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  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)  where LBC30min is the soil’s LBC from Eq  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  can be rearranged to LR (mg kg-1) = LBC30min [(target pHCaCl2 + 0.6) – pHCaCl2]  3. An alternative to the use of Eq  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)  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.