Published in Soil Sci. Soc. Am. J. 68:1228-1233 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
DIVISION S-2—SOIL CHEMISTRY
Soil Lime Requirement By Direct Titration with Calcium Hydroxide
Min Liu,
D. E. Kissel*,
P. F. Vendrell and
M. L. Cabrera
Dep. of Crop and Soil Sciences, 3111 Plant Sciences, Univ. of Georgia, Athens, GA 30602
* Corresponding author (dkissel{at}uga.edu).
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ABSTRACT
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A simple method for titrating acidic soils could be used by soil testing laboratories as an alternative method for routinely determining the lime requirement (LR) of soils. Because soil pH as a function of added base is linear in the pH range of 4.5 to 6.5, it may be possible to determine the slope of this relationship by regression analysis using a minimum number of base additions. The LR could then be determined by calculating the corresponding amount of lime required to reach the target pH using the regression equation. Our objectives were to determine reaction times needed for saturated Ca(OH)2 solution to reach an equilibrium pH with 17 widely different soils, and then test a simple titration procedure for its ability to predict the LR, determined first by a full titration curve and then by a standard incubation method. All titrations were performed in a 1:1 soil/water ratio with calcium hydroxide as the base. Using the regression procedure to estimate LR, equilibration times of 30 min between base additions gave greater LRs than 15-min equilibration, but the same as 45-min equilibration. When all 17 soils were considered, the LR from extrapolation of the linear regression yielded the same, as did the full titration curve (TC). The LR from titration with three base additions and 30-min equilibration time between additions estimated an average of 80% of the LR determined by the 3-d incubation. The Adams-Evans (AE) buffer procedure overestimated and underestimated the LR for soils with relatively low- and high-LRs, respectively. Based on these results, titration appears promising for routine use, but it needs more research to determine whether it can be simplified further.
Abbreviations: AE, Adams Evans • LR, lime requirement • MLRA, Major Land Resource Area • TC, titration curve
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INTRODUCTION
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BUFFER METHODS are typically used by soil testing laboratories in the USA for the prediction of the LR of acid soils. For example, the AE buffer procedure (Adams and Evans, 1962) is used widely in the southeastern and mid-Atlantic regions of the USA, and the SMP buffer (Shoemaker et al., 1961) is widely used in the Midwest regions (Sims, 1996). The accuracy of these buffers has been widely studied (Follett and Follett, 1980; Tran and van Lierop, 1981; Alabi et al., 1986). A recent concern about buffers is the potential toxicity of their components. Both the AE and SMP buffers contain p-nitrophenol, a potentially toxic compound.
A possible alternate method for determining the LR of acid soils is by direct titration with a base, although these methods are time-consuming and would need to be simplified for routine use. Dunn (1943) studied direct titration to predict the LR of acid soils and focused on the time to reach equilibrium for the reaction between the added base and soil acids. He found that 4 d were needed for pH values to reach equilibrium when a 0.022 M Ca(OH)2 solution was added to acid soils. He also discovered that shaking affected the time required for soil pH values to reach equilibrium. He reported that a constant pH was reached within 8 h when the soil suspensions were mixed by shaking, whereas 4 d were required for the suspensions to obtain a stable pH without shaking. Finally, he suggested a direct titration method for LR by incorporating acid soils with different rates of 0.022 M Ca(OH)2 for 4 d. The Ca(OH)2 titration method suggested by Dunn (1943) for measuring the LR was widely accepted as a reliable method for evaluating buffer methods that were developed for estimating the LR (Follett and Follett, 1980; Alabi et al., 1986; McConnell et al., 1991; Owusu-Bennoah et al., 1995). However, Dunn's method was also considered to be a time-consuming procedure and not suitable for routine use in soil testing laboratories. Many studies focused on the TC itself. Magdoff and Bartlett (1985) concluded that the relationship between pH and OH– added is nearly linear within the pH range of most agricultural soils (4.5–6.5). Weaver et al. (2004) also reported a linear relationship between pH and base added for a series of Georgia soils.
