Mehlich and Modified Mehlich Buffer for Lime Requirement David H. Hardy Application and Principle The development of the Mehlich buffer was to estimate acidity actually affecting plant growth in North Carolina s mineral (sandy, low CEC coastal plains soils, clay piedmont soils) and organic soils (Histosols, mineral soils with Histic epipedons) in order to provide a lime requirement (LR). Earlier methods to measure acidity and estimate lime requirements for mineral soils were based on 1N KCl extraction of exchangeable Al +3 or soil solution Al +3 as relevant to soils with high OM contents (Evans and Kamprath, 1970; Kamprath, 1970). Hence, the Mehlich buffer method (Mehlich, 1976) was designed for high volume, routine soil testing labs where measurements by earlier methods were too tedious and time-consuming. The Mehlich buffer measures primarily exchangeable acidity (H + and Al +3 ) through displacement of acidic cations by Ba 2+ and NH 4+. Subsequent appearance of H + in solution occurs through direct displacement from the CEC or through exchange of Al +3 followed by hydrolysis. The underlying premise is that a decline in ph of the buffer is linearly related to exchangeable acidity. Since the ph is buffered at 6.6, some nonexchangeable acidity is assumed measured also. In his published work (Mehlich, 1976), Mehlich s measurement of buffer acidity (AC) was related to exchangeable acidity (ACe = Al 3+ and H + ) measured with an unbuffered neutral salt (BaCl 2 ) and residual acidity (ACr) measured after ACe, using BaCl 2 and triethanolamine at ph 8.25. The residual acidity is referred to as nonexchangeable acidity today. The relationships found between AC and ACe were: Mineral Soils ACe (meq / 100 cm3) = -0.54 + 0.96 AC r = 0.966, n = 91 Note: ACe ranges 0.3 to 9.2 meq ACe / 100 cm 3 soil. Histosol Soils ACe (meq / 100 cm3) = -7.54 + 1.6 AC r = 0.956, n = 100 Note: ACe ~ 0 when AC was < 4.8 meq / 100 cm 3 soil Lime response curves were developed using the above soil acidity relationships in relation to growth response of soybeans, maize, and barley exposed to varying levels of acidity. Mineral Soils LR (lb / acre) = 0.1 (AC) 2 + AC Note: For plants with slight to moderate tolerance for ACe with soil reaction in water < 5.8 to < 6.5. For plants more sensitive to ACe (soil reaction < 6.5), a multiplier of 1.5 was to be applied to the equation. Histosols or Mineral Soil with Histic Epipedon ( 20% OM)
LR (lb / acre 90% CCE) = [- 7.4 + 1.6 AC] 1.3 Note: For soil reaction in water < ph 5.2 to 5.5 and with soil weight per volume 0.75 to 0.95 g/cm3. When sandy, silt or clay texture soils were considered, use the equation for mineral soils. Of importance is to recognize that Mehlich specified no specific target ph in calculating lime requirements but made reference to ph mentioned in aforementioned equations as related to other knowledge about soil acidity in these soils as related to study in concomitant field and greenhouse studies. Before his death in 1984, Mehlich modified the above equations into one equation currently used by North Carolina to make lime recommendations with target ph based primarily upon three soil classes mineral, mineral-organic, and organic as presented in the Calculations section of this chapter. In 2005, the modified Mehlich buffer originated and is used today by soil testing programs in Virginia, Pennsylvania, and Maine. The Modified Mehlich buffer functions under the same principles, except calcium chloride is replaced by barium chloride in the buffer. The reason for this change is related to disposal of barium specific to state water quality regulation. Lime recommendations given by these states are based upon equations derived from lime incubation studies of their native soils (Wolf et al, 2008). Equipment and Apparatus 1. 2-mm sieve 2. Scoop, 10 cm 3 volume and leveling rod 3. Cup glass, plastic, or waxed paper; 50 ml or of similar size 4. Syringe pump or pipette; 10-mL volume 5. Stirring apparatus 6. ph meter. AC or battery operated with reproducibility to at least 0.05 ph units 7. ph electrode equipped with auto. Typically combination electrode with calomel reference electrode Reagents Mehlich Buffer 1. Sodium Glycerophosphate (Na 2 C 3 H 5 (OH) 2 PO 4 5 ½ H 2 O), fw 315.15; CAS# 1555-56-2
