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A simple quantitative method for determination of active organic
carbon in soils. Dr Tim Apps, Apps Laboratories Preparation of digestion solution. This note
describes an adaptation of a method for estimating reactive (relatively
fresh) soil organic matter in soil. The method is simple and uses a low cost
photometer to obtain quantitative results. It is aided by a spreadsheet model
that allows flexibility in sample size and dilution factor for final readings
thus allowing for soils with a wide range of organic matter levels to be
tested. Soil moisture content can be incorporated into the model thus
providing higher accuracy. Background
Conventional
soil testing has tended to address issues of nutrition by directly estimating
availability of individual nutrients. However this often doesn’t tell much
about how the soil functions for example to provide structure, water holding
capacity or nutrient supply. Recently efforts have been made to understand
and measure soil ‘quality’ and soil ‘health’ factors that can be related to
the ability of the soil to store and supply nutrients and in turn, to farming
practices. Soil organic matter (SOM) is one of those key factors. Amongst the
well known benefits of soil organic matter is the ability to hold and slowly
release nutrients especially nitrogen as it breaks down. An excellent
overview of the role of soil organic matter is (Hoorman and Islam 2010) Most
methods for determination of organic matter in soils focus on either the
humus fraction or on total organic carbon. Humus is sometimes estimated by
alkaline EDTA extraction. A typical kit is the Lamotte 5012 Humus kit. Humus
concentration in this kit is estimated visually in a range consisting of 5
levels. Total
organic matter can be estimated by methods such as acid – dichromate digestion.
An example of a test kit that performs this is the Lamotte 5020 Organic
matter kit. Acid digestion is slightly hazardous and requires at least some
basic lab facilities. Soil
organic matter can also be estimated by digestion with 30% hydrogen peroxide
(H2O2 ). This method determines only a portion of the
total organic matter and this can vary from 20% to 90% depending on the type of soil and the
sample depth. Some representative figures obtained by this method compared to
more complete digestion eg using acid / dichromate are: Forest soils 30% at
25 cm depth to 90% near the surface and for pasture soils 50% at 20 cm depth
to 60% near the surface (Apps 1987). The method uses 1.5 ml peroxide for 1 gm of soil.
A correction can be made for water content which should be determined on a
duplicate sample. Typical overall figures for organic matter from this method
range from 3% – 6 % of dry weight of soil.
Humus
content is known to be a significant factor in nutrient holding capacity and
therefore is a good qualitative indicator of exchange capacity. H2O2
digestion provides a combined estimate of both reactive and partly decomposed
organic matter. Neither of these tests estimate active SOM and therefore are
limited in providing feedback on management strategies such as those that are
seasonal and that include tillage, grazing regime, amendments or residue
retention. Potassium
permanganate (KPM) digestion has been proposed as a way to estimate a more
reactive fraction of soil organic matter. KPM is a moderately strong oxidant
and is non hazardous at the solution strengths that are needed to oxidize
organic matter in soils. If organic
matter is added to a KPM solution some of the KPM will be chemically reduced
and its purple / magenta colour will fade in proportion to the amount of
organic matter oxidized. The change in colour can be used as a qualitative
measure of reactive SOM. A simple procedure is described in (DPI Victoria 2012). Weil et
al. have described in detail a simple quantitative method for estimating
reactive SOM by permanganate digestion. The method describes the use of a
spectrophotometer to measure concentration of stock and reacted KPM solution
to estimate reactive SOM in a sample (Weil,
Islam et al. 2003). Theory
KPM will
oxidize reactive relatively fresh SOM. “Specifically, slightly alkaline KMnO4 is known
to hydrolyze and oxidize simple carbohydrates, amino acids, amine/ amide
sugars, and C-compounds containing hydroxyl, ketone, carboxyl, double-bond
linkages and aliphatic compounds, to give a light pink color” (Weil, Islam et al. 2003). To obtain
a quantitative estimate for organic carbon, KPM has to be determined
preferably by photometer. KPM absorbs strongly in the approximate range 500
to 600 nm. (Weil, Islam et al.
2003) argue that 570 nm is the preferred wavelength.
