Active Carbon (POXC)

For the entire detailed protocol, see the full downloadable manual

Active carbon, also known as Permanganate oxidizable carbon (POXC), is a soil fertility indicator that can be used to measure impact of soil fertility interventions. Active carbon consists of the available or labile portion of Soil Organic Matter (SOM), of which carbon in organic molecules is the backbone and about 45% by weight. Soil carbon/ SOM enhances water-holding capacity of soils, feeds soil life that binds soil particles together, and enhances the storage of plant nutrients in soils, so that the active fraction of soil carbon measured by this test is linked to many ecological functions of the soil. The video and instructions below detail how this test is done.

Instructional Video:

Materials you will need:

  • Centrifuge tubes or similar container with a lid that seals, 50-100 mL
  • Bottled water or clean tap water
  • Air-dried soil, 2.5 grams per analysis
  • Test solution: 0.015 M Potassium Permanganate + 0.1 M CaCl2 IN THE SAME SOLUTION. See the downloadable manual for the recipe to make this solution
  • 11-mL clear vials, with 0.75 inch diameter, for use with the Hanna high-range phosphate colorimeter
  • Additional 50-mL centrifuge tubes, or similar containers in which to dilute the digested solution and also prepare a diluted version of the initial, undigested solution without soil.
  • Graduated transfer pipet, these can be bought with 0.5 mL graduations.  A graduated dropper can also be prepared by marking the level where it contains exactly 0.50 mL or 0.50 g of water, using an accurate balance

Method step by step:

  1. Mix 2.5 g soil in 20 mL Potassium Permanganate + Calcium Chloride solution. Note that this could be more soil if we estimate at the outset that there are very low levels of active C, or less soil for very high organic matter soils. Regardless, the amount should be recorded on a datasheet. See the video and manual for strategies on a datasheet and timing of multiple samples.
  2. Shake 2 minutes.
  3. Let settle 10 minutes. The CaCl2 will flocculate the soil clays so that they settle out and leave a clear solution, except for the remaining purple color of the KMnO4.
  4. Then, take 0.5 mL of the settled, purple and add to 30 mL water in centrifuge tube or other small container, and mix well, rinsing the dropper by taking up and expelling water from the tube. Really try to use exactly 0.5 mL on this step, since it affects the precision of the test in a direct way. The tubes or containers with 30 mL water can be pre-weighed before the analysis (weighing out 30 g water = 30 mL water).
  5. Transfer some of the diluted solution to the 11 mL, 0.75 inch diameter vial which is used as a cuvette for the colorimeter.  Read the sample with the Hanna high range colorimeter and note the reading. Briefly, here are the instructions for reading using the Hanna colorimeter (see image below):
    • Press button to turn on (press and hold to turn off first if just finishing another measurement)
    • Wait for C1, insert control vial with water, and press the button again.
    • Wait for C2, insert purple-colored vial with permanganate, and press button again.
    • Take a reading.
Above: procedure for taking a reading on the Hanna phosphate colorimeter
  1. Using the colorimeter, you will also need to read a 100% check sample using the same procedure above in step 5, which you make by diluting 0.5 mL unreacted Permanganate + Calcium Chloride solution (straight from the bottle without soil addition) in the exact same way as the sample into 30 mL water in a bottle or vial (see picture below).  Re-measure the 100% control value of this solution every few samples.
Above: procedure for taking a reading on the Hanna phosphate colorimeter


You should have recorded three values from each sample: (1) the weight of the soil going in; (2) The reading from the sample on the colorimeter in step 5, and (3) the reading from the 100% KMnO4 check solution in step 6 (unreacted with soil and diluted in the exact same way as the samples).

Let’s say that we weighed out 2.51 g soil to analyze,  the reading of the soil sample is 13.2, and the reading of the 100% check solution is 17.6.

Then, as follows:

  1. The known concentration of the 100% solution is 0.015 M
  2. The proportion of remaining KMnO4 in the reacted soil solution is found as the ratio of the readings since the reading is proportional to the KMnO4 in solution: this is 13.2/17.6 or 75%, since 13.2/17.6 = 0.75
  3. This means that the concentration of permanganate ion consumed by the soil and lost in the solution (proportional to the presence of active carbon) is (100%-75%) or just 25% of the initial 0.015 M in terms of concentration.  Multiplying 25% x 0.015 we get 0.25 x 0.015 = 0.00375 M, which is how much the concentration of permanganate was lowered in the solution because of carbon in the soil.
  4. We want to turn this change in concentration into an absolute amount in moles of permanganate, and we can do this by multiplying this change in concentration by the volume of the solution: 0.00375 M x 0.020 L = 0.000075 moles, which can also be expressed as 7.5 x 10 -5 Moles (note that 20 mL = 0.020 L which we must use since molarity is defined as moles / L )
  5. To convert this to mg of active carbon in soil oxidized by the KMnO4, the developers of the test determined that a conversion factor of 9000 can be used:
    mg C that was oxidized = 7.5 x 10 -5 moles KMnO4 x 9000 = 0.675 mg C
  6. This amount of “active” carbon can then be divided by the initial mass of dry soil that was weighed out for the analysis (here we also need to express the g soil weighed out initially as kg, dividing by 1000):
  7. So, POXC or active carbon content = 0.615 mg / 2.51 g = 0.615 / 0.00251 kg, which equals 269 mg C/kg soil, rounding to the nearest whole number. This is a relatively low level of POXC; The highest levels seen in soils can approach 1500 mg C/kg or even more, but even lower levels down to 100 mg/kg are seen in sandy and/or highly impoverished soils.