The national formulary quality has proven satisfactory and is less costly than the crystal Beta form used in the original method. Function: Serves as the major buffering component given its solubility over a wide range of soil ph. 2. Triethanolamine ([HOCH 2 CH 2 ]3N), fw 149.14; 1.117-1.125 g/ml density CAS# 102-71-6 Function: Serves in conjunction with acetic acid to extend the linear range of the buffer from 3.8 to 5.2. It is also used to adjust the final ph of the buffer if needed. 3. Glacial acetic acid, (CH 3 COOH), 99.5 %, 17.4 N CAS# 64-19-7 Function: Serves as a buffering agent over the range of 3.8 to 5.2, thus lowering ph and giving linearity to the unbuffered portion of sodium glycerophosphate within this ph range. It is also used to adjust ph the of final buffer solution if needed. 4. Ammonium chloride (NH 4 Cl), fw = 53.5, CAS# 12125-02-9 Function: Displaces exchangeable acidity and also reduces the ph of the unbuffered portion of sodium glycerophosphate. In the presence of sodium glycerophosphate, aluminum chloride serves to enhance the linearity over the ph range of 5.2 to 7.0. 5. Barium chloride dihydrate (BaCl 2 2H 2 O), fw = 244.32, CAS# 10326-27-9 Function: Supplements the ammonium chloride in the displacement and also serves as a preservative; it prevents bacterial and fungal if stored for long periods of time. 6. Aluminum chloride (AlCl 3 6H 2 O), fw = 241.48, CAS# 12125-02-9 Function: Used in the verification of the Mehlich buffer as explained below. 7. Hydrochloric acid, HCl, 36 %, 12.1 N, CAS# Function: Used in the verification of the Mehlich buffer as explained below. Mixture for 2-L of buffer solution Add about 1500 ml of distilled water to a 2-L volumetric flask. Add 5.0 ml of CH 3 COOH; add 9.0 ml of (HOCH 2 CH 2 )3N as 18 ml of a 1:1 aqueous solution for ease of delivery; add 86 g NH 4 Cl and 40 g of BaCl 2 2H 2 O. Mix these materials until in solution.
In a separate container, dissolve 36 g of Na 2 C 3 H 5 (OH) 2 PO 4 5 ½ H 2 O in about 400 ml of distilled water. After dissolution, add this solution to the 2-L flask containing the other ingredients. Allow the solution to cool and then make to final volume with distilled water. Verification of ph of final Mehlich buffer solution Calibrate ph meter with standard buffer solutions using standard ph buffers of ph 4.0 and 7.0. Dilute an aliquot of the Mehlich buffer solution with an equal volume of distilled water (10 ml Buffer + 10 ml d. water) and measure ph while stirring. The ph should be 6.60. Adjust for each 0.01 ph unit above 6.60 by adding CH 3 COOH crop-wise until desired ph is obtained. For each 0.01 ph unit below 6.60, add 1:1 aqueous (HOCH 2 CH 2 )3N drop-wise until desired ph is obtained. Verification Mehlich Buffer using Standard Acid The final concentration of the Mehlich buffer is verified by adding 10 ml of buffer with 10 ml of distilled water with 10 ml of 0.1 N HCl-AlCl 3 standard acid. The ph of this mixture should be 4.1 +/- 0.05. The 0.1 N HCl-AlCl 3 standard acid is prepared by mixing 0.05 N HCl with 0.05 N AlCl 3 6H 2 O which is prepared by dissolving 4.024 g AlCl 3 6H 2 O in 0.05 N HCl and mixing well. Modified Mehlich Buffer The only change is the substitution