Comparison of a combination pH electrode and field effect transistor electrode for the determination of soil pH and lime requirement

Abstract

A commercial laboratory robot has been used to compare the responses of two different types of electrode for the determination of soil in water pH (pH_w), and soil in SMP buffer solution pH (pH_SMP). The latter is integral to a popular method for the determination of soil lime requirement. A combination pH electrode is compared with a solid state, ion selective field effect transistor electrode. Response times and hysteresis effects are discussed.     

Evaluation of lime requirement methods for Delaware soils

Abstract

Rapid, accurate identification of the lime required to attain a desired pH is essential for the coarse‐textured soils of the Atlantic coastal plain to avoid micronutrient deficiencies (Mn, Zn) in sensitive crops and to insure herbicide efficacy. The University of Delaware Soil Testing laboratory is one of only seven of the 25 states in the Northeastern and Southern regions that does not use a buffer solution to make lime requirement determinations. The present method bases lime recommendations on soil pH in water, combined with an estimate of buffering capacity obtained by hand texturing soils. This approach is time‐consuming and includes the potential for considerable operator variability in obtaining the textural estimate. A study was initiated to compare four buffer solutions (Adams‐Evans, Mehlich, SMP‐single buffer, SMP‐double buffer) with the current approach and the actual lime requirement as determined by incubation of 19 Delaware soils with six rates of CaCO₃ for six months. Soil pH effects on Mn and Zn availability were determined by extraction of all samples from the incubation study with the Mehlich I (.05M HCl + .0125M H₂SO4) soil testing solution. Results indicated that organic matter was the primary soil component responsible for pH buffering in Delaware soils, and that the Adams‐Evans or Mehlich buffers were the best predictors of actual lime requirement. The appropriate target pH range for the coarse‐textured soils of Delaware, based on Mn and Zn availability, was determined to be 5.5–6.0. Liming soils to pH values greater than 6.0 is, for most crops, unnecessary and will reduce Mn availability below critical levels for sensitive crops such as soybeans and small grains.     

Evaluation of Four Buffer Solutions for Determining the Lime Requirement for Ohio Soils

Lime is used as a soil amendment to achieve the optimum pH suitable for good crop growth. Buffer pH (BpH) measurements have been calibrated to relate the linear drop in pH of the soil–buffer system to the amount of lime needed to neutralize soil to a certain pH level. The amount of lime required to neutralize soil acidity, called the lime requirement (LR), is obtained from soil–limestone (CaCO₃) incubations. In this study, 13 soils from Ohio were incubated with CaCO₃ for a period of 1 month to determine the LR to achieve different target pHs. This LR was then regressed with the different BpHs of four buffer solutions [(1) Shoemaker, McLean, and Pratt (SMP), (2) Sikora, (3) Mehlich, and (4) modified Mehlich] to obtain calibration equations. The Sikora and modified Mehlich buffers are variations of the SMP and Mehlich buffers, respectively, but they are designed to promote buffering without use of any hazardous constituents [i.e., chromium(VI) in SMP buffer and barium in the Mehlich buffer]. This study was done to verify the applicability of the buffers that do not contain any hazardous constituents and to calibrate these buffers for predicting lime requirement needs for Ohio soils. Comparing the calibrated equations of the SMP and Sikora buffers with CaCO₃‐incubation LR recommendations revealed that the SMP and Sikora buffer solutions were not significantly different, and a single calibrated equation can be used for these two buffers to determine LR predictions in Ohio. The Mehlich and modified Mehlich calibration equations differed significantly from the SMP calibration equations and were not as highly correlated with CaCO₃‐incubation LR recommendations using a linear model (r² < 0.54). Thus, it is possible to use the Mehlich and modified Mehlich for determining lime recommendations, but they require a correction factor such as inclusion of the initial soil pH to improve the precision of the LR prediction. We also found the various buffers tested in this study were better able to predict LR rates for greater LR soils than low LR soils. In conclusion, successful laboratory tests to predict LR for Ohio soils are possible using alternative buffers that do not contain hazardous constituents.     

