Composting In Schools

Monitoring Compost pH

Why is compost pH worth measuring? Primarily because you can use it to follow the process of decomposition. Compost microorganisms operate best under neutral to acidic conditions, with pH's in the range of 5.5 to 8. During the initial stages of decomposition, organic acids are formed. The acidic conditions are favorable for growth of fungi and breakdown of lignin and cellulose. As composting proceeds, the organic acids become neutralized, and mature compost generally has a pH between 6 and 8.

If anaerobic conditions develop during composting, organic acids may accumulate rather than break down. Aerating or mixing the system should reduce this acidity. Adding lime (calcium carbonate) generally is not recommended because it causes ammonium nitrogen to be lost to the atmosphere as ammonia gas. Not only does this cause odors, it also depletes nitrogen that is better kept in the compost for future use by plants.

At any point during composting, you can measure the pH of the mixture. In doing this, keep in mind that your compost is unlikely to be homogeneous. You may have found that the temperature varied from location to location within your compost, and the pH is likely to vary as well. You therefore should plan to take samples from a variety of spots. You can mix these together and do a combined pH test, or test each of the samples individually. In either case, make sure to make several replicate tests and to report all of your answers. (Since pH is measured on a logarithmic scale, it doesn't make sense mathematically to take a simple average of your replicates.)

pH can be measured using any of the following methods. Whichever method you choose, make sure to measure the pH as soon as possible after sampling so that continuing chemical changes will not affect your results:

Soil Test Kit

Test kits for analysis of soil pH can be used without modification for compost samples. Simply follow the manufacturer's instructions.

pH Paper

If your compost is moist but not muddy, you can insert a pH indicator strip into the compost, let it sit for a few minutes to soak up water, then read the pH using color comparison.

Compost Extractions

Using a calibrated meter or pH paper, you can measure pH in a compost extract made by mixing compost with distilled water. It is important to be consistent in the ratio of compost to water and to account for the initial moisture content of the compost, but there is no universally accepted protocol specifying these procedures.

One approach is to read the pH in oven-dried samples that have been reconstituted with distilled water.

  1. Spread compost in a thin layer in a pan, and dry for 24 hours in a 105-110°C oven.
  2. Weigh or measure 5 g samples of oved-dried compost into small containers.
  3. Add 25 ml distilled water to each sample.
  4. Mix thoroughly for 5 seconds then let stand for 10 minutes.
  5. Read the pH with a calibrated meter or with pH paper and record as compost pH in water, or pHw.

An alternative is to measure pH in samples that have not been dried. In this case, the amount of water that you add will need to vary to compensate for the varying moisture content of the compost. You will still need to dry some of the compost in order to measure moisture content, but you can take the pH readings on samples that haven't been altered by drying.

  1. Calculate the % moisture of your compost:
    a) Weigh a small container.
    b) Weigh 10 g of compost into the container.
    c) Dry the sample for 24 hours in a 105-110°C oven, or for 5 minutes in a microwave oven. If you use a microwave oven, place a beaker containing 100 ml of water in the oven during the drying to protect the oven's magnetron.
    d) Reweigh the sample, subtract the weight of the container, and determine the moisture content using the following equation:

    M = ((Ww-Wd)/Ww) x 100
    in which:
    M = moisture content (%) of compost sample
    WW = wet weight of the sample, and
    Wd = weight of the sample after drying.

  2. Use the % moisture to figure out how much water to add.
    For example, if your compost sample is 40% moisture, you will compensate by adding only 60% of the water you would need if the sample were air dried (0.60 x 5 ml = 3 ml water needed).
  3. Weigh or measure 5 g samples of compost into small containers.
  4. Add the calculated amount of distilled water to each sample.
  5. Mix thoroughly for 5 seconds.
  6. Let stand for 10 minutes.
  7. Read the pH with pH paper or a calibrated meter and record as compost pH in water, or pHw.


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