Walkley-Black Method
Rao Mylavarapu
Application and Principle
The Walkley Black (WB) method determines Soil Organic Matter (SOM) by
quantifying the amount of oxidizable soil carbon as determined by the reaction with
acidic dichromate (Cr2O72-). The oxidation step is then followed by titration of the excess
dichromate solution with ferrous sulfate. The SOM is calculated using the difference
between the total volume of dichromate added and the amount of unreacted dichromate
determined through titration with ferrous sulfate after the reaction. The method is based
on upon that described previously by Mebius (1960) and is an estimate since not all
organic matter is oxidized. Problems with this procedure include excessive organic matter
in the soil (the limit for this procedure is approximately 6%) and difficulty with the end
point determination, which can be found in dark-colored soil solutions. The use of a
lighted stir plate can be of assistance in the end-point determination. The WB procedure
also results in production of chromate, which is categorized as a hazardous chemical and
requires regulated disposal in compliance with USEPA regulations.
Equipment and Apparatus
1. Analytical balance, resolution ± 0.01 g
2. 250-mL wide mouth graduated Erlenmeyer flask
3. Fume Hood
4. Titration stand and burette
5. Stir plate with light
6. Stirring rods
7. Weighing vessel
Reagents
1. Deionized water
2. 0.16M Potassium dichromate (K2Cr2O7)
3. 1.0M Ferrous Sulfate (FeSO4 • 7H2O)
4. 1, 10-Phenathroline Ferrous Sulfate complex
5. Concentrated Sulfuric Acid (H2SO4)
6. 0.16M Potassium dichromate (K2Cr2O7), 1N - Dissolve 98.08 g of ovendried/desiccated Potassium dichromate in approximately 1500 mL of pure water
and dilute to 2 L. After preparation of this solution, transfer to a clean glass bottle
for use with a repipetter. Do not mix old Potassium dichromate solution with the
new solution.
7. 1.0M Ferrous Sulfate (FeSO4.7H20), 0.5N - Dissolve 278.02 g of Ferrous Sulfate
in approximately 1500 mL of pure water. Carefully add 30 mL of concentrated
Sulfuric Acid, mix, cool, and dilute to 2 L. After preparation, this solution may be
transferred to a clean 8-L plastic carboy. Do not mix old Ferrous Sulfate solution
with the new solution. The tubing, stopcock, and attachments to the burette should
be rinsed three times with new Ferrous Sulfate solution before titrating any blanks
or samples. Prepare a new solution every 30 days.
Procedure
1. Titrate two blank samples (no soil) before proceeding with any unknown samples
in order to standardize the Ferrous Sulfate solution. If the difference between the
two blanks is not within 0.2 mL of Ferrous Sulfate solution, clean the burette and
associated tubing. Reanalyze two more blanks to determine if the problem has
been eliminated.
2. Weigh 1.0 g of mineral soil into a 250-mL wide mouth graduated Erlenmeyer
flask.
3. Pipet 10.0 mL of the Potassium dichromate solution into each flask containing
and mix by carefully rotating the flask to wet all of the soil.
4. Under a fume hood, carefully add 20 mL of concentrated Sulfuric Acid to each
flask and mix gently.
5. Allow flasks to stand for 5 min under the fume hood.
6. Add pure water to each flask such that the final volume is approximately 125-mL.
Mix by swirling gently.
7. Allow the samples to cool and return to room temperature and recheck volume
after 30 minutes.
8. Add 5 or 6 drops of Phenanthroline complex and immediately titrate with the
Ferrous Sulfate solution. Use a mixing bar to properly mix the sample as it is
titrated. As the titration proceeds, the solution will take on a green color that will
change abruptly to reddish-brown when the endpoint of the titration is reached.
9. Record each volumetric reading to the nearest tenth of an mL.
Calculation
1. Organic C (OC) is determined from the meq of K2Cr2O7 reacting with the soil to
oxidize organic C, which is the difference between total meq of K2Cr2O7 added to
soil and the meq of FeSO4 that titrated the remaining K2Cr2O7 after the reaction.