Comparison of Shoemaker–McLean–Pratt and Modified Mehlich Buffer Tests for Lime Requirement on Pennsylvania Soils

Abstract

The Shoemaker–McLean–Pratt (SMP) buffer test is commonly used in Pennsylvania and throughout the United States to determine the lime requirement (LR) of acid soils. The buffer contains potassium chromate, a carcinogen, and all waste must be collected for disposal in a hazardous waste facility. An alternative to the SMP buffer is the Mehlich buffer. Although the Mehlich buffer contains barium chloride (BaCl₂), also a hazardous and regulated compound, calcium chloride (CaCl₂) has been shown to be an effective substitute. The goal of this study was to compare the SMP buffer and the modified Mehlich buffer (CaCl₂ substituted for BaCl₂) for estimating LR on PA soils and to determine if the modified Mehlich buffer could provide an effective alternative to the SMP test. Twenty‐two agriculturally important Pennsylvania soils with pH values ranging from 4.5 to 6.4 were collected, and the actual LR of each soil was determined by incubating soils for 3 months with calcium carbonate. The modified Mehlich buffer was a more accurate predictor of the lime required to raise soils to either pH 6.5 (r²=0.92) or 7.0 (r²=0.87) in comparison to the SMP buffer (r²=0.87 and 0.82, respectively). Comparison of calibration equations for Mehlich buffer versus lime requirement derived in this study were similar to those developed on soils from other states and geographic regions.     

Principles underlying the practice of determining lime requirements of acid soils by use of buffer methods

Abstract

A buffer is generally a mixture of a weak acid and a salt of the same weak acid. Hence it can neutralize both acids and bases, and thus resists marked changes in pH of a system. Yet systematic change in pH of a buffer caused by addition of an acidic substance can be used to indicate the total acidity represented by the change in buffer pH. Since acid soil is itself a buffer, when it is added to a buffer mixture for the purpose of measuring its acidity or lime requirement (LR), the resulting double‐buffer suspension (soil‐buffer) is a relatively complex system. Much of the complication in interpreting the changes in buffer pH brought about by mixing soil and buffer stems from the facts: i) that much of the acidity is pH‐dependent, and ii) that quick‐test methodology involves reaction of only a fraction of the total soil acidity with the buffer. Marked change in relative amounts of H ions dissociating from the soil‐SMP‐buffer system at soil‐buffer pH 6.9 and above accounts for relatively wide variations between buffer‐indicated and CaCO₃ incubation‐measured LR of low LR soils. Similarly, decreased reactivity of H⁺ in high organic matter soils and increased reactivity of H in acid‐leached soils cause errors in buffer‐indicated LR. Awareness of these principles helps avoid pitfalls of existing buffer methods, and has led to incorporation of the double‐buffer feature for improving the SMP method.     

Improved SMP buffer method for determining lime requirements of acid soils

Abstract

Single buffer‐two pH and two‐buffer adaptations were compared as double buffer features of the SMP method using a group of 54 soils of wide range in lime requirement (LR). Data from both methods were highly correlated both with each other and with Ca(OH)₂‐titrated acidity.

Formulas for LR based on the schematics of similar triangles relating differences in measured pH vs corresponding acidities for the double buffer system were developed. A regression equation relating buffer‐indicated LR and Ca(OH)₂ titrated acidity was used to adjust the quick‐test double buffer‐indicated values to levels nearer the actual ones. A recommended SMP double buffer procedure, and a formula for computing LR from soil‐buffer pH's measured by the double buffer, quick‐test method are presented.     

Effect of calcium carbonate as determined by lime requirement buffer pH methods on soil characteristics and yield of sorghum plants

Abstract

A primary limit to crop production in extended regions of northern Greece is the infertility of acid soils, especially nutrient element unavailability or toxicity. An experiment was conducted to determine under greenhouse conditions which buffer pH method selected in a previous laboratory experiment is best suited to predict the lime requirement (LR) of acid soils which is most appropriate in relation to plant growth and nutrient element uptake of sorghum plants. The lime needs of three naturally occurring acid soils were estimated by three methods: Adams‐Evans (AE), Shoemaker‐McLean‐Pratt single buffer (SMP‐SB), New Woodruff (NWOOD), and the calcium hydroxide [Ca(OH)₂] equilibration procedure. Two greenhouse experiments were conducted: (i) Experiment I during the 1996 season with ORESTIAS a sandy loam soil, and (ii) Experiment II during 1997 and 1998 season with XANTHI a loam sandy soil and DRAMA a sandy clay loam soil, respectively. The following results were obtained. Differences were noticed between the soils as well as within the LR methods. The ORESTIAS soil needed 63% more calcium carbonate (CaCO₃) than the XANTHI soil and 70% more than DRAMA soil to achieve the target pH. Among the three LR methods, results showed that in two of the three soils the highest LRs were determined by the NWOOD and the lowest by the Ca(OH)₂ methods. After six weeks of incubation, no one method gave exactly the needed amounts of CaCO to achieve the target pH, the estimated amounts being mostly higher than tiiat needed except for the DRAMA soil. Among the methods, in general the SMP‐SB method predicted lime rates that raised the soil pH nearest to the target pH and the NWOOD soil seemed to be the more consistent for the three soils. The smallest LRs were predicted by the Ca(OH) method. Based upon plant production and nitrogen (N) uptake in the 1996 season, the shoot yields were significantly higher using the SMP‐SB method and lower with the NWOOD method. Similar results were obtained for the XANTHI and DRAMA soils during the 1997 season. On the contrary in the 1998 season (2nd experimental year), the highest yields were obtained with the NWOOD buffer method. For the 1996 and 1997 seasons, tissue N concentrations were partly significantly higher using the SMP‐SB method. In the 1998 vegetation period, the N concentrations were low and the control plants had significantly higher N contents.     