If the relationship between soil pH and added base is linear for a wide range of soils, only a few data points would be needed to determine a soil's buffering capacity, which could then be used for calculation of the lime requirement. The objective of this study was to determine if titration might be simplified in this way, so that titration could be used routinely for determining the LR of acid soils.
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MATERIALS AND METHODS
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Seventeen soil samples with a wide range of clay and soil organic C contents were collected from the Ap horizon of agricultural soils and the A horizon of forested soils in January of 2002. Approximately 5 kg were collected at each location, with samples taken from five of the major land resource areas (MLRA) in Georgia (Fig. 1), with samples taken approximately proportional to the areas of agricultural land in the various regions. The soils were oven-dried at a temperature of 35°C, crushed, and then sieved (2-mm) to remove gravel and non-decayed crop residue, which consisted of <1% of the soil by weight. Soils were then stored in sealed Ziploc bags (S.C. Johnson, Racine, WI) until analyzed. A subsample of each soil was analyzed for C and N by dry micro-Dumas combustion with a Leco CNS 2000 Analyzer (Leco, St. Joseph, MI). Particle-size distribution was determined with the pipette method described by Kilmer and Alexander (1949). Four of the soil samples contained more than 300 g kg–1 clay and five had clay contents in the range from 100 to 200 g kg–1. Eight contained <100 g kg–1 clay. Three of the soil samples contained more than 20 g kg–1 total C and eight soil samples contained in the range from 10 to 20 g kg–1 total C. The others had <10 g kg–1 total C (Table 1).
Saturated Ca(OH)2 solution (0.022 M) was used as the standard base to titrate the soils. In a 12-L polyethelene carboy, 50 g of powdered reagent grade Ca(OH)2 was added to 10 L of deionized water, stirred with a glass rod, and then allowed to settle for 4 d at room temperature. The carboy was fitted with an ascarite trap to prevent carbon dioxide from reacting with Ca(OH)2.
The soil pH measurements and titrations were performed in a 1:1 soil/water suspension while being stirred. A propeller shaped stirrer was adjusted to fit into the 120-mL polypropylene beakers so that all soil particles were suspended throughout the titration. A Visco Alpha Multi-task version 2.0 digital titrator (Visco Alpha, Middletown, NY) was used to titrate each soil. Through programming the titration software, the number of aliquots, the interval time between two aliquots, and the stirring speed during titration can be adjusted. A Titronic Universal Piston Burette (SCHOTT Glas Business Segment Labware, Hattenbergstraße, Mainz, Germany) was used to add the Ca(OH)2 solution with an accuracy of ±0.01 mL. A SCHOTT glass pH electrode (SCHOTT Glas Business Segment Labware, Hattenbergstraße, Mainz, Germany) with a colomel reference was used to determine pH. The pH meter was calibrated with standard pH 4.00 and 7.00 buffers before each titration.
Time Interval Study
Based on our preliminary observations, and as noted by Weaver et al. (2004), soil pH was relatively stable by 10 to 15 min following the addition of base. Therefore the times selected between additions of 0.022 M Ca(OH)2 were 15, 30, and 45 min. Because TCs are approximately linear within the pH range (4.5–6.5) of most agricultural surface soils, three aliquots of base were used to develop the slopes of the titration curves for each soil. Titrations were performed in a 1:1 soil/water ratio with 30 mL of deionized water added to 30 g of soil. The soil pH was measured while being stirred. One milliliter of 0.022 M Ca(OH)2 per addition was used for less buffered soil samples No.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 14 (Table 1), which generally were lower in soil organic C (typically <12000 mg C kg–1) and typically had soil pH buffering capacities of less than 2.5 cmol (+) kg–1 pH–1. A measure of the soils' pH buffering capacity was determined from the slope (b) of the linear regression of pH vs. mmol OH–1 added from the Ca(OH)2 incubation study (described below), and expressed as 1/b with units of cmol (+) kg–1 pH–1. The values of pH buffering capacity are given in Table 1. Three milliliters of 0.022 M Ca(OH)2 per addition was used for soil samples No. 12, 13, 15, 16, and 17, which typically had pH buffering capacities >2.5 cmol (+) kg–1 pH–1. These soils were relatively higher in soil organic C (typically >12000 mg C kg–1). For each titration, a linear regression equation was fitted to the relationship between base added (abscissa) and the soil pH (ordinate). A pH of 6.5 was selected as the target pH because it is a typical target pH for liming recommendations. The LRs were then calculated based on the slopes of the linear regression equations and the pH difference between initial pH (y intercept) and pH 6.5 as shown by Eq. [1]
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where int pH is the y intercept from the linear regression, and b is the slope of the linear regression of pH vs. lime added as shown in Fig. 2. The b is a measure of the buffering capacity of the soil, and as noted earlier, can also be expressed as 1/b with units of cmol (+) kg–1 pH–1. The LRs calculated from data for each of the time intervals using Eq. [1] were compared to determine if they were statistically different by using the Paired t test analysis in SAS software (SAS Institute, 1985). Comparisons were LRs calculated from data with time interval of 15 min between base additions vs. time interval of 30 min between additions and time interval of 30 min vs. time interval of 45 min between additions.
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Fig. 2. Example of lime requirement predicted from three aliquots of Ca(OH)2 with a 30-min time interval between two additions, for Soil No. 9.
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Full Titration Curves
The buffer curves of each soil sample were established by titrating 30 g of soil in water (1:1 soil/water ratio) with either 1 mL (generally low-LR soils) or 3 mL (generally high-LR soils) of 0.022 M Ca(OH)2 per addition. Thirty minutes were chosen as the time interval between additions based on the results from the time interval study. The soil suspension was continuously stirred during the titration and the pH was measured while being stirred at the end of each time interval. For electrode safety, the pH electrode was stored in the 7.00 buffer standard between intervals.
Calcium Hydroxide Incubation Study
Each soil sample was also incubated for 4 d with three amounts of a 0.022 M Ca(OH)2 solution added to 30 mL of deionized water and 30 g of each soil in a 120-mL polypropylene beaker. After thoroughly mixing for 30 min by stirring with a glass rod, the initial pH of each sample was measured while stirring at the same speed as that used for titration. Then the three amounts of 0.022 M Ca(OH)2 solution equivalent to 0.5, 1, or 1.5 times the LR to the target pH of 6.5, calculated from the full titration curves, were added to each soil. Three drops of chloroform were added to depress microbial activity. The samples were then covered with PARAFILM (Royal Purple, Ltd., Porter, TX) to reduce evaporation. A 10-mm long slit was cut through the film for air exchange. A glass stirring rod was inserted through the opening for mixing the soil periodically. The soil samples were incubated for 4 d at room temperature (23 ± 2°C). The pH was measured at 24, 48, 72, and 96 h while being stirred. Approximately half of the soil treatments were duplicated to determine precision. The relationship of soil pH versus Ca(OH)2 added (expressed as the equivalent CaCO3) was fitted for each soil by nonlinear regression using Table Curve 2D (Systat Software Inc., Richmond, CA) and the Ca(OH)2 incubation LR to pH 6.5 was calculated from this equation.
Adams-Evans Buffer Procedure
The A-E buffer procedure was used to predict the LR of each soil sample. Twenty milliliters of deionized water was added to 20 g of each soil. After 40 min, the pH was measured while being stirred. Then, 20 mL of A-E buffer was added to each soil suspension. The soil suspensions were shaken for 10 min at 400 excursions min–1 and then allowed to stand for 0.5 h. The buffer pH was then measured while being stirred. The A-E procedure was duplicated for each soil and the mean value was used in the analysis.
