Acid/Alkalinity Issues

pH is the measure of the acidity or alkalinity of soil, with 7 considered “neutral” and numbers below acidic and above alkaline. Composting feedstocks have a pH, which will fluctuate during the composting process.

The initial pH of garbage, yard clippings, manure, and other compostable material is likely between 5.0 and 7.0 unless it contains ash or other highly alkaline materials. If the material has begun putrefying before being received for composting, the pH will be near the lower value, since anaerobic organisms produce acids. When the initial pH is between 6.0 and 7.0, the pH of the composting material may drop a little during the first two or three days of aerobic composting, also due to the formation of acids. 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. Adding alkaline material is rarely necessary in aerobic decomposition. In fact, it may do more harm than good because the loss of nitrogen by the release of ammonia as a gas will be greater at a 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 are normal.

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 feedstocks available for composting present no problem of pH control.