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.
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.
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.
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.
M = ((Ww-Wd)/Ww) x 100
M = moisture content (%) of compost sample
WW = wet weight of the sample, and
Wd = weight of the sample after drying.
Cornell Waste Management Institute ©1996
Ithaca, NY 14853-5601