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RESULTS AND DISCUSSION
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Plots of soil pH as a function of Ca(OH)2 added (30 min equilibration time and expressed as equivalent amount of Mg CaCO3 ha–1) were generally linear when only the three data points were fit using linear regression. The initial pH was omitted from the regression because it was well below the y intercept in >50% of the soils. The three data values gave r2 values >0.992 for 15 of the 17 soils. Even the two soils with the poorest fit gave r2 values of 0.915 (Soil 11) and 0.937 (Soil 16). Slope values for the titrations covered an approximately 10-fold range from a minimum value of 0.549 Mg CaCO3 ha–1 pH–1 (Soils 1 and 6) to a high of 5.587 Mg CaCO3 ha–1 pH–1 (Soil 13). The LR can be estimated from the titration data by extrapolating to the target pH, as shown in Fig. 2, or it can be calculated based on the slope, the target pH and the intercept using Eq. [1].
Time Interval Study
The main objective of this part of the study was to determine the equilibration time needed between two additions of Ca(OH)2 to reach an equilibrium pH. Three time intervals of 15, 30, and 45 min between additions were tested. The LRs calculated with Eq. [1] for the three time intervals are given in Table 2. The mean values of LR from the 15-, 30-, and 45-min time intervals were 2.093, 2.591, and 2.539 Mg ha–1, respectively. PROC Paired t test analysis in SAS showed the 45 min LR values and the 30 min LR values were not significantly different with a P-value of 0.5958. However, there was a significant difference between the 15-min LR values and the 30-min LR values (P value = 0.0062). The results indicated that the 30-min time interval between two additions of base was adequate for Ca(OH)2 to reach a constant pH.
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Table 2. Comparison of the predicted titration lime requirements (LR) values (Mg ha–1) among three levels of time interval between two additions of base.
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Test of the Three-Addition Method
Using only enough titration data points to establish the slope of the linear equation for calculation of the LR might make titration feasible for routine laboratory use. To test its feasibility, LRs from the three-addition method were compared with the LRs from the complete titration curve for all 17 soils. The complete titration curve for Soil No. 4 is shown in Fig. 3 along with the nonlinear regression equation fit to the titration data. The complete titration curve for this soil is somewhat curvilinear, and had among the poorest agreement with the LR predicted using the three-addition method (1.87 vs. 2.10 Mg ha–1 from the complete TC). Yet for routine soil testing, this difference may be acceptable, as it represents only an 11% lower LR than the value from the complete TC. The comparison of LR for all soils by the two methods is shown in Fig. 4. The relationship between the two methods indicates close agreement between the two methods for the full range of soils with a slope of 0.996 and an intercept of 0.053 (not different from 1 and 0, respectively). Further, the Proc Paired t test analysis in SAS showed the TC LR values and the LR values from the linear regression and Eq. [1] were not significantly different with P value of 0.4729. In summary, the results indicate that the use of three data points and their linear regression on extrapolation to pH 6.5 worked well for the soils in this experiment.
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Fig. 3. The complete titration curve to pH 7 for Soil No. 4 using a 30-min time interval between additions of Ca(OH)2. Results are expressed as Mg CaCO3 ha–1.
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Fig. 4. Comparison of the lime requirement (LR) values (Mg ha–1) from the complete titration curves and LR values predicted from the first three aliquots of base.
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Calcium Hydroxide Incubation Study
The 4-d Ca(OH)2 equilibrium was selected as a standard to test the validity of titrations with a 30-min time interval between additions. The change in soil pH of each soil sample treated with 1 LR level is shown in Table 3 With perfect agreement between the two methods, the equilibrium pH values would be 6.5. For most soil samples, the soil pH decreased from the first day to the second day, with the average pH being about the same on Day 2 and 3. Soil pH then increased from the third day to the fourth day. The results were consistent with observations by Dunn (1943), which found that minimum and stable pH values were reached after 4 d of incubating acid soils with standard Ca(OH)2 solution without mixing during the incubation. For samples shaken continuously, equilibrium was reached in 8 h in all samples. Because the soils were stirred occasionally in our study, we would have expected a minimum pH to be reached between 8 h and 4 d. The soil pH increased from the third day to the fourth day for most of the soil samples, possibly because of the reduction of Fe and Mn minerals, given that the soil samples were kept in reduced conditions, and perhaps as a result of increased ammonification reactions from the addition of chloroform.