OC, % =
(meq K2Cr2O7 – meq FeSO4) x (0.03 g C/meq) x 100 x cf
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g oven-dry soil

The oxidation state of C in organic matter is assumed to be 0. The C in organic matter is
oxidized to CO2 with an oxidation state of +4. Thus, the equivalent weight of C is 3 g.

An oxidation correction factor (cf) is required since not all the organic C in soil is oxidized
with room temperature oxidation. Correction factors can range from 1.14 to 1.32.
2. The difference in total meq of K2Cr2O7 reacting with soil and the meq remaining
after C oxidation can then be shown as:
(meq K2Cr2O7 – meq FeSO4) = (10 – mLS) (10) (1 N / mLB) =
10 (1 – mLS/mLB)
where, mLB is the volume of the blank titration and mLs is the volume of
the sample titration
Substituting the above into the equation for OC, % results in:
OC,% =
10 x (1 – mLS / mLB) x (0.03 g C / meq) x 100 x cf
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g oven-dry soil
Using a correction factor (f) of 1.3 results in:
OC, % = 10 x ( 1 – mLS/mLB) x 0.039
mg oven-dry soil
3. Since there is approximately 58% C in soil organic matter, soil OM can be
determined as:
OM, % = 10 x (1 – mLS/mLB) x 0.039 x 1.72
g oven-dry soil
S = Volume of Ferrous Sulfate solution required to titrate the sample, in mL.
B = Average Volume of Ferrous Sulfate solution required to titrate the two blanks, in mL.
10 = conversion factor for units.
1.72 = a factor derived from the conversion of % organic carbon to % organic matter
Analytical Performance
Range and Sensitivity
1. The Walkley-Black method of determining organic matter using 1 g of soil is
accurate in mineral soils where total OM is 6% or less. Soils with more than 6%
SOM should be tested using quantities less than 1 g to avoid difficulties in
observing the color change at the titration endpoint and exhausting all the
K2Cr2O7 available to oxidize the OM.
Precision and Accuracy
1. Each volumetric reading is recorded to the nearest 10th of a mL. This titrant
volume change corresponds to 0.03% OM.
2. To ensure precision, the person titrating must be able to view the reddish-brown
color change at the endpoint to stop the titration. The aid of a stirring bar and a
well lit stirring plate can improve the ability to view the end point.
Interferences
1. Ferrous iron and chloride in soil can result in positive errors in OM% since these
constituents can result in reduction of K2Cr2O7. Ferrous iron should not be a
problem in soils thoroughly air-dried since this ensures nearly complete oxidation
of ferrous iron to ferric iron. Chloride should not be a concern in well drained
soils without a recent fertilizer application. Options for treating soils with high
chloride can be found in Nelson and Sommers (1996).
2. Manganese oxide (MnO2) can compete with K2Cr2O7 in oxidizing soil OM
resulting in negative errors in OM%. This is a rare occurrence and only occurs
with freshly precipitated MnO2.
Safety and Disposal
1. Disposal of chromate waste by-products of WB titration method must be properly
managed in accordance to EPA regulations.
References
Magdoff, F.R., M.A. Tabatabai, and E.D. Hanlon. 1996. Soil Organic Matter: Analysis
and Interpretation. Soil Sci. Spec. Pub. No. 46:21-31.
Mylavarapu, R. 2009. UF/IFAS Extension Soil Testing Laboratory (ESTL) Analytical
Procedures and Training Manual. Circular 1248, Soil and Water Science Department,
Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences,
University of Florida.
Nelson, D.W. and L.E. Sommers. 1996. Total carbon, organic carbon, and organic
matter. p. 961-1010. In D.L Sparks et al. (eds.) Methods of soil analysis. Part 3.
Chemical Methods. SSSA Book Series No. 5, SSSA and ASA, Madison, WI.
Walkley, A. and I.A. Black. 1934. An examination of the Degtjareff method for
determining soil organic matter, and a proposed modification of the chromic acid titration
method. Soil Science 37:29-38.