The Mehlich buffer method for determining exchangeable acidity (Ac ) was developed under the premise that an increase or decrease in exchangeable acidity can be quantified by measuring the corresponding increase or decrease in the pH of a buffered reagent.
In order for this method to accurately assess quantity of acidity, the buffer must be linear over a wide range in exchangeable acidity values. Linearity of the Mehlich buffer was indeed confirmed by titration with a standard acid. The standard acid was composed of a mixture of
0.05N HCl and 0.05N AlCl3 • 6H2O,
which equates to a 0.1N solution of the composite acid mixture. The standard acid mixture contains 50% H+ and 50% Al+3 and is designed to measure the predominant forms of acidity common in North Carolina soils. Research shows that aluminum (Al+3) and hydrogen (H+) ions are the predominant forms of acidity in mineral and organic soils, respectively.
The buffer reagent should also have a capacity that is not influenced by the soil yet is sensitive enough to measure low acidity values common in soils with a low cation exchange capacity (CEC), that is, with values less than 5.0 meq/100 cm3. The same requirement was sought in the Adams-Evans buffer (Adams and Evans, 1962).
The capacity of the Mehlich buffer method ranges from zero to 10.4 meq of acidity/100 cm3 of soil. This range is equivalent to 10.4 metric tons/hectare of pure calcium carbonate or 5.1 U.S. tons/acre of lime with a 90% calcium carbonate equivalent. The buffer ranges from pH 6.6 (no acidity) to pH 4.0, which is equivalent to 10.4 meq of acidity/100 cm3 of soil.
The buffer pH range (4.0 to 6.6) contains 25 one-tenth pH units, which equates to 0.4 meq acidity/100 cm3 (25 x 0.4 meq Ac per tenth unit buffer-pH depression). This, in turn, equals 10.4 meq of total acidity over the entire buffer pH range from 4.0 to 6.6. Given the sensitivity of 0.4 meq Ac /100 cm3, the BpH depression method can measure as little as 0.4 metric ton per hectare or 356 pounds per acre (based on 100% CaCO3).
The buffer reagent is composed of
- sodium glycerophosphate (C3H5(OH)2PO4 Na2 • 2.5H2O), fw 216.05;
- triethanolamine (TEA), 1.117 to 1.125 g/mL density;
- glacial acetic acid (CH3COOH), 17.4N (99.5%);
- ammonium chloride (NH4Cl), fw 53.5; and
- barium chloride (BaCl2 • 2H2O), fw 244.32.
Functions of buffer reagents
Sodium glycerophosphate is used as the major buffering component because it is soluble over a wide range of pH.
Ammonium chloride functions in the displacement of exchangeable acidity. It is similar in this regard to Normal KCl-extractable acidity. Ammonium chloride also reduces the pH of the unbuffered portion of sodium glycerophosphate. In the presence of sodium glycerophosphate, Normal NH4Cl has a well-defined pH range from 5.2 to 7.0. The NH4Cl gives linearity to sodium glycerophosphate within this pH range.
Barium chloride supplements ammonium chloride in displacing exchangeable acidity. It also serves as a preservative against fungal growth during prolonged storage periods of the buffer reagent. However, during periods of high sample volume, the frequency of buffer preparation eliminates any concern of fungal growth.
Acetic acid functions as a buffer within the pH range of 3.8 to 5.2. It reduces the pH and provides linearity to the unbuffered portion of sodium glycerophosphate within this pH range. Acetic acid is also used to adjust the final buffer pH to 6.6 in the initial buffer preparation (see Preparation of the buffer reagent).
Triethanolamine (TEA) is used in conjunction with acetic acid to extend the linear range of the buffer from pH 3.5 to 5.2. TEA is also used to adjust the buffer pH to 6.6 during initial buffer preparation.
Preparation of the buffer reagent
To prepare 2 liters of buffer reagent, mix the following materials in a 2-liter volumetric flask until they are in solution:
- 1500 mL of distilled water,
- 5.0 mL of glacial acetic acid,
- 9.0 mL of triethanolamine (for ease of delivery, add 18 mL of 1:1 aqueous mixture),
- 86 g NH4Cl and 40 g BaCl2 • 2H2O.
