
Ca(OH)2 added should be enough to give an increase in pH of at least 0.3 pH
units.
3. Shake the soil Ca(OH)2 mixture on an end to end shaker for five minutes and then
let stand an additional 25 minutes before measuring pH. If using an automated pH
analyzer, vigorously stir the soil and Ca(OH)2 mixture and allow the suspension
to equilibrate for 30 minutes before taking the soil pH measurement.
4. Ensure room temperature is between 20 and 25oC before proceeding with
pH measurement.
5. Place electrode in the soil slurry to measure pH. Measurement may be taken
with or without continuous stirring. If measurement is made without
continuous stirring, stir the sample with a stir bar before placing electrode in
the sample. Allow adequate time for pH to reach a stable reading. Stability
can be ascertained by pH meter settings for manual measurements or
software settings for automated instruments (see Chapter 4).
Analytical Performance
Range and Sensitivity
The single addition titration can be used on very low buffered sandy soils to
highly buffered fine-textured soils with high organic matter. For soils that
vary widely in their lime buffer capacity (LBC), the addition of 2.7 mL of
0.023 M Ca(OH)2 to 20 g of soil provides an adequate sensitivity for soils. With
2.7 mL Ca(OH)2, a highly buffered soil with an LBC of 1000 mg CaCO3 kg-1pH-1
would have a difference of 0.31 pH units between initial pH and pH taken 30 min
after Ca(OH)2 addition. Less buffered soils would have a larger difference in pH;
for example, an LBC of 500 mg CaCO3 kg-1 pH-1 would have a pH difference of
0.62 pH units.
Precision and Accuracy
1. pH measurements can be made to the nearest 0.01 pH unit. Calculation of the
LBC should be based on pH values to the nearest 0.01 pH unit. The greater
accuracy in pH is needed to maximize accuracy of LBC which is calculated from
the initial pH minus the pH taken after Ca(OH)2 addition. It is also advisable to
take the initial pH and second pH with the same pH electrode.
2. Typical measurements of precision for one electrode for ten consecutive
measurements of a check soil LBC are shown below.
Method
Single Addition Ca(OH)2
Number of
measurements
10
Mean
LBC
189
Standard deviation
9.1
Interferences
1. There are no known interferences with the Ca(OH)2 titration method.
2. Soil samples do not reach pH equilibrium by 30 min after application of Ca(OH)2;
therefore, adjustments are needed in the calculation of lime requirement (see
Interpretation 2, and 3, given below).
3. The electrode should have a protective sleeve when measuring pH in sandy soils
to protect the glass pH bulb from abrasive sand particles. Electrode life is
lengthened with the protective sleeve.
4. The Ca(OH)2 solution has an undetermined but long shelf life due to its high pH
greater than 12. It must be protected from CO2 with an ascarite trap to scrub CO2
from air that enters the carboy as solution is withdrawn.
Interpretation
1. Calculation of lime requirement (LR) requires the calculation of the soil’s pH
buffer capacity which, for convenience and better understanding by clients, is
expressed in units of CaCO3 and is called lime buffer capacity (LBC). The LBC is
calculated with the following equation
LBC30min = (V x M x 100.1 g/mole) (kg soil)-1(pH2-pH1)-1
[1]
where LBC30min has units of mg CaCO3 (kg soil)-1pH-1, V is the volume (mL) of
Ca(OH)2 added, M is the molarity of the saturated Ca(OH)2, 100.1 g/mole is the
molecular weight of CaCO3, pH2 is soil pH 30 min after adding Ca(OH)2, and pH1
is the initial soil pH. The equation assumes that CO32- anions react identically to a
chemically equivalent amount of OH- ions (Kissel et al., 1988 and Liu et al.,
2008).
2. The LR, expressed as reagent-grade powdered CaCO3, is calculated from the
following equation developed by Kissel et al. (2007) but with metric units and
without corrections for lime quality and depth of soil incorporation:
LR (mg kg-1) = LBC30min (target pHw – pHCaCl2)
[2]
where LBC30min is the soil’s LBC from Eq [1] above, after 30 min equilibration
with Ca(OH)2, target pHw is the target pH in water, and pHCaCl2 is the pH
measured in 0.01 M CaCl2 before the addition of Ca(OH)2. The target pHw is
used rather than target pHCaCl2 because, as noted by Liu et al. (2005), the 30-min
equilibration time for the Ca(OH)2 is not sufficient to reach a final equilibrium pH
with the soil acidity. Since the target pHw is numerically greater than a target
pHCaCl2, the value of (target pHw – pHCaCl2) is larger and makes up for a smaller
value of LBC due to the lack of complete equilibrium. Based on the data
presented by Kissel et al. (2009), the average difference in pHw and pHCaCl2 for
approximately 1200 soil samples was 0.6 units. Because pHw is approximately 0.6
units greater than pHCaCl2, Eq [2] can be rearranged to
LR (mg kg-1) = LBC30min [(target pHCaCl2 + 0.6) – pHCaCl2] [3]
3. An alternative to the use of Eq [3] for calculating LR is to estimate the LBC for
which a soil is at pH equilibrium (LBCequil) after adding Ca(OH)2. This may be
done by obtaining a calibration equation for LBCequil as a function of LBC30min.
Obtaining this calibration with a five day laboratory incubation of acid soil with
Ca(OH)2 is described by Thompson et al. (2010). In that study, the ratio of
LBCequil/LBC30min was a linear function of LBC30min. This LBC ratio can be used
as a multiplier to calculate the LBCequil from the LBC30min that is determined in
the laboratory. Once the LBCequil has been calculated, the LR equation then
becomes
LR (mg kg-1) = LBCequil (target pHCaCl2 – pHCaCl2)
[4]
Effects of Storage
1. Air-dried soils may be stored several months without affecting the soil-buffer pH
measurement provided they are stored in an ammonia-free environment or in a
tightly sealed container.
2. The apparatus used for determining the pH should be maintained and stored
according to the manufacturer instructions.
Safety and Disposal
1. The chemicals used in this procedure pose no safety risk and therefore can be
stored and disposed of according to routine laboratory procedures.
2. After reaction of Ca(OH)2 with soil, only soil and approximately 0.01 M CaCl2
remain, which can be disposed of safely according to routine laboratory
procedures.
References
Kissel, D. E., M. L. Cabrera, and R. B. Ferguson. 1988. Reactions of ammonia and urea
hydrolysis products with soil. Soil Sci. Soc. Am. J. 52:1793-1796.
Kissel, D.E., R.A. Isaac, R. Hitchcock, L. Sonon, and P.F. Vendrell. 2007.
Implementation of soil lime requirement by a single addition titration method.
Communications in Soil Science and Plant Analysis, 38:1341-1352.
Kissel, D.E., L.S. Sonon, P.F. Vendrell, and R.A. Isaac. 2009. Salt concentration and
measurement of soil pH. Commun. Soil Sci. Plant Anal. 40:179-187.
Liu, Min, D.E. Kissel, M.L. Cabrera, and P.F. Vendrell. 2005. Soil lime requirement by
direct titration with a single addition of calcium hydroxide. Soil Sci. Soc. Amer. J.
69:522-530.
Liu, Min, D.E. Kissel, M.L. Cabrera, L.S. Sonon, and P.F. Vendrell. 2008. Effects of
biological nitrogen reactions on soil lime requirement determined by incubation. Soil Sci.
Soc. Am. J. 72:720-726.
Thompson, J.S., D.E. Kissel, M.L. Cabrera, and L.S. Sonon. 2010. Equilibration reaction
from single addition of base to determine soil lime requirement. Soil Sci. Soc. Am. J. 74:
663-669.