Soil pH values from the incubation on Day 3 (Table 3) ranged from the lowest value of 6.01 to the highest value of 6.63. The average pH value of the 17 soil samples was 6.36. Because the average initial pH was 4.81, and because the titrations can be represented by a linear equation, the average pH of 6.36 suggested that the direct titration with a 30-min time interval between additions neutralized about 90% [(6.36 – 4.81)/(6.50 – 4.81) x 100] of the soil acidity between the soils' initial pH and the target pH of 6.5. A relationship between the predicted LR values from the first three aliquots of Ca(OH)2 with a time interval of 30 min (Y, Mg ha–1) and the 3-d incubation LR values (X, Mg ha–1) was also established to generate a liming factor for the direct titration procedure with 30 min time interval between two additions (Fig. 5). The linear regression equation was generated under a 99.9% confidence interval, with a 0 intercept and slope of 0.80, which indicates that the titration with a 30-min time interval measured on average about 80% of the LR that was measured by the 3-d incubation. The slow release of H+ from the soil complex was possibly responsible for the small amount of acidity not measured, which occurred during the 2- to 3-d period after Ca(OH)2 was added.
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Fig. 5. Relationship between Ca(OH)2 incubation lime requirement (LR) values and the LR values predicted from the first three aliquots of a Ca(OH)2 titration using 30-min time interval between additions of Ca(OH)2.
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Adams-Evans Buffer Procedure
The 3-d Ca(OH)2 incubation was also used as the standard for comparison with the LR values from the AE procedure. The relationship between the AE lime requirement values and the LRs values from Ca(OH)2 incubation for all 17 soils is shown in Fig. 6. This regression resulted in the linear relationship:
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Fig. 6. Comparison of the lime requirements (LRs) between the Adam-Evans procedure and the 3-d Ca(OH)2 incubations.
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Notice that the y intercept of the relationship was relatively large with a value of 0.979 Mg ha–1, which indicated that at least for the soils in this study, the AE method overestimated the LRs of soils with low LRs. In contrast, those soils with LRs > 4.5 Mg ha–1 had low values by the AE method when compared with the incubation method.
In contrast to the AE buffer method, the LRs from the titration procedure were more closely related to the LR determined by a 3-d incubation. The agreement was consistent across all 17 samples, with an intercept of 0 and r2 = 0.9637. The titration procedure appeared to estimate the LR of soils with both low and high LRs equally well.
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CONCLUSIONS
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Plots of soil pH as a function of Ca(OH)2 added (expressed as equivalent amount of kg CaCO3 ha–1) were generally linear. A 30-min time interval between additions of Ca(OH)2 was adequate for the added base to react with the soil acids. A linear extrapolation of the first three data points of the titration predicted, on average, 80% of the soil acidity measured by the 3-d Ca(OH)2 incubation, which was considered the standard method for determining the LR of acid soils. The AE procedure gave a higher estimate than Ca(OH)2 incubation for those soils with a low LR, but for soils with LR > 4.5 Mg ha–1, AE gave lower LRs, when compared with the 3-d Ca(OH)2 incubation.
The lack of uniformity of agreement between the AE and the 3-d Ca(OH)2 incubation is apparently related to the fact that the LR from AE is based on a calibration with many Alabama soils (Adams and Evans, 1962), whereas the 3-d Ca(OH)2 incubation actually provides a direct measure of a soil's acidity. This was supported by the fact that the LR estimated from the linear extrapolation using three titration data points was in much better agreement with that measured by the Ca(OH)2 incubation across all 17 soils. Although only 80% of the LR from the incubation was estimated with the extrapolation of three data points, the fit in Fig. 3 was good (r2 = 0.96) and the intercept of the relationship was zero. The slope of the titration data also provides a measure of the soil's pH buffering capacity, which is a fundamental property of the soil. Whether this approach could be simplified further cannot be determined from this study, but it deserves further work.
Received for publication September 19, 2003.
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REFERENCES
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ABSTRACT
INTRODUCTION