However Apps Labs tests show that absorbance of KPM solutions is
significantly higher at 520 nm than at 570 nm. Normally
a spectrophotometer or photometer than can measure absorbance at different
wavelengths is an expensive addition to a lab. A simple and cheap solution
especially for field use is a Hanna Instruments Checker photometer. These are
very compact and relatively inexpensive (<$100) single wavelength
photometers for testing a range of individual analytes in water. Models are
available for a few different wavelengths including 525 nm and 575 nm. They
are analyte specific photometers. For example the Phosphate Checker reads
phosphate in the range 0.00 to 2.50 ppm using a 525 nm wavelength. However a
calibration curve can be constructed showing the concentration of potassium
permanganate, [ KPM] against ppm phosphate. The Checker then becomes a tool
for measuring KPM in solution. See Checker
calibration Weil et
al have tested various shaking and settling times for samples and have found
that 2 minutes shaking time followed by 10 minutes settling times gives
results that are consistent and that can be related well to management
activities. Method
overview
This
method is a adaptation of (Weil,
Islam et al. 2003). The main differences are: To reduce handling, the
recommended stock solution is prediluted 0.02 mol/L KPM with 0.1 mol/L CaCl2.
The latter is included as a flocculant. This solution is used without further
dilution. See Preparation of digestion solution. Air dried or soil
at field moisture can be used but in the latter case soil water content
should be determined on a duplicate sample so that organic matter can be
reported on a wt / dry weight of soil. In time, at a given site it may be
possible to estimate soil water content based on look and feel alone with
acceptable accuracy. A sample
of soil between 1 and 2.5 gm is added to 20 mls KPM / CaCl2
solution, shaken then allowed to settle. The amount of soil used will
depending on organic matter content and should be determined by experiment. A
spreadsheet calculator allows the amount of soil added to be varied along
with the dilution factor necessary for obtaining a reading by the Checker
photometer. The soil sample can be measured by weight or volume. If volume is
used then a bulk density (gm/cc) measurement is required but this can be
estimated if required – some representative figures are provided in the
calculator. Because
KPM solution may degrade over time, it is useful to run a control test using
the photometer with each batch of samples to determine the current [KPM]. As
long as the current ‘Control’ information is included in the model the model
will automatically take the starting [KPM] into account when calculating KPM
used by each sample. The
sample is mixed with KPM solution in a suitable sized test tube or similar.
The mixture is shaken for 2 minutes at around 100 cycles per minute followed
by 10 minutes standing. For consistency shaking and settling times should not
be varied. Absorbance is taken in a diluted subsample of the digestion mix. A
suitable dilution of the test solution so it can be read by the Checker
phosphate photometer is 60 – 180 depending on how much unreacted KPM is left
in the test solution after shaking. The Checker photometer uses 10 ml tubes
so to achieve these dilutions a volume of between 0.05 and 0.67 mls should be
added to the tubes and made up to 10 mls with distilled or RO (reverse
osmosis) water. At Apps Labs we use graduated plastic droppers. These deliver
1/18 ml per drop. One drop in 10 mls is equivalent to a 1 : 180 dilution, 3
drops in 10 ml is a 1 : 60 dilution. Checker
calibration
The
calibration curve should be constructed using fresh accurately prepared KPM
solution. For convenience the stock 0.02 mol/L KPM solution can be used. For
each individual Checker photometer the calibration curve should be stable
over time. Either use a pre calibrated Checker or calibrate each new Checker
before starting any testing. The
Checker reads [KPM] up to around 0.00012 mol/L which is approximately a x180
dilution from 0.02 mol/L. If using 0.02 mol/L KPM (same as stock digestion
solution) add approximately 1 drop from a calibrated pipette to a colorimeter
tube and make up to 10 mls with distilled or RO water. The dilution factor is
calculated as 10/(mls delivered in 1 drop). For example if the pipette
delivers 1/18 ml per drop then the dilution is 10/(1/18) = 180. A second
calibration point can be found by first diluting the stock solution x2 then
taking a 1 drop subsample. For a third point dilute the stock solution by x4
then take the subsample. At Apps Labs we calibrated a phosphate Checker and
have included the calibration curve on the calculations spreadsheet. For each
individual photometer new calibration data can be entered in the model. Method
summary
1. Add 20
mls of 0.02 mol/L KPM solution to a test tube. Record the actual volume used. 2. Weigh
a field moist soil sample – between 1 and 2.5 gms is suitable. Experience
will show how much to use. Record the weight. If a balance is not available
then a scoop can be used. Record the volume used, estimate the bulk density
(BD) (table is included on the calculations spreadsheet) then multiply volume
x BD to get weight. 3.