A Modification to the Adams‐Evans Soil Buffer Determination Solution

Abstract

Many soil analysis labs routinely determine lime requirement of acidic soils using different buffer solutions for optimum plant growth. The Adams‐Evans lime determination solution was introduced more than 40 years ago and has been used by many soil analysis labs. Even though many buffer solutions have been developed since then, very little attention has been paid to address the toxic nature of chemicals involved in buffer solutions. The most commonly used buffer solutions, such as the Adams-Evans, Shoemaker‐McLean‐Pratt (SMP), Woodruff, and others, contain p‐nitrophenol, which is toxic to humans and the environment. Use of p‐nitrophenol requires prescribed containment and disposal procedures, that creates extra burden on soil analysis labs that provide their invaluable service at low cost. Replacing p‐nitrophenol with monobasic potassium phosphate (KH₂PO₄), which has similar buffering capacity but with no known toxicity, is beneficial to soil testing labs and the environment. The original Adams‐Evans buffer solution was compared with the modified Adams‐Evans buffer solution with soils of different pH, cation exchange capacity and lime requirement. The linear regression between the buffer pH values and lime recommendations made by Adams‐Evans and the modified Adams‐Evans solutions were highly significant. Thus, the modified Adams‐Evans buffer solution can be used without loss of established recommendation criteria as the original buffer solution.     

Performance characteristics in the determination of soil‐water pH and soil‐SMP buffer pH using a flow‐through junction combination electrode

Abstract

A laboratory robot has been used to assess the performance of a commercially available flow‐through junction pH electrode. The electrode was tested for accuracy and precision in the determination of soil‐water pH (pH_w) and the Shoemaker, McLean, and Pratt buffer solution pH (pH_SMP). The significance of the pH_SMP determination lies in subsequent calculations of soil lime requirement (LR). Response times and hysteresis effects were compared under different operating conditions.     

Calcium requirements of citrus

Abstract

A close relationship was found between the pH of soil suspensions in the SMP buffer solution (pH_smp) and the potential acidity of soils (H + Al) extracted by a neutral calcium acetate solution (r = 0.98), for twenty six soil samples of the State of Sao Paulo, Brazil, This relationship was represented by the equation lnY = 7.76 ‐ 1.053X, which allowed for the calculation of H + Al directly from the values of pH_smp.

With the values of H + Al and the sum of bases, calcium, magnesium and potassium, the cation exchange capacity (CEC), and the base saturation (V) were calculated. Relationships between the base saturation of the soils and the active acidity of soil suspensions were close, both for pH determined in water (r=0.94) and pH determined in 0.01M CaCl₂ solution (r ‐ 0.97). Thus the lime requirement (LR) of soils could be calculated, for given values of pH or base saturation, using the equation LR = CEC (V₂ ‐ V₁)/100, in which V₁ is the base saturation of the soil and V₂ is the expected value upon liming.

The predicted values for lime required to increase the soil pH in water to either 5.5 or 6.0 were comparable to those obtained by the direct use of the SMP buffer method, and were, respectively, two and four times higher than the amounts required to neutralize exchangeable aluminum, considering the criterion LR = Al × 1.5.

The proposed method to determine lime requirement of soils is described in detail and the advantages of its use are discussed.     

