The initial pH of garbage, refuse, manure, and other compostable material is likely between 5.0 and 7.0 unless the waste contains ash or other highly alkaline materials. If the material has undergone putrefaction before being received for composting, the pH will be near the lower value. When the initial pH is between 6.0 and 7.0, the pH of the composting material will usually drop a little during the first two or three days of aerobic composting, owing to the formation of some acid. If the pH is 5.0 or 5.5, there will be little change during this period.

After two to four days the pH usually begins to rise and will level off at between 8.0 and 9.0 towards the end of the process. The control of the pH in composting is seldom a problem requiring attention if the material is kept aerobic, but large amounts of organic acids are often produced during anaerobic decomposition on a batch basis. Ash, carbonates, lime or other alkaline substance will act as a buffer and keep the pH from becoming too low. However, the addition of alkaline material is rarely necessary in aerobic decomposition and, in fact, may do more harm than good because the loss of nitrogen by the evolution of ammonia as a gas will be greater at the higher pH. Since the optimum pH for most organisms is around 6.5 to 7.5, it would probably be beneficial if the pH could be maintained in that range. However, since composting is necessarily a batch-process operation, minor changes in the pH must be expected.

Apparently, initial pH values of 5.0 to 6.0 do not seriously retard initial biological activity since active decomposition and high temperatures develop rapidly after material is placed in the stack. Temperatures do appear to increase a little more rapidly when the pH is in the range around 7.0 and above. The usual waste materials available for composting present no problem of pH control.