Calculate or estimate the water content of the soil sample. To estimate the
water content use the table included on the calculations spreadsheet.
Calculating the water content by weighing a sample before and after air
drying, allowing for the weight of the container is more accurate and
therefore preferred. 4. Cap /
seal and shake the tube 2 mins at around 100 cycles per minute. Allow to
settle for 10 mins but don’t re-shake before sampling. Take care not to spill
KPM as it can stain benches, clothes and skin. Use a cooking grade citric
acid solution to reduce staining especially on glassware. 5. Fill
the Checker colorimeter tube to just below the 10 ml line with distilled or
RO water. Add one to 3 drops of clear sub sample from near the top of the KPM
/ soil solution using a pipette (less drops for deeper colored solutions).
The pipette must be calibrated so you know how many drops there are in 1 ml,
that is how many mls per drop. Fill the colorimeter tube to the 10 ml line.
Calculate and record the dilution factor. 6. Follow the instructions for the Checker to
take the absorbance as mg/L PO4. The blank is 10 mls of the same distilled or
RO water. Record the colorimeter readout. 7. With
every series of tests take an absorbance reading of the unreacted KPM
solution, that is without a soil sample. To do this take 1 drop of the KPM
from the stock solution and add to the colorimeter tube, making up to 10 ml
with distilled or RO water. For a blank again use distilled or RO water. 8. Use
the spreadsheet calculator to estimate organic carbon. Various units are
shown but mg/kg based on soil dry weight including mg/kg. The calculator is
an Excel spreadsheet and can be found at www.appslabs.com.au/Reactive_soil_organic_matter_model.xls. Fill test
tubes and colorimeter tubes with a citric acid solution and let stand to
clean off any KPM deposits (may take overnight). Control
The
control measurement establishes the starting concentration of KPM so that the
amount removed by organic matter digestion can be calculated. To do this add
1 drop of the stock 0.02 mol/L KPM (1/18 ml) to 10 ml RO water (10 ml total)
using a calibrated dropper. Take the ‘absorbance’ using the HI713 Checker
then use the spreadsheet model to calculate a ‘starting’ [KPM]. Every
calculation needs to have a ‘control’ [KPM] estimate included but the actual
test only needs to be carried out periodically to take into account any
variation in the stock solution. Preparation
of digestion solution.
The
digestion solution is 0.02 mol/L KPM plus 0.1 mol/L CaCl2 (in the one solution). This can be
prepared by adding 1.58 gm KPM and 7.35 gm of CaCl2 to 500 mls distilled or RO water.
The pH of the solution is raised to pH 7.2 to improve stability. This
requires approx 0.3 mls of 0.1 mol/L sodium hydroxide (NaOH) per 500 mls of
KPM solution. Typical
results.
Results
for this test can be reported as organic matter or organic carbon either as %
w/w or as w/w for example as mg/kg. Organic carbon can be approximated to 55%
of SOM. Table.1 shows some representative values for organic carbon
in soils using potassium permanganate and hydrogen peroxide digestion.
Table.1 Selected values for soil organic carbon by hydrogen peroxide and potassium permanganate digestion. * (Weil, Islam et al. 2003), ** (Apps 1987), *** Apps Laboratories internal records. References:
Apps, G. J. T. (1987). Disturbance
effects on carbon and phosphorus levels and decomposition in a krasnozem soil
of south-eastern Australia. Botany Department, Monash University. DPI Victoria (2012). Quick
Reference Guide: Potassium Permanganate Test for Active Carbon. Hoorman, J. J. and R. Islam
(2010). Understanding soil microbes and nutrient cycling. Fact Sheet
SAG-16-10, Ohio State University Agriculture and Natural Resources. Weil, R. R., K. R. Islam, et al.
(2003). “Estimating active carbon for soil quality assessment: A simplified
method for laboratory and field use.” American Journal of Alternative
Agriculture 18: 3-17. |