of BaCl 2 with CaCl 2 on a molar basis. Procedure 1. Scoop 10 cm 3 of air-dry, screened (2-mm) soil into a 50-mL cup. Add 10 ml distilled water and mix for 5 seconds. Wait 1-hr and read ph after calibrating meter with 4.0 and 7.0 standard buffer solutions. Add 10 ml of Mehlich buffer solution to cup and gently swirl 5 sec. Read ph of soil buffer mix (BpH) after sitting 30 min. Calculations 1. Calculate exchangeable acidity (AC) by the following formula: (6.6 BpH ) 0.25 = meq acidity / 100 cm 3 = AC where 6.6 is the ph of the Mehlich buffer alone. Note: For every 0.1 drop in BpH from 6.6, 0.4 meq AC / 100 cc is measured. Analytical Performance
Range and Sensitivity 1. The capacity of the Mehlich buffer ranges from 0 to 10.4 meq AC / 100 cm 3 soil, which is equivalent to 5.1 U.S tons of 90% CCE lime per acre. The sensitivity is 0.4 meq AC / 100 cm 3 soil. 2. Its use appears to be suitable on a wide range of soils of varying texture and organic matter. Precision and Accuracy 1. Soil buffer ph can be read to 0.01 ph unit. 2. Based on North Carolina s laboratory of four check soils being analyzed, the following data were attained. Check Soil Mean BpH Std. Deviation X 6.41 0.02 A 6.30 0.02 E 5.34 0.07 H 6.43 0.04 n = 63 Interpretation 1. Calculate the lime recommendation by the following formula: Lime recommendation (tons / acre) = AC [(target ph soil ph water ) / (6.6 target ph)] 2. In North Carolina, the lime recommendation is determined by the Mehlich buffer exchangeable AC in conjunction with the target ph for individual crops and / or specific soil class. The minimum lime recommendation given is 0.3 tons per acre. In North Carolina, desired target ph for mineral soils is crop specific (i.e. target ph = 6.0 for corn; target ph = 6.2 for cotton); for mineral-organic and organic soil classes, the target ph is 5.5 and 5.0, respectively, regardless of crop. For further explanation of soil class, refer to the chapter on colorimetric determination of humic matter within this publication or visit http://www.ncagr.gov/agronomi/obpart1.htm#sca. 3. The derivation of the factor that is multiplied by the AC value in the above equation is unknown. It is believed to be a compilation and of Mehlich s early findings in working with this buffer as noted in the principles and application section and Mehlich et al., 1976).
Calculations as supplied by VA are found at http://www.soiltest.vt.edu/pdf/recommendation_guidebook.pdf. Effects of Storage 1. Air-dry soil can be stored indefinitely without affecting Mehlich buffer ph measurement. The Mehlich buffer solution itself if made with BaCl 2 has an indefinite storage period. The modified Mehlich buffer solution made with CaCl 2 when un-refrigerated has a shelf life of about 1 to 2 weeks due to growth of bacteria or fungi depending on lab temperature and light conditions. Safety and Disposal 1. When used with proper precaution and personal protection equipment (lab coat, gloves, and eye protection), there is no special safety concern with either buffer or chemical components. In some states, there may be concerns over barium waste. References Evans, C. E. and E. J. Kamprath. 1970. Lime response as related to percent Al saturation, solution Al, and organic matter content. Soil Sci. Soc. Proc. 29:893-896. Kamprath, E. J. 1970. Exchangeable aluminum as a criterion for liming leached mineral soils. Soil Sci. Soc. Proc. 34 252-254. Mehlich, A. 1976. New buffer ph method for rapid determination of exchangeable acidity and lime requirement of soil. Commun Soil Sci Plant Anal 7(7):637-652. Mehlich A, Bowling SS, Hatfield AL. 1976. Buffer ph acidity in relation to nature of soil acidity and expression of lime requirement. Commun Soil Sci Plant Anal 7(3):253 263.