A simple quantitative method for determination of active organic carbon in soils.
Dr Tim Apps, Apps Laboratories
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.
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).
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.
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.
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.
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).
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.
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.
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.
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.