In a separate container, dissolve 36 g of sodium glycerophosphate in 400 mL of distilled water. Add this solution to the 2-liter volumetric flask. Allow the mixture to cool, then fill to volume with distilled water. This volume is enough to analyze about 200 samples.
At this point, check the pH of the buffer reagent. Mix equal amounts of buffer solution and distilled water (e.g., 10 mL buffer solution and 10 mL distilled water). The pH should read 6.6 ±0.04. If the pH is greater than 6.64, add acetic acid by the drop to the original buffer solution and mix. Retest the pH as described until it is 6.6. If the pH is less than 6.56, add 1:1 aqueous TEA by the drop to the original buffer solution. Retest the pH, and repeat the process until the desired pH is achieved.
It is also a good idea to check the buffer against a standard acid just to make sure that appropriate amounts of all the ingredients were added. The standard acid is a mixture of 0.05N HCl + 0.05N AlCl3 • 6H2O (total = 0.1N). Prepare the standard acid by dissolving 4.024 g of aluminum chloride in 0.05N HCl. Mix well before using. Use the standard acid to check the final concentration of the buffer mixture as follows: combine 10 mL of buffer, 10 mL of distilled water and 10 mL of the prepared standard acid. The pH of this mixture should be 4.1 ±0.05. If the pH is not within these limits, check preparation of the buffer reagent to make sure all ingredients were added.
Laboratory procedure for determining buffer acidity
- Measure 10 cm3 soil (screened to a particle size of less than 2 mm) into a 50-mL,
wax-coated paper cup or other appropriate container.
- Add 10 mL distilled water, mix and allow to stand for at least 1 hour.
- Read soil pH and record as water pH.
- Add 10 mL of buffer solution to the soil-water mixture with enough force to mix.
- Let stand for 30 minutes.
- Read pH while stirring and record as buffer pH (BpH).
- Calculate the exchangeable acidity.
Calculating exchangeable acidity
meq Ac ÷ 100 cm3 = (6.6 – BpH) ÷ 0.25
- 6.6 = pH of the buffer and
- BpH = pH of the soil and buffer mixture.
Example: Assume a soil-buffer mixture has a pH of 4.1, i.e. BpH = 4.1. The exchangeable acidity is calculated as follows:
(6.6 – 4.1) ÷ 0.25 = 10 meq Ac ÷ 100 cm3
The lime requirement for North Carolina soils is determined by a combination of Mehlich-buffer exchangeable acidity (Ac), the target pH for the crop and/or the optimal soil pH based on soil class. The target pH by soil class is 6.0 for mineral soils, 5.5 for mineral-organic soils and 5.0 for organic soils. Factors used for reducing the lime rates based on the target pH are shown in Table 1. Lime rates calculated from this table are based on 90% calcium carbonate equivalent (CCE).
North Carolina lime recommendations assume use of liming materials that have a 90% CCE and that meet the size specifications outlined in Table 2.
Table 3 shows relationships among BpH, exchangeable acidity and lime equivalents.
Adams F, Evans CE. 1962. A rapid method for measuring lime requirement of red-yellow podzolic soils. Soil Sci Soc Proc 26: 355–7.
Hardy DH, Tucker MR, Stokes CE. 2012. Crop fertilization based on North Carolina soil tests. Raleigh (NC): North Carolina Department of Agriculture and Consumer Services, Agronomic Division. Agronomic Division Circular No. 1.
Mehlich A. 1976. New buffer method for rapid estimation of exchangeable acidity and lime requirement. Commun Soil Sci Plant Anal 7(7): 637–52.
Van Lierop W. 1990. Soil pH and lime requirement determination. In: Westerman RL, editor. Soil testing and plant analysis. Madison (WI): Soil Science Society of America. p 73–126. SSSA Book Series 3.