Comparative study of selected lime requirement methods for some acid Ghanaian soils

Abstract

The acid soils of the western region of Ghana which hitherto have been relegated to forest and tree crops production are becoming increasingly important for agricultural food crop production in the country. However, on account of their strongly acidic properties, there is the need to apply agricultural lime to the soils to improve upon their productivity. At present, however, information on the lime requirement and appropriate liming practice for these soils is lacking. The objective of this study was to compare the suitability of selected chemical methods for the determination of the lime requirement to predict lime needs of these naturally occurring acid soils. The lime requirement of six acid soils were determined by calcium hydroxide [Ca(OH)₂] titration, exchangeable aluminum (Al), and Shoemaker, McLean, and Pratt (SMP) buffer methods. Correlation analysis showed that all the methods were highly correlated with one another. The SMP method was found to be somewhat better than either exchangeable Al or Ca(OH)₂ titration method for estimating the lime requirement of the soils. Hence, the SMP method is recommended for use as the diagnostic index of lime requirement of these soils because of its speed and simplicity. Regression studies on the lime requirement values by the three methods and selected soil properties showed that exchangeable Al and organic carbon were the most important soil factors contributing to the lime requirement of these soils. Clay content was significantly correlated only with the Ca(OH)₂‐based lime requirement values (r = 0.81*).     

Modification of the Mehlich Lime Buffer Test

Routine use of the Shoemaker, McLean, and Pratt (SMP) lime buffer method resulted in chronic problems with electrode reference junction degradation, stability of readings relative to contact time, and generation of hazardous waste. During method recalibration, the Mehlich buffer was chosen for parallel evaluation with SMP, with the goal of improving method performance and eliminating of hazardous waste. The Mehlich buffer was modified by substituting calcium (Ca) for barium (Ba). The modified Mehlich (MM) buffer was found to be identical to the original with respect to buffering power and linearity over an extended pH range of 3.0–6.6. Seven agronomic soils were incubated with eight rates of calcium carbonate for 90 days. Regression analysis was performed to predict lime requirement (LR) to several target pH levels, based on pH measured in each buffer alone or in both water and buffer. Slightly better predictability was obtained using multiple regressions, with R²>0.95 in all cases. Significant but minor differences occurred between the newly calibrated buffers in extended comparisons. The MM buffer was superior to the SMP during routine usage, with fewer adverse effects on electrodes and the elimination of hazardous waste.     

Lime requirement determination of tropical peat soil using buffer‐pH methods

Abstract

A study was conducted to calibrate and evaluate five buffers for the lime requirement (LR) determination of tropical peat soil. The buffers tested were the Shoemaker‐McLean‐Pratt (SMP); Mehlich; 0.1M ammonium acetate (NH₄OAc); 0.1M barium acetate [(Ba(OAc)₂]; and 0.1M calcium acetate [Ca(OAc)₂]. Calibration was done by comparing the precision of linear regression equations adjusted to the relationships between the LR rates required to achieve pH 5.0 measured in a 1: 4 (soiltwater) ratio as determined by incubation and soil‐buffer pH values. Incubation LR using calcium carbonate (CaCO₃) to achieve pH 5.0 by peat soil was utilized to calibrate each buffer. Evaluation was carried out by assessing the LR from the calibrated buffers which estimate the LR nearest to the target pH of 5.0. The calibration study showed that the SMP and Mehlich buffers were less precise than the Ba(OAc)₂, NH₄OAc, and Ca(OAc)₂ buffers. The evaluation study indicated that the Ba(OAc)₂ buffer is the most accurate, followed by NH₄OAc and Ca(OAc)₂ buffers. The Ba(OAc)₂ buffer method is recommended for LR determination of tropical peat soil and NEUOAc or Ca(OAc)₂ as an alternative method.     

Calibration of the single‐ and double‐buffer SMP lime requirement methods by root elongation bioassay

