Soil Health Assessment

potato field showing soil

Method to analyze Permanganate Oxidizable Carbon (POXC, active carbon)

Introduction

Active carbon, also known as permanganate oxidizable carbon (POXC), is a soil health indicator that can be used to measure the impact of practices that try to build soil health, or diagnose a soil for a lack of soil organic matter. Active carbon consists of the available or labile portion of Soil Organic Matter (SOM), and is also usually correlated to the total amount of carbon (SOC) in soils . 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.

Instructional video:

Materials:

  1. Potassium permanganate (KMnO4)
  2. Calcium chloride (CaCl2)
  3. Water: clean water from a faucet, or clean bottled water, is acceptable.
  4. Graduated cylinder, 25 mL or 30 mL.
  5. Centrifuge tubes, 50 mL or similar small bottles or tubes, for extracting samples.
  6. A small, graduated dropper or transfer pipet that has been calibrated to contain 0.5 mL (weigh it empty and then mark the line where it contains 0.50 g water)
  7. Hanna colorimeter, model HI-717, ‘checker’ model for high range phosphate (the same model as used in the available phosphorus test in this tool kit)
  8. Vials that fit the colorimeter, these are 11 mL vials with a diameter of 17 mm (3/4 inch)

Procedure:

Making the Permanganate + Calcium Chloride Solution.

For each 100 mL of solution:

  1. Measure or weigh 100 mL water (equal to 100 g) in a transparent water bottle (an amber colored dark bottle or one wrapped in tape or foil is also good, to protect the solution from light.
  2. Add 1.11 g CaCl2 for each 100 mL of water.
  3. Mix the solution well until all CaCl2 is dissolved.
  4. To this same solution of water + CaCl2, add 0.237g KMnO4 (potassium permanganate); or 0.24 g if you have a scale with only 0.01 g precision). Mix well.
  5. Making a volume of solution greater than 100 mL will give greater precision in the concentrations, for example, 500 mL of solution with 5.55 g CaCl2 and 1.185 g KMnO4 (multiply 1.11g and 0.237g which correspond to 100 mL by 5) will mean that the weighing errors for permanganate are not as great.
Carrying out the measurement
  1. Add 2.5 g of soil, sieved to 2 mm or with stones removed, to 20 ml permanganate solution, in a centrifuge tube (50 ml) or similar bottle. Be sure to record the weight of the soil.
  2. Close the tube well and shake for 2 minutes.                                                                
1. Shake well to mix the soil and permanganate solution for 2 minutes.
2. After shaking, shake the container 2 or 3 times in an upright position, to wash bits of sand and organic matter from its lid and sides, and let it rest for 10 minutes.
  1. Before allowing the solution to settle, swirl the capped tube once, without turning upside down, to wash any remaining soil from the cap and sides of the tube.
  2. Let sit for 10 minutes. The clays will settle out because of the calcium chloride, leaving a purple solution without suspended soil.
  3. During this time, or prior to the analysis, prepare a tube with 30 mL of tap water to dilute the permanganate solution, and at least one additional tube of 30 mL to prepare the 100% dilution of the standard.
  1. At the end of the 10 minutes settling time, withdraw 0.5 mL of the overlying soil-free solution with the dropper from the tube with permanganate and soil, as precisely as possible (Fig. 3). Add this to the tube with 30 mL of water to dilute the color for reading with the colorimeter. Rinse the dropper with the water, drawing and releasing a few times, to ensure that all the color is transferred to the tube with water.
3. Transfer 0.5 mL of the purple sample solution to 30 mL of clean water. Repeat for raw solution without reacting with soil.
  1. Also prepare the same dilution of 0.5 mL + 30 mL of water in the other tube previously prepared, but with 0.5 mL of raw permanganate solution directly from the reagent bottle and without having been exposed to soil. Call this the ‘100% standard’. Only one of these tubes is needed for the entire procedure, although preparing two can improve accuracy. It is used to compare the concentration of KMn04 with and without reacting with soil.
  2. Take the reading with the colorimeter. For readings, you will need vials with the diluted sample and the diluted 100% standard you just prepared, and a blank vial with just water. Pour the samples and 100% standard from the tube where it was diluted into the appropriate vials for the colorimeter (3/4 inch diameter). Make sure all vials are clean and free of dirt or stains on the outside, wiping clean if necessary.
  3. For each reading, carry out the procedure shown the figure below:
    • Turn on the colorimeter by pressing the button and wait until ‘C1’ shows.
    • Insert the blank vial with clear water and press the button.
    • Wait for ‘C2’ to appear.
    • Take out the blank and insert the sample to read, and press the button again.
    • Wait for the reading and then record it, for example ‘12.3’
  4. Repeat the procedure with the 100% standard, which should have a higher reading than the sample, usually between 19 and 22.
4. Reading of the sample reacted with the soil sample.
5. Reading the 100% standard.
  1. To convert the readings into the level of active carbon in soil, you can use the online calculator just below.  If you want to go through the calculations in detail, these can be found in the manual downloadable from this website. In addition, there are data forms available that will carry out the calculation, through an open data kit (ODK) survey app available at the soil health data platform at https://soils.stats4sd.org.  After calculating the value of active carbon, which will generally range from 0 to 1200 mg C/kg soil, you can interpret the results using the scoring table at the end of this webpage.
  2. Note on performing the test on multiple samples: The timing of the reaction steps in this test is important. For greater precision, it is essential not to vary the shaking (2 min.) and decanting (10 min.) times by more than 10 or 15 seconds. For this reason, to analyze several samples in a row, they should be organized into groups. One strategy is to work in groups of four samples. A single stopwatch is started and a table like the one below can be set up to record the designated timings and record any variations that occur. Sample codes and result values can also be recorded in this table to maintain a hard copy. In this way, about 10 samples per hour can be made and confusion over timing is avoided.

Qualitative scoring table for active carbon or POXC: