Active Carbon (POXC)

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. Soil carbon provides a fabric which binds soil particles together, and 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.

You will need:

● Centrifuge tubes
● Bottled water or clean tap water
● Air-dried soil
● Test solution: 0.015 M Potassium Permanganate + 0.1 M CaCl2 IN THE SAME SOLUTION.
● 11-mL clear vials, with 0.75 inch diameter, for use with the Hanna high-range phosphate colorimeter.
● A second 50-mL centrifuge tube, or similar container to dilute the digested solution in
● Graduated transfer pipet, these can be bought with 0.5 mL graduations.  A dropper can also be calibrated by marking the level where it contains exactly 0.50 mL or 0.50 g of water, using a balance.

Method

  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 OM 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 so that during settling, the clays will settle out and leave a clear solution, except for the 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.  These tubes or containers can be pre-weighed with 30 mL of water before the analysis (weighing out 30 g water = 30 mL water). For reference, this is a dilution specialized for a Hanna high-range phosphate colorimeter, which gives a good range of values. For the Hanna low-range colorimeter the dilution should be 1 mL in 110 mL water, or 0.5 mL in 55 mL; for other colorimeters the dilution should be tested to give a full-scale reading for the unreacted 100% control solution (see next step) which will maximize the sensitivity of the test.
  5. Transfer some of the diluted solution to the 11 mL, 0.75 inch diameter vial which will serve as a cuvette for the colorimeter.  Read the sample with the Hanna high range colorimeter and note the reading.
  6. You will also need to read a 100% check sample, which is just unreacted Permanganate / Calcium Chloride solution (straight from the bottle without soil addition) diluted in the exact same way as the sample.  Re-measure the 100% control value of this solution every few samples.

Calculations:

You should have recorded two values from each sample: the weight of the soil going in, and the reading from the sample on the colorimeter. In addition you should have the 100% KMnO4 check solution reading (unreacted with soil and diluted in the exact same way as the samples).

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

then, as follows:

    1. The known concentration of the 100% solution is 0.015 M
    2. The remaining KMnO4 in the reacted soil solution is 13.2/17.6 x 0.015 M or 75% x 0.015 M, since 13.2/17.6 = 0.75
    3. This means that the amount of permanganate ion consumed by the soil and lost in the solution (the important factor linked 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 amount in moles of permanganate, and we can do this by just 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):
      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 fairly low level of POXC; The highest levels seen in soils can approach 1500 mg C/kg or even more.

 

 

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