Abstract

Chemical and biological lime requirement (LR) reference values of 154 soils were obtained by six months of incubation of each soil with five levels of calcium carbonate (CaCO₃). Levels of CaCO₃ addition differed among soils according to their characteristics. Chemical LR values were based on the individual neutralization curves to achieve a desired pH (pH_d) value of 5.5, 6.0, and 6.5 in water or 5.0, 5.5, and 6.0 in 0.01M calcium chloride (CaCl₂). Biological LR values were estimated to achieve 90% relative root elongation on each soil after a growth period of 48 h using wheat cv Abe. Chemical values of LR suggest that SMP method is valid for a wide range of mineral soils from different geographic regions. However, the proportion of soil acidity reacting with the buffer is not constant. Results indicate that values obtained with the routine methods need to be calibrated with equations different from the originals. The use of curvilinear models to adjust one single pH of the soil‐buffer system improved substantially its accuracy, allowing the single‐buffer (SB) results to be comparable to the more time consuming and labourious double‐buffer (DB) technique. No advantage was noticed with the use of curvilinear equations for DB technique. The adoption of pH_d in 0.01M CaCl₂ leads to an increase of precision of the predicted LR. Regression equations are provided for calculating LR rates to different pH_d values. Accuracy is high (r²=0.887) even for pH values (5.0 in 0.01M CaCl₂) lower than normally considered in methods based on buffer solutions. An overestimation of biological LR values was observed with both SMP methods. Notwithstanding, a calibration can be also made with the root bioassay, adjusting the chemical values to lime rates based on biological constraints related with aluminum (Al) toxicity. Regression equations are provided. Once more, the use of quadratic model for SB method allows an accuracy (r²=0.836) comparable with the DB technique (r²=0.850).     

Lime requirement for agricultural soils: A review of the current and potential methods for routine use

Liming is one of the key agronomic practices to improve crop yields in acid soils because, among other things, it reduces aluminum toxicity and creates favorable conditions for crop growth. For an effective liming program, the methods to determine lime requirement should be as precise as possible. This paper reviews the existing lime requirement methods and discusses the potential of a new one suitable for routine use in the laboratory to test most agricultural soils. The most widely used lime requirement methods can be categorized into four groups: titration, incubation, buffer, and field methods. Other methods such as spectroscopy method or the use of empirical equations have also been adopted. Although some methods are highly reliable, they are not optimal for routine use because they are inconvenient during the laboratory procedures or cannot be validated for all conditions. Based on the linearity between soil pH and the added base in the pH range from 4.5–6.5 in most agricultural soils, a titration-based method on 1:1 soil:0.01 M CaCl₂ slurry of a single sample appears to be a promising candidate for routine use. In further studies, this generally applicable method should be evaluated to provide a better comparison to established methods for lime requirement determination.     

New buffer pH method for rapid estimation of exchangeable acidity and lime requirement of soils

Abstract

A new buffer pH method (BpH) for the rapid estimation of unbuffered salt‐exchangeable acidity (ACe) and lime requirement (LR) has been developed. The buffer reagent, consisting of sodium glycerophosphate, acetic acid, trletlianolamine, ammonium chloride and barium chloride, was useful within the pH range 3.8 to 6.6. Delta values from BpH were converted into buffer pH acidity values (AC) and calibrated against ACe of 91 mineral soils and 100 acid Histosols. The correlation coefficients between AC and ACe were 0.966 and 0.956 for the mineral soils and Histosols, respectively. The corresponding regression equations in terms of meq/100 cm were ACe ‐ ‐0.54 + 0.96 AC and ACe = ‐7.4 + 1.6 AC for mineral soils and Histosols, respectively.

To predict lime requirement of mineral soils a curvilinear equation was required. The equation, LR in meq CaCO₃/100 cm³ = 0.1 (AC)² + AC, was tested successfully against rates of lime carried out under laboratory conditions and against crop response in the greenhouse. Field studies on acid Histosols with maize and soybeans showed optimum yield when the rate of lime added was approximately equivalent to ACe.     

Evaluation of liming and magnesium materials for increasing soil pH and available magnesium

Abstract

Four liming and Mg materials were compared in a greenhouse experiment with soybeans for their ability to raise soil pH, supply Mg, and their effect on the availability of Mn, Cu, Fe, and Zn. Three materials were added at rates of 0, 1, and 2 times the lime requirement, calcitic lime, dolomitic lime, and Hydra‐Mag (an industrial by‐product containing 20% Mg). Sul‐Po‐Mag was the fourth material added as a plus Mg check at a rate based on an equivalent amount of Mg to that supplied by Hydra‐Mag. Plant growth, plant tissue element content and extractable soil elements were determined after growing the soybeans for 5 weeks. Plants in treatments where no lime/Mg materials were added were very small due possibly to Mg deficiency and Al toxicity. The 1 and 2 times rates of the materials gave about equal growth except that the high Sul‐Po‐Mag rate caused salt injury. Hydra‐Mag increased soil pH more than calcitic lime which increased soil pH more than dolomitic lime. Soil and plant Mg levels were increased more by Hydra‐Mag than dolomitic lime when applied at equivalent rates based on the lime requirement. Dolomitic lime gave very good plant growth indicating that it made adequate amounts of Mg available. Hydra‐Mag reduced plant and extractable soil Zn, Cu, and Fe but no more so than calcitic or dolomitic lime. Hydra‐Mag reduced plant Mn more than for the other limes.     

Implementation of Soil Lime Requirement by a Single‐Addition Titration Method

Abstract

Buffers for determining a soil's lime requirement (LR) sometimes contain hazardous chemicals. Our objective was to implement a single‐addition titration with calcium hydroxide [Ca(OH)₂] to determine the LR of soils. The soil pH buffering capacity is calculated from the rise in pH from a single addition of base. The LR is calculated from the soil pH buffering capacity, the target pH, and initial soil pH. The LR of 531 randomly selected client samples determined by single‐addition titration were slightly higher than by the Adams–Evans (AE) buffer procedure when LRs were less than 1800 lb per acre. The new procedure recommended about 11% less lime than AE at LRs greater than 1800 lb per acre. Independent evaluations of samples that gave the most widely different LR revealed that the single‐addition titration was more accurate and more precise than the AE buffer.     

A comparison of lime requirements by five methods on grassland mineral soils in Ireland

Liming is necessary for good nutrient availability and crop growth. Lime use in Ireland is now the lowest in half a century. A recent study shows that grassland mineral soils in Ireland has a mean pH of 5.4 and mean lime requirement (LR) of 9.3 t/ha ground limestone. There have been a number of studies in the USA to re-evaluate LR, but little activity in the European Union (EU) in recent years. The primary aim of our research was to compare five methods for estimating LR, which included the Shoemaker–McLean–Pratt (SMP) buffer method currently used in Ireland (IRL), the Sikora buffer method used at the University of Kentucky (UKY), Ca(OH)₂ titration used at University of Georgia (UGA), the modified Mehlich buffer method used at Penn State University (PSU) and the UK RothLime model, using 57 representative grassland mineral soils from Ireland with a pH range from 4.8 to 6.6. The secondary aim was to explore an alternative to the SMP buffer that does not involve the use of toxic chemicals. The results show good agreement between the pH measured by the Irish and three US laboratories and reasonably good agreement in LR estimated by five methods. The main conclusions are: (1) a significant proportion of grassland on mineral soils in Ireland would benefit from liming to increase soil pH, (2) on average, LRs as recommended in Ireland are higher than those advised elsewhere , ( 3) the target pH in Ireland is high compared with that in other countries and should be reduced from pH 6.5 to 6.2, (4) the SMP buffer method should be replaced by a suitable alternative and, in principle, any of the four methods studied would be suitable, (5) to find the most suitable alternative for accurate LR advice it would be necessary to compare the different methods to the actual LR from incubation of representative soils with calcium hydroxide.     

Development of buffer methods and evaluation of pedotransfer functions to estimate pH buffer capacity of highly weathered soils

The pH buffer capacity of a soil (pHBC) determines the amount of lime required to raise the pH of the soil layer from its initial acid condition to an optimal pH for plant growth and the time available under current net acid addition rate (NAAR) until the soil layer acidifies to a critical pH leading to likely production losses. Accurate values of pHBC can also be used to calculate NAAR from observed changes in soil pH. In spite of its importance, there is a critical shortage of pHBC data, likely due to the long period of time needed for its direct measurement. This work aimed to develop quick, simple and reliable methods of pHBC measurement and to test these methods against a slow (7‐day) titration used as benchmark. The method developed here calculates pHBC directly from the pH buffer capacity of the buffer solution and the increase in soil pH and corresponding decrease in pH of the buffer solution following mixing and equilibration. The pHBC values calculated using Adams and Evans or modified Woodruff buffers were in accord with those measured by slow titration. Buffer methods are easily deployed in commercial and research laboratories as well as in the field. The advantage of using buffer solutions to calculate pHBC instead of lime requirement is the broad application of this soil property. The pHBC of a soil is an intrinsic property that would not be expected to need remeasurement over periods of less than decades. Recurring lime requirement can be calculated from the soil's pHBC, initial and target pH values. A large proportion of the variability in pHBC was explained by the soil organic carbon content. This relationship between pHBC and soil organic carbon content allowed us to develop local pedotransfer functions to estimate pHBC for